Nutrient Removal from Aqueous Solutions Using Biosorbents Derived from Rice and Corn Husk Residues: A Systematic Review from the Environmental Management Perspective

Abstract

This review critically analyzed the use of biosorbents derived from rice husks and corn residues for nutrient removal from aqueous solutions. Additionally, this review highlighted the use of such biosorbents in wastewater treatment. Furthermore, novel approaches for sustainable nutrient removal from aqueous solutions were identified. A comprehensive understanding of the implementation of biosorption processes using agro-industrial residues based on corn and rice crops is critical for the sustainable management of residues and water bodies in the world to protect and conserve natural resources. Specifically, the review focuses on the exploration, preparation and application of innovative biosorbents to remove various forms of nutrients such as total nitrogen, total phosphorus, nitrates, ammonium and phosphates from aqueous solution, analyzing the sustainability of treatments applied to biomass, such as thermal transformation or chemical modification to reduce environmental impacts. It was found that 95 to 99% of nitrogen and phosphorus can be removed with biosorbents made from rice husks and corn residues, analyzing approximately 50 scientific articles related to these plant materials. Research opportunities were identified, such as the recovery of removed nutrients for soil improvement, life cycle analysis to assess the concept of zero waste, among other aspects. Finally, a scheme is proposed for the selection and application of sustainable biosorbents for the removal of nutrients from aqueous solutions.

1. Introduction

Water is essential for life. Aquatic ecosystems depend on water quality because of the involved transport of essential nutrients for aquatic life [1,2] and oxygen for aquatic aerobic organisms, such as fishes and macroinvertebrates [3]. However, anthropic activities cause pollutant discharge in high concentrations into water. Such discharge poses a considerable risk to aquatic life. Wastewater discharge without effective treatment severely degrades water quality. Regulations governing the discharge of wastewater into waterbodies aim to protect water sources and minimize the environmental impacts on the health of aquatic ecosystems and humans.

Pollutants from wastewater, such as heavy metals, organic pollutants, pathogens, nitrogen, and phosphorus, end up in water sources, causing various harmful effects to aquatic organisms and humans. However, nutrients are also defined as the chemical elements that an organism requires for their growth and development [4]. Nonetheless, phosphorus and nitrogen can cause water eutrophication, favoring the excessive growth of algae, which biodegrade through aerobic processes, limiting oxygen for aquatic life and endangering its survival [5,6]. The main sources of nutrients in aquatic systems are (a) natural sources, such as atmospheric deposition, including wet atmospheric depositions (dissolved in drops and deposited through precipitation) and dry (particles) transport of nitrogen and phosphorus into bodies of water; and (b) anthropogenic sources, including fertilizer applications, septic systems, and nutrient discharges from industrial wastewater [7]. Engineers have designed and implemented processes in wastewater treatment plants (WWTPs) to avoid the discharge of pollutants, such as phosphorus and nitrogen, in high concentrations. However, processes for the removal of these contaminants from wastewater, such as chemical precipitation, biological, oxidation, and membrane processes (e.g., electrodialysis and reverse osmosis), are expensive [8,9,10]. This limits the applicability of such processes in developing countries, such as Colombia. Such countries do not have WWTPs with conventional processes because of inadequate financial resources. Therefore, implementing high-cost operating technologies is not feasible.

Low-cost processes, such as biosorption, have been used for the removal of phosphorus and nitrogen. Such processes are easy to operate and maintain. The most widely used biosorbent is activated carbon, which can remove various contaminants from water (e.g., metals, colorants, among others). A growing interest has been directed towards activated carbon for removal of nitrates and phosphates from aqueous solutions [11,12]. However, when using nonrenewable materials, adsorption is not sustainable from economic and environmental perspectives. Studies have focused on other low- or no-cost materials (different from mineral carbon) based on live or dead biomass, including readily available agro-industrial residues. Many agro-industrial residues, such as rice husks and corn residues, have been used to produce biosorbents for removing phosphorus, nitrogen, and other specific contaminants, such as heavy metals, from aqueous solutions [13,14,15]. For instance, rice crop residues have been successfully utilized for the removal of metals. In a study conducted by Bar et al. [13], removals of Cu(III) ranging from 50 to 85% were achieved in aqueous solution using untreated rice husk, which had only been washed and dried. Sarkar et al. [14] concluded that rice bran can be employed for the removal of heavy metals from wastewater. These agricultural residues have also proven to be efficient in eliminating phenols.; Mandal et al. (2022) [15] reported phenol removals between 70 and 90% for initial phenol concentrations of 5 to 300 mg/L using rice husk ash as a sustainable biosorbent, proving the significance of biosorption from natural sources in water decontamination.

In terms of nutrient removal, preliminary studies by Shukla et al. [9], Fatima et al. [16], Kizito et al. [17], and Zhu et al. [18] have achieved removal of phosphorus and nitrogen from aqueous solutions at high efficiencies of 50–80% using rice husk–derived biochar. Fan and Zhang [19] and Hollister et al. [20] studied the efficiency of biochar derived from corn stalk cellulose and corn stover for nutrient removal from aqueous solutions and revealed that corn residues, such as corn straw, stalk, and cob, can achieve good efficiencies. Another relevant aspect is the regeneration of biosorbents from agro-industrial residues, which may seem unimportant due to the abundance and low-cost availability of these residues. [15]. Nevertheless, nutrient-laden biosorbents might be used as soil fertilizers, as discussed in subsequent sections. When biosorbents are not utilized, they are typically incinerated at temperatures exceeding 800 °C, and the resulting ashes could be employed in road repair, landfills, or brick manufacturing in various regions [15].

Therefore, the significance of utilizing biosorbents derived from agro-industrial residues, such as rice husk, and their importance in mitigating environmental impacts in the biosorption process of nutrients from aqueous solutions should be considered. However, in some developing countries, such as Colombia, the application of biosorbents obtained from these types of waste is yet to be comprehensively studied. The efficiency and applicability of the agro-industrial residues of corn and rice for nutrient removal from wastewater should be understood to achieve effective global wastewater management. In Colombia, the rice industry produces 400,000 tons/year of rice husk residues, which are not sufficiently exploited. One of the largest rice production areas is located in Tolima, Colombia, in the Andean region, with production of 758,244 tons per year (38% of the national total production) in 2017–2018. According to the 2015 data reported by the Departamento Nacional de Estadística, corn corresponds to the largest planted area in the country, with a total of 313,820 hectares, followed by potatoes with 165,733 hectares. Rice is a transitory crop from Cundinamarca (department of Colombia), which, together with potatoes and peas, has an estimated biomass production of 129,860 tons/year, approximately half of that corresponding to corn (approximately 60,000 tons/year) [21].

Rice is one of the most important crops because it is the third-largest planted crop in the world. Numerous people depend on its harvest. Corn is one of the most consumed cereals in the world [22]. Therefore, rice and corn residues have garnered considerable research attention to minimize their environmental impacts. In addition to easy access to rice and corn husk residues (such as straw) because of the involved large cultivation area and consumption, these wastes have attracted considerable research attention because of their potential for developing low-cost and easy-to-prepare biosorbents, synergy between water treatment and other environmental applications (recovery of wastewater substances and soil improvement based on nutrient-laden biomass), and biodegradability [23,24]. Rice husks exhibit certain characteristics that make them suitable for study as a biosorbent of nutrients from aqueous solutions. For example, rice husks have a high silica content and can be transformed into biochar to remove nutrients from aqueous solutions and improve soil conditions [25]. Furthermore, rice husks have functional groups that improve their biosorption capacity, such as carboxyls [26], enabling increased efficiency with regard to nutrient biosorption.

Corn straw contains a considerable amount of lignocellulose with active functional groups for biosorption [27], and its biosorption capacity can be improved via physical, chemical, and biological modifications [28,29,30]. Corn stalks are rich in cellulose and exhibit a high content of reactive hydroxyl groups with the capacity for ion exchange, which can be utilized in the preparation of various functional polymers to remove ionic contaminants from water [31]. Therefore, evaluating the development of low-cost biosorbents from these agricultural residues is critical for nutrient removal from wastewater. The objective of this review was to critically analyze the current state of knowledge with regard to the application of biosorbents derived from rice husks and corn residues in nutrient removal from aqueous solutions, highlight the application of the biosorbents in wastewater treatment, and identify novel approaches for sustainable nutrient removal from wastewater.

Therefore, a bibliographic search of scientific articles published between 2013 and 2023 was performed in the following databases: ISI Web of Science, Scopus, Google Scholar, Science Direct, Redalyc, and SciELO. Combinations of the following keywords were used: biosorbents, biosorption, nutrient removal, nutrient recovery, phosphorus removal, nitrogen removal, wastewater, rice hulls and corn residues. Around 150 scientific articles related to the biosorption of nutrients from agro-industrial residue were considered, of which 33% corresponded to the elimination of nitrogen or phosphorus in aqueous solution using biosorbents obtained from rice husks and corn residues.

Research articles were selected considering the following criteria: (1) the study uses one of the two biosorbents of interest (rice husks or corn residues) for nutrient elimination or recovery from aqueous solutions (50 articles), (2) the focus is oriented toward wastewater treatment (50 articles), (3) the data analyses are focused on biosorption, either batch or column biosorption (fixed bed), at laboratory and pilot plant scales (50 articles), (4) biosorption is integrated with other wastewater treatment processes or technologies, (10 articles), and (5) mixtures of both residues are considered for removing nutrients from wastewater (5 articles). This review of the selected articles involved investigating the applicability of low-cost treatments to nutrient removal or recovery from wastewater to provide a tool for related decision-making.

2. Effects of Nutrients Dissolved in Water on Human Health

In addition to the negative environmental effects of eutrophication in aquatic systems, nutrients in high concentrations or excesses of algae in water for human consumption can cause diseases. Examples of the negative effects of such water on human health are presented in Table 1. It is important to emphasize that cyanotoxin produced by algae is not a nutrient; its presence in the water is an effect of eutrophication [32]. The presence of phosphates could indirectly affect people’s health when eutrophication occurs, leading to the proliferation of harmful algae. These algae have been associated with negative effects on human health, including paralytic effects, neurotoxic effects, and hypoxia [33]. This highlights that the presence of nutrients in the water can promote the growth of algae species, which in high concentrations can be toxic to human health.

Effects of nutrients dissolved in water have been studied, and there is evidence that nitrate, phosphate and ammonia produce certain diseases in high levels; for example, phosphates are associated with urine damage and osteoporosis [34]. Additionally, there is scientific evidence that ammonium can affect the developing central nervous system when it enters the human body [35]. Nitrate is the most common contaminant in the world’s underground aquifers [36], and in high concentrations in water ingested by people, it can cause hepatic damage, hypertension and cancer [37,38,39]. Nitrate can be converted to nitrite in the human body, producing diseases such as cancer and methemoglobinemia [40], the latter being a disease that affects the ability of hemoglobin to adsorb oxygen (especially babies under three months of age are more prone to this adverse effect) [36].

Table 1. Effect of high-nutrient water consumption on human health.

Nutrient or Contaminant	Effect on Health (Disease)	Reference	Permissible Limit
Phosphate	Urine damage	[34,41]	0.5 mg/L [42]
	Osteoporosis	[34]
Nitrate	Infantile cyanosis syndrome (blue baby syndrome)	[38,43,44]	10 mg/L [45,46]
	Cancer	[43,44]
	Hepatic damage	[39]
	Vomiting	[38]
	Hypertension	[38]
	Diarrhea	[38]
	Respiratory tract disease	[38]
	Spontaneous abortions (miscarriages)	[38]
Ammonium	Hyperammonemia (affects developing central nervous system)	[35]	1.5 mg/L [47]
	Liver failure	[48,49]
Nitrite	Cancer	[40]	0.2 mg/L [50]
	Methemoglobinemia	[40]
	Effect on the thyroid gland	[40]
	Urinary tract tumors	[40]
Algae cyanotoxin	Paralytic effects	[33,51]	1-µg/L Microcystin [52,53]
	Diarrhea	[33,51]
	Neurotoxic affectation	[33,51]
	Hypoxia	[54]

Table 1. Effect of high-nutrient water consumption on human health.

Nutrient or Contaminant	Effect on Health (Disease)	Reference	Permissible Limit
Phosphate	Urine damage	[34,41]	0.5 mg/L [42]
	Osteoporosis	[34]
Nitrate	Infantile cyanosis syndrome (blue baby syndrome)	[38,43,44]	10 mg/L [45,46]
	Cancer	[43,44]
	Hepatic damage	[39]
	Vomiting	[38]
	Hypertension	[38]
	Diarrhea	[38]
	Respiratory tract disease	[38]
	Spontaneous abortions (miscarriages)	[38]
Ammonium	Hyperammonemia (affects developing central nervous system)	[35]	1.5 mg/L [47]
	Liver failure	[48,49]
Nitrite	Cancer	[40]	0.2 mg/L [50]
	Methemoglobinemia	[40]
	Effect on the thyroid gland	[40]
	Urinary tract tumors	[40]
Algae cyanotoxin	Paralytic effects	[33,51]	1-µg/L Microcystin [52,53]
	Diarrhea	[33,51]
	Neurotoxic affectation	[33,51]
	Hypoxia	[54]

3. Importance of Biosorption in Removing Phosphorus and Nitrogen from Wastewater

The removal of nitrogen and phosphorus from wastewater has been widely studied using efficient technologies and processes, such as chemical precipitation, adsorption, biological (e.g., A²O process, Bardenpho process, sequential batch reactor-based process, and microalgae-based bioremediation), oxidation, and membrane processes (e.g., electrodialysis and reverse osmosis) [9,55,56,57,58,59]. However, many of these processes, particularly membrane processes, result in high operating costs [55].

Notably, the current trend is focused on biosorption, which has gained considerable popularity in the scientific community as an efficient, cost-effective technology that involves simple designs and is easy to operate for the removal of nutrients and other contaminants from wastewater [60].

Adsorption refers to the interaction between an adsorbate and an biosorbent at gas–solid or liquid–solid interfaces (depending on the phases involved); it involves several mechanisms, such as van der Waals force-based bonding, complexation, hydrogen bonding, ion exchange, and coprecipitation [61].

Activated carbon–based biosorption is the preferred conventional method because of the large surface area, porosity, and high biosorption capacity of activated carbons, as well as the ease of the involved design and operation [62]. However, commercial activated carbons are expensive because the high energy requirements for (1) thermal (high-temperatures) or (2) chemical activation processes combining carbonization and activation necessitate expensive recycling. Therefore, producing activated carbons from low-cost raw materials processed at low temperatures remains challenging [63,64,65].

Studies have focused on developing alternative, low-cost biosorbents for the removal of nutrients and other pollutants from wastewater. When working with biomass (living or dead), the materials used for biosorption are called biosorbents, and the process of adsorption with biosorbents is called biosorption [66,67,68,69]. Examples of low-cost biosorbents for removing contaminants in aqueous solutions are shown in Figure 1.

Elimination of nutrients in water is relevant because although nitrogen and phosphorus are fundamental for life, human activities are altering the biochemical cycles that affect aquatic and terrestrial ecosystems, considering that pollution by nutrients can be considered a problem with irreversible consequences [70].

The United States, Asia and Europe present a high risk of affecting the nitrogen cycle. As for phosphorus, in addition to these geographic areas, South America and Oceania are included as critical points [71]. In the United States, it is estimated that about 60% of rivers and bays have deteriorated due to nutrient pollution [70]. This problem is mainly related to the deficient management of urban wastewater, and it is estimated that worldwide, 16.6 Tg of nitrogen and 3.0 Tg of phosphorus [72] are released to water bodies. Therefore, it is important to protect natural systems by removing and recovering nutrients and obtain income from recovered nutrients [72].

There are technologies to efficiently remove nutrients from water in wastewater treatment plants, but their application depends on the available economic resources. For example, ion exchange has achieved removals of 80 to 90% of phosphates, which can be compared to 91% of phosphates removed via biosorption (which is considered cheaper than ion exchange) [73]. Likewise, efficiencies of up to 99% have been found using biosorbents made from corn waste and rice husk (Table 2 and Table 3), so biosorption with this biomass is relevant for scaling up wastewater plants.

In addition, the adsorption equilibrium is determined by analyzing adsorption kinetics, allowing an investigation of the involved mass transfer mechanisms, which is crucial for an optimal design of adsorption systems and for operational, safety, and maintenance considerations of WWTPs [68,69]. The adsorption kinetic models used in the research reported in Table 2 and Table 3 were mainly pseudo-first order, pseudo-second order, intra-particle diffusion and liquid film diffusion. Meanwhile, the adsorption equilibrium models applied were the isotherms of Langmuir, Freundlich, Langmuir–Freundlich, Temkin and Dubinin–Radushkevich.

4. Agro-Industrial Residues and Biosorbents

The biomass and residues generated from agro-industrial processes have been considered relevant inputs for producing bioproducts or materials with potential environmental applications, such as soil remediation or treatment of atmospheric and wastewater pollutants [74].

Agro-industrial wastes are of low cost and locally available in countries where agricultural activity contributes to the national economy [75,76], such as several Latin American countries, including Colombia. Certain agro-industrial residues are mishandled, negatively affecting the local and global environments as follows: (1) their inadequate soil disposal alters ecosystems and contributes to greenhouse gas emissions due to breakdown through biological processes [77]; (2) when these residues accumulate, they promote the appearance of infectious vectors such as rats, pests, and cockroaches [78]; (3) greenhouse gas emissions are caused by uncontrolled burning of some agro-industrial residues, such as rice husks [26,79]; and (4) they can limit photosynthetic activity when discharged into waterbodies [6]. Such discharges can harm the environment and should be studied to ensure the sustainability of natural resources.

Because untreated agro-industrial waste is harmful to the environment, the scientific community is focused on seeking uses of these wastes. One use is as biosorbents for treating contaminated water to protect water sources from specific pollutants that cause eutrophication of water bodies, such as heavy metals, emerging pollutants, and nutrients.

Instead of decontaminating water, nutrients can be desorbed and recovered for environmental applications (such as their use as slow-release organic fertilizers), or the spent biosorbent can be reused to improve soil properties for cultivating and restoring highly degraded soils and for achieving ecological generation of biogas for energy use purposes [16,80,81]. This reuse of nutrient-laden biosorbents has allowed agro-industrial waste to play a crucial role in the circular economy. The final residue of the material employed in water treatment can be used to improve the conditions of soil to increase its productivity as well as potentiate the ecosystem services provided by the land and its relationship with the natural environment [57,82,83].

Biosorbents obtained from agricultural residues, such as rice husks [84,85], sugarcane bagasses [86,87], coconut shells [88,89], wheat straw [89,90], and corn cobs and stalks [91,92], have been comprehensively studied with regard to the removal of phosphorus and nitrogen from aqueous solutions. These materials have been directly applied, chemically modified or transformed into biochar through pyrolysis, in nutrient biosorption in continuous or batch processes. Table 2 shows examples of different biosorbents, with nutrient elimination in aqueous solution of up to 99%, highlighting their importance for application in water purification. Figure 2 illustrates experimental setups used to perform laboratory biosorption studies.

Table 2. Characteristic of different biosorbents obtained from agro-industrial residues.

Biosorbent/Classification	Operation Mode	Geographic Location	Type of Water Loaded with Nutrients	Pollutant	Removal Efficiency (%)	Data for Biosorption Capacity	Reference
Rice husk biochar/No activation used	Batch	Fukuoka (Japan).	Synthetic water solutions	Nitrates and phosphates	5–25	Co = 5–20 mg/L.	[85]
						Da = 5 g/L q = 2.1–5 mg/g
Wheat straw chemically modified with Epichlorohydrin/Biosorbent used after chemical activation	Batch	China	Synthetic water solutions	Nitrates	50	Co = 50 mg/L.	[93]
						Da = 4 g/L q = 2.1 mmol/g
Corn straw biochar chemically modified with Fe3O4/Biosorbent used after chemical activation	Batch	China	Synthetic water solutions	Phosphates	55–95	Co = 20 mg/L.	[94]
						Da = 4 g/L q = 2–20 mg/g
Eggshell and rice straw biochar chemically modified with CaO/Biosorbent used after chemical activation	Batch	China	Synthetic water solutions	Phosphates	45	Co = 5–200 mg/L.	[95]
						Da = 0,2 g/L q = 231 mg/g
Date palm wastes (surface fibers)/No activation used	Batch	Iraq	Synthetic water solutions	Phosphates	85	Co = 50 mg/L.	[96]
						Da = 5 g/L q = 8–9 mg/g
Date palm wastes (date stones)/No activation used	Batch	Iraq	Synthetic water solutions	Phosphates	87	Co = 50 mg/L.	[96]
						Da = 5 g/L q = 8–9 mg/g
Pine cone (raw and sodium hydroxide-modified)/Biosorbent used after chemical activation	Batch	Turkey	Synthetic water solutions	Ammonium	19–99	Co= 50 mg/L.	[97]
						Da = 0.5–10 g/L q = 35–55 mg/g
Pomegranate peel (raw)/No activation used	Batch	Hungary	Synthetic water solutions	Ammonium	27–97	Co = 5–90 mg/L.	[98]
						Da = 0.5–10 g/L q = 4–5 mg/g

Note: [Co] = Initial concentration of the adsorbate (nutrient) in mg/L. [Da] = Biosorbent dosage in g/L. [q] = biosorption capacity in mg/g.

Table 2. Characteristic of different biosorbents obtained from agro-industrial residues.

Biosorbent/Classification	Operation Mode	Geographic Location	Type of Water Loaded with Nutrients	Pollutant	Removal Efficiency (%)	Data for Biosorption Capacity	Reference
Rice husk biochar/No activation used	Batch	Fukuoka (Japan).	Synthetic water solutions	Nitrates and phosphates	5–25	Co = 5–20 mg/L.	[85]
						Da = 5 g/L q = 2.1–5 mg/g
Wheat straw chemically modified with Epichlorohydrin/Biosorbent used after chemical activation	Batch	China	Synthetic water solutions	Nitrates	50	Co = 50 mg/L.	[93]
						Da = 4 g/L q = 2.1 mmol/g
Corn straw biochar chemically modified with Fe3O4/Biosorbent used after chemical activation	Batch	China	Synthetic water solutions	Phosphates	55–95	Co = 20 mg/L.	[94]
						Da = 4 g/L q = 2–20 mg/g
Eggshell and rice straw biochar chemically modified with CaO/Biosorbent used after chemical activation	Batch	China	Synthetic water solutions	Phosphates	45	Co = 5–200 mg/L.	[95]
						Da = 0,2 g/L q = 231 mg/g
Date palm wastes (surface fibers)/No activation used	Batch	Iraq	Synthetic water solutions	Phosphates	85	Co = 50 mg/L.	[96]
						Da = 5 g/L q = 8–9 mg/g
Date palm wastes (date stones)/No activation used	Batch	Iraq	Synthetic water solutions	Phosphates	87	Co = 50 mg/L.	[96]
						Da = 5 g/L q = 8–9 mg/g
Pine cone (raw and sodium hydroxide-modified)/Biosorbent used after chemical activation	Batch	Turkey	Synthetic water solutions	Ammonium	19–99	Co= 50 mg/L.	[97]
						Da = 0.5–10 g/L q = 35–55 mg/g
Pomegranate peel (raw)/No activation used	Batch	Hungary	Synthetic water solutions	Ammonium	27–97	Co = 5–90 mg/L.	[98]
						Da = 0.5–10 g/L q = 4–5 mg/g

Note: [Co] = Initial concentration of the adsorbate (nutrient) in mg/L. [Da] = Biosorbent dosage in g/L. [q] = biosorption capacity in mg/g.

Biochar, a solid-state porous carbonaceous material, is produced through pyrolysis of various types of biomass (including agro-industrial waste) occurring under anoxic conditions at temperatures between 300 °C and 1500 °C [99]. Biochar can be used as a biosorbent, particularly for removing nutrients from aqueous solutions with high efficiency [100]. In addition, it can be useful for soil improvement and carbon sequestration to mitigate global warming because of its favorable properties, such as a large surface area, high porosity, cation exchange ability, and pH buffering capacity [101].

Biosorbents have been prepared through chemical modification to improve their surface properties and surface chemical reactivity by incorporating functional groups (e.g., hydroxyl, carboxyl, and ether) on their surface, enhancing their interaction with polluting ions to eliminate these ions from residual water [102]. Therefore, the efficiency of these porous materials in the elimination of contaminants, including phosphorus and nitrogen, from wastewater increases. The chemical modification involves reacting a biosorbent with chemical substances, such as hydrogen chloride (HCl) [17], magnesium oxide (MgO) [103], sodium hydroxide (NaOH) [26], and ferric chloride (FeCl₃) [104] before its application in wastewater treatment.

In the case of ammonia, biochar derived from various natural fibers exhibits a negative charge, high cation exchange capacity and a low point of zero net charge (PZNC). These characteristics enable the attraction of cations, such as ammonia, through electrostatic interactions and, therefore, chemical sorption [85]. The opposite can be expected for anions such as nitrate and phosphate, where the repulsion of negatively charged ions can be anticipated. However, different investigations have shown that biochar generated under high temperature show an anion exchange capacity increase [85]. This may be attributed to the generation of oxonium functional groups during biochar production. These groups carry a pH-independent positive charge and enhance the anion exchange capacity of the sorbents [105,106], as observed for nitrate [85].

The pH of a solution plays a crucial role in the biosorption process for phosphate, influencing the species of ions present in the aqueous solution. The capacity of sorbents from natural sources to retain phosphate is limited, especially in low-concentration solutions with high pH [86]. The removal of phosphate diminishes as the solution’s pH increases, primarily due to the deprotonation of amine functional groups, and the net surface charge becomes positive [86]. On the other hand, competition may arise between the negative charges of phosphate ions and hydroxyl ions, leading to a reduction in biosorption at pH above the point of zero charge (PZNC) [85]. Phosphate solubilization occurs through naturally occurring leaching from coconut husk biochar after thermal or acidic treatment [85,89]. Notably, untreated coconut shell extracts 90% of phosphorus (P) [107]. Additional findings reveal that the biosorption of phosphate in Fe-impregnated biochars increases with reaction time, reaching equilibrium after 4 h, primarily through a swelling process, with weak ion exchange biosorption [92].

Amphoteric straw cellulose presents modified functional groups that interact through electrostatic attractions with phosphate ions. Furthermore, when the sorption capacity of ammonia increases, the equilibrium of biosorption capacity of phosphate decreases in the amphoteric material. This phenomenon is attributed to the stronger electrostatic attraction of the carboxymethyl group in the amphoteric biosorbent and ammonia compared to the interaction with phosphate ions [89]. Biochar with the presence of Mg can exhibited phosphate sorption. A synergistic effect was observed among biomass fiber, alumina, and magnesium oxide of layered bimetallic hydroxide, which enabled both physical and chemical biosorption of phosphate from water. The positively charged surface of layered bimetallic hydroxide attracted negatively charged phosphate ions, and weak interactions between Mg–Al and biomass can be replaced due to the presence of interlamellar water and laminate anions. Subsequently, the combination of magnesium oxide and alumina oxide interacted with adsorbed phosphate, leading to the formation of salt and accomplishing a chemical biosorption [88].

5. Rice Husks in Nutrient Removal from Aqueous Solutions

Rice husk as biosorbent can be an interesting alternative for nitrate, phosphate, and ammonia removal in aqueous solution because it is non-toxic and does not require much energy to develop a removal system with this biosorbent [25,26]. Other advantages of its application are its low acquisition cost, high biosorption capacity for contaminants in the water, and the ability to apply regeneration [23]. Nonetheless, it also presents disadvantages such a decomposition over time, reducing its efficiency in the elimination of nutrients [108]; and it may be inefficient in removing anions such as nitrates and phosphates, so it will require chemical modification that increases the costs [85].

Rice husk is a by-product generated in rice milling, and it constitutes 20–22% of rice [109]. This agricultural residue is available in large quantities in rice-producing countries [18] and corresponds to the natural pods that form in grains during their growth; it represents one-fifth of the annual raw rice production, approximately 545 million tons in the world [110]. Rice is grown on all continents, with 113 countries producing rice [111]. In Colombia, rice is the third most important crop, the first and second being coffee and corn, respectively, according to the order of relevance in the national economy [112].

Rice husk is a fibrous material containing approximately 50% cellulose, 30% lignin, and 20% silica [113]. In addition, rice husk contains abundant floristic fibers, proteins, and some functional groups, such as carboxyl and amidogen, allowing removal of ionic dyes [114]. If rice husk is pretreated, it can be conditioned to remove heavy metals and nutrients, among other contaminants, from wastewater [23,115,116].

Given the characteristics of rice husk as a biosorbent, studies have focused on using it for nutrient removal from aqueous solutions. These studies have been conducted in various scenarios depending on the type of water to be treated and with different research objects (Table 3). Rice husks have been investigated for the removal of phosphorus and nitrogen and used in various formats, including crushed, chemically modified and unmodified, transformed into biochar, and mixed with other biomasses in various presentations for treating different types of water, such as synthetic water (to simulate nutrient-laden wastewater and surface water runoff) and water from pork husbandry. The removal processes are performed in two modes of operation, namely batch and fixed-bed operations.

Studies have revealed the high efficiency of rice husk as a biosorbent without the application of any treatment (only washing and evaporation) for the removal of total nitrogen and phosphorus in a fixed-bed operation, achieving efficiencies of up to 97% [5] and 95% [78], respectively. These results are critical for understanding water treatment and for agriculture because excellent yields are obtained through the application of biosorbents loaded with nutrients in the growth of plants on an experimental scale, promoting recycling and sustainable crop production.

The biochar loaded with phosphates after biosorption can be used as a raw material for producing biogas through anaerobic digestion because phosphate is an essential nutrient for microorganisms in charge of biomethanization, as revealed by Yadav et al. [82]. This result is relevant for using waste derived from biosorption in obtaining energy and promotes the circular economy.

Studies by Shukla et al. [9], Fatima et al. [16], Kizito et al. [17], and Zhu et al. [18] are related to the preparation of biochar from rice husks and its use as a biosorbent for removing nutrients from aqueous solutions. In these studies, the biosorbent was not chemically modified, either before or after collecting the obtained biochar, and efficiencies between 50% and 80% were achieved in the removal of phosphorus and nitrogen, respectively.

Rice husk biochar after chemical modifications using various substances, such as (3-chloro-2-hydroxypropyl)-trimethylammonium [117], NaOH [26], and HCl [10], has been studied for improved efficiencies with regard to phosphorus and nitrogen removal from aqueous media. Efficiencies between 69% and 87% were achieved, which are an improvement over the efficiencies obtained using unactivated biochar. Chemically modified biosorbents are loaded with functional groups (surface chemistry) that improve the ionic exchange between an adsorbate and the biosorbent, increasing the efficiencies in the removal of the studied contaminants from wastewater.

Although chemical modifications of biosorbents (either agro-industrial waste or biochar) can substantially improve efficiencies of nutrient removal from aqueous solutions, sustainability is essential when considering usage of activating agents. For example, although Fe and Al cations have been widely used to positively charge biosorbents [118,119], these materials can cause negative environmental effects due to their low recovery and toxicity [120]. For example, high ingested concentrations of iron can lead to health problems such as liver cirrhosis, diabetes, nausea, and other heart-related issues [121]. On the other hand, elevated ingested concentrations of aluminum are associated with brain injury in humans [122].

Ramola et al. [123] proposed the use of biochar obtained from the pyrolysis of a mixture of rice husks and calcite for phosphate biosorption from aqueous solutions and achieved related maximum efficiencies of approximately 90%. By integrating a material that is abundant, profitable, and nontoxic, such as calcite, an adequate pH range can be obtained for recovering various nutrients from aqueous solutions [124].

Magnesium oxide (MgO) has attracted considerable attention for activating rice hull biosorbents because of its low cost, low toxicity, and high efficiency in ammonium and phosphate removal. Tran et al. [74] removed up to 71% of ammonium and phosphate from synthetic residual water prepared in the laboratory using the biochar obtained from rice husks and corn cobs, both infused with MgO.

Alam et al. [125] mixed rice husks with other low-cost materials, such as recycled concrete aggregates and crushed glass, to remove the nitrates and phosphates in water from runoff for mitigating diffuse or nonpoint pollution, such as that occurring in agriculture. Fertilizers dispersed in soil through the surface and subsurface runoff contaminated water bodies and aquifers. Herein, nitrate and phosphate removal efficiencies of between 88% and 99% were obtained for a simulation of runoff water biosorption in fixed-bed columns using the aforementioned biomass. Thus, this result demonstrated the potential of rice husk as a biosorbent for the elimination of certain ionic pollutants (for example: metals, nutrients, among others) from aqueous media.

Analyzing this biosorbent particularly in Colombia and Latin America would be important, primarily for social reasons. According to a report from UNICEF and WHO in 2019 [126], the proportion of water treated solely with secondary treatment in Latin America ranges from 15 to 82%. This highlights the need to reduce various contaminants in wastewater, including nutrients in water, by considering the application of low-cost technologies such as rice husk, an agro-industrial waste, as sorbents. Additionally, there is insufficient scientific information to determine the optimal operational conditions for the implementation of nitrate and phosphate biosorption (especially in continuous flow mode or fixed-bed operation) using biosorbents derived from rice husk as an alternative wastewater treatment. This represents a knowledge gap; ideal operational conditions in columns for removing these contaminants through rice husk as a biosorbent, including flow rate, bed height, biosorbent mass, breakthrough, and saturation time, among other parameters, have not yet been established. Furthermore, the association of these operational variables with the characteristics of the resulting biosorbent material (such as surface area, zero-point charge, infrared spectroscopy, among others) has not been determined. Studying these associations is crucial to optimize both the nitrate and phosphate biosorption process and the rice husk-based biosorbent.

6. Corn Residues for Treating the Nutrients in Aqueous Solutions

Corn residues can be used as nutrient biosorbent in aqueous solution mainly because they are abundant, since corn is a highly productive crop, and its residues can be obtained at low cost [23]. Different parts of the corn crop residues have important characteristics, like lignocellulose content that can be used to improve the absorption process [28,29,30]. Corn stalk has high content of hydroxyl groups with ion exchange capacity that favors the biosorption of nutrients in aqueous solution [31]. Moreover, corn stalk is non-toxic and low energy is needed to develop a removal system with this biosorbent [127].

However, one disadvantage that presents itself is the potential for cost increase in regards to the removal of anions in water due to the requirement for chemical modification to charge its surface positively to favor biosorption [19,128].

Corn is one of the oldest known food grains and one of the most planted crops in the world. Corn is cultivated in more than 170 regions worldwide, and its production is highly concentrated in regions such as Asia, as well as North and South America [129]. In 2014, 1,060,107,470 tons of corn were produced worldwide, with the industrial corn production centered on obtaining starch because corn grains are rich in starch, accounting for 60% to 80% of the weight of the corn grain [130].

However, some residues such as the straw and corn stalks in corn crops are valuable for treating residual water and in environmental management because of their biosorption properties in aqueous solutions and their low- or no-cost collection (Table 3).

A study by Liu et al. [99] revealed that 80–99% of total phosphorus removals from synthetic runoff water passed through fixed bed–packed columns with biochar obtained from corn straw chemically modified with ferrous sulfate (before pyrolysis). This phenomenon confirmed that chemical modifications and thermal treatment of the residues increased the removal efficiency of the studied adsorbates.

Iron can enhance the biosorption of total phosphorus in aqueous solutions through the mechanism of chemical adsorption. This metal can form complexes by reacting with phosphates, thereby improving their removal on the material’s surface [131]. Ionic exchange can also occur between iron ions present in the biosorbent and phosphates as adsorbates, facilitating the removal of phosphorus from the aqueous solution [132].

Additionally, the metallic ion Fe⁺³ exhibits a strong binding capacity with the phosphate radical, thereby increasing the capacity for removal of this water contaminant [104]. However, it is important to assess other variables affecting the biosorption process, including the impregnation of metals such as iron in the biosorbent and the pH, biosorbent dose, reaction time, etc.

However, the activating agents of the biosorbents have certain requirements, such as null toxicity and low cost, to be attractive for application in removing nutrients from aqueous solutions.

Therefore, Jiang et al. [128] converted residues, such as corn and banana straw, into biochar after modification using a low-cost magnesium chloride (MgCl₂) compound and applied it for the removal of total ammonium and phosphorus from aqueous solutions. The best results were obtained using biochar prepared with the highest magnesium content. The results revealed that raw material selection plays a vital role in an effective application of biochar in biosorption because the chemical composition (functional groups) affects surface chemistry and the interaction between the biosorbent and adsorbate.

Zhuo et al. [133] conducted another study using a similar approach and incorporated an economical and innocuous activating agent. In this approach, biochar was obtained from corn stover previously modified with calcium to eliminate phosphates from aqueous solutions and achieve removal efficiencies of up to 90%.

Studies have focused on evaluating components of corn crops for nutrient removal from aqueous solutions, such as magnesium-modified ground corn biochar [134], cellulose biochar from corn stalks [19], biochar from corn stover [20], biochar from corn cobs [76], and amine-modified corn cobs [75]. These globally applied approaches are crucial decision-making tools because they indicate the various corn residues that have been used to mitigate the effects of eutrophication from the discharge of wastewater laden with nutrients in natural waterbodies.

However, understanding several processes or unitary operations for treating wastewater is crucial for eliminating contaminants because the selection of a set of processes depends on the characteristics of wastewater and the intended uses of the treated water according to regulatory water-quality requirements. Priyanka et al. [135] provided a novel approach in this regard by studying the integration of sequential batch reactors (SBRs) by implementing simultaneous nitrification, denitrification, and phosphorus removal with subsequent biosorption treatment with corn cob biochar to remove organic compounds and nutrients from gray water. The results of this study provide a notable scientific contribution because (1) SBRs had not been previously applied for gray water treatment, and (2) high-efficiency removal of phosphates was obtained from gray water (approximately 65%) through biosorption with corn cob biochar as a complement to the proposed SBR. This study revealed a promising novel process for phosphate elimination from gray water and a low-cost alternative.

Gotore et al. [136] investigated the integration of artificial wetlands (working with common reed plants) with biosorption. They added corn cob biochar to a substrate with various supplies of oxygen in swine wastewater to eliminate ammonium and phosphate from an aqueous solution. This study solved the problem of low-efficiency nutrient removal from anaerobically digested wastewater from pig farms [137,138] and achieved ammonium and phosphate removal efficiencies of 70% and 80%, respectively. Thus, they proposed a novel approach based on a combination of two low-cost technologies (biosorption with natural biosorbent coupled with artificial wetlands). This novel approach can be implemented in developing countries with limited economic resources, as the required materials can be locally produced. Even biosorbents made with agro-industrial waste could be coupled with technologies widely used in these communities, such as upflow anaerobic sludge blanket reactors, oxidation lagoons, and septic tanks, providing feasible alternatives for the removal of various wastewater pollutants, including nutrients. Therefore, these processes could be used to solve nutrient contamination generated by dumping swine wastewater without effective treatment, which is an important source of contamination in eutrophication of water bodies.

However, when applying biosorption, the efficiencies may be relatively low compared to the quality of the effluent required by a certain environmental regulation for the discharge of liquid waste to water systems [18,139]. Therefore, combined processes could potentially be useful, such as those described above, including artificial wetland coupled with biosorption, in order to improve the quality of the effluent. Couplings between biosorption and other water treatment processes could also be considered, such as, for example, electrocoagulation [140] or electrochemical oxidation [141], among others. However, the selection of these couplings will depend mainly on the water quality required in the effluent and the available economic resources [42].

Table 3. Characteristic of different biosorbents obtained from rice husks and corn residues.

Biosorbent/Classification	Biosorption Method (Operation Mode)	Geographic Location	Type of Water Loaded with Nutrients	Pollutant	Removal Efficiency (%)	Data for Biosorption Capacity	Reference
Rice husk biochar/No activation used	Batch	Khordha (India)	Synthetic water solutions	Nitrates and phosphates	65–75	Co = 0.5–10 mg/L. Da = 200 g/L. q = 0.07–0.5 mg/g	[16]
Rice husk: 1) ground, 2) chemically modified with (3-chloro-2-hydroxypropyl)-trimethylammonium/Biosorbent used after chemical activation	Batch	Chonburi (Thailand)	Wastewater from swine processes	Nitrates and phosphates	65–84	Co = 0.5–10 mg/L. Da = 5–30 g/L. q = 2–12 mg/g	[117]
Rice husk/No activation used	Fixed bed	Iraq	Synthetic water	Total phosphorus	95	Co = 1 mg/L. bh = 1 m	[78]
A layer mix of recycled concrete aggregate (RCA), crushed glass, and rice husk/No activation used	Fixed bed	Rio Grande Valley of Texas (United States)	Synthetic waters to simulate storm runoff	Nitrates and phosphates	88–99	Co = 6–75 mg/L. bh = 0.5 m	[125]
NaOH-modified rice husk biochar/Biosorbent used after chemical activation	Batch	Besut, Terengganu (Malaysia)	Synthetic water	Phosphates	97	Co = 2–10 mg/L. Da = 0.2–1 g/L. q = 1.8 mg/g	[26]
Rice husk biochar/No activation used	Batch	Pakistan	Synthetic water	Nitrates and phosphates	40–95	Co = 50–100 mg/L. Da = 1 g/L. q = 47–95 mg/g	[16]
Rice husk biochar/No activation used	Batch	Beijing (China)	Residual water from a pig-manure digester plant	Nitrates and nitrites	50–80	Co = 35–60 mg/L. Da = 1–50 g/L. q = 40–45 mg/g	[17]
HCl-modified rice husk biochar/Biosorbent used after chemical activation	Batch	Panipat (India)	Synthetic water	Phosphates	89	Co = 10 mg/L. Da= 0.4–4 g/L. q= 0.8–1.4 mg/g	[10]
Rice husk/No activation used	Fixed bed	Iraq	Synthetic water	Total nitrogen	97	Co = 1 mg/L. bh = 0.4 m	[5]
Rice husk biochar/No activation used	Batch	Huzhou (China)	Synthetic water	Ammonium Ion	10–50	Co = 40 mg/L. Da = 0.8–8 g/L. q = 0.6–2 mg/g	[18]
Rice husk biochar treated with chemical solutions, such as NaOH and H2SO4/Biosorbent used after chemical activation	Batch	Benarés (India)	Synthetic water	Phosphates	60–97	Co = 10–30 mg/L. Da = 2–7 g/L. q = 9–13 mg/g	[82]
Rice husk biochar/No activation used	Batch	Juja (Kenya)	Wastewater from an animal slaughterhouse	Nitrates and nitrites	35–65	Co = 13–130 mg/L. Da = 0.8–8 g/L. q = 0.3–13 mg/g	[39]
Silica compound MCM-41 synthesized from rice husk/Biosorbent used after chemical activation	Batch	Egypt	Synthetic water	Phosphates	36–76	Co = 0.5–2.5 mM of Na2HPO4·2H2O. Da = 2–8 g/L. q = 11–16 mg/g	[142]
Rice husk biochar mixed with calcite/Biosorbent used after chemical activation	Batch	Jiaxing (China)	Synthetic water	Phosphates	54–87	Co = 25–125 mg/L. Da = 0.25–0,35 g/L. q = 11 mg/g	[123]
Rice husk biochar activated with MgO/Biosorbent used after chemical activation	Batch	Thai Nguyen (Vietnam)	Synthetic water	Ammonium ion and phosphates	44–71	Co = 100 mg/L. Da = 1 g/L. q = 17–118 mg/g	[74]
Rice husk biochar with and without a mixture of sludge/No activation used	Batch	Ho Chi Minh City (Vietnam)	Synthetic water	Phosphate and ammonium	46–74	Co = 50–100 mg/L. Da = 1–1.2 g/L. q = 61–67 mg/g	[143]
Mixture of rice husk biochar and oyster shell/No activation used	Batch	Jiaxing (China)	Synthetic water and domestic sewage	Total phosphorus	93–99	Co = 3–100 mg/L. Da = 0.2 g/L. q = 150–200 mg/g	[144]
Rice husk biochar activated with Ca/Biosorbent used after chemical activation	Batch	Taiwan	Synthetic water	Nitrates	20–55	Co = 100 mg/L. Da = 0.1 g/L. q = 2.4–32 mg/g	[145]
Corn straw biochar chemically modified with ferrous sulfate/Biosorbent used after chemical activation	Fixed bed	Henan (China)	Synthetic water to simulate runoff	Total phosphorus	80–99	Co = 1.9–2.5 mg/L. bh = 0.5 m. q = 0.7 mg/g	[99]
Corn cob biochar/No activation used	Batch	Beijing (China)	Synthetic water	Ammonium Ion	7–15	Co = 100 mg/L. Da = 10 g/L. q = 0.6–1.1 mg/g	[76]
Corn stalk biochar chemically modified with Mg/Biosorbent used after chemical activation	Batch	Beijing (China)	Swine plant wastewater	Total phosphorus	83–95	Co = 84–2600 mg/L. Da = 10 g/L. q = 7–18 mg/g	[134]
Cellulose extracted from corn stalks chemically modified with dimethylformamide, pyridine, and diethylamine/Biosorbent used after chemical activation	Batch	Shaanxi (China)	Synthetic water	Nitrates and phosphates	10–60	Co = 0.5–100 mg/L. Da = 2 g/L. q = 14–23 mg/g	[19]
Corn stover biochar/No activation used	Batch	New York (United States)	Synthetic water	Nitrates and phosphates	98–99	Co = 0.1–10 mg/L. Da = 10 g/L	[20]
(1) Corn cobs modified with graft amines. (2) Unmodified corn cob./Biosorbent used after chemical activation	Batch	Sydney (Australia)	Synthetic water	Nitrates	10–50	Co = 20 mg/L. Da = 0.1–1 g/L. q = 50 mg/g	[75]
Raw granular corn cob (GCC)/No activation used	Batch	Baghdad (Iraq)	Domestic wastewater	Ammonium Ion	56	Co = 5–100 mg/L. Da = 3 g/L. q = 2–10 mg/g	[146]
Corn cob biochar/No activation used	Batch	Bogor (Indonesia)	Synthetic water	Ammonium ion, nitrate, and phosphate	90	Co = 0.1–50 mg/L. Da = 8 g/L. q = 0.18 mg/g	[107]
Corn cob biochar/No activation used	Batch	Bhubaneswar (India)	Synthetic gray water	Phosphates	39–63	Co = 16–22 mg/L. Da = 0.1–0.55 g/L. q = 4–13 mg/g	[135]
Corn straw biochar/No activation used	Batch	China	Synthetic water	Ammoniacal nitrogen	45–89	Co = 30 mg/L. Da = 33 g/L. q = 0.8–1 mg/g	[147]
Corn straw biochar modified with magnesium chloride (MgCl2) /Biosorbent used after chemical activation	Batch	Guilin (China)	Synthetic water	Ammonium ion and total phosphorus	30–80	Co = 20–350 mg/L. Da = 5 g/L. q = 10–25 mg/g	[128]
Corn stalk biochar assembled with double-layer hydroxyls (Ni–Fe, Mg–Al, and Zn–Al)/Biosorbent used after chemical activation	Batch	Harbin (China)	Synthetic water	Phosphates	30–93	Co = 50 mg/L. Da = 0.25 g/L. q = 152 mg/g	[52]
Corn stalk biochar, previously modifying the stalk with FeCl3/Biosorbent used after chemical activation	Batch	China	Real eutrophic water	Total nitrogen and phosphorus	35–85	Co = 2.5–50 mg/L. Da = 1.25 g/L. q = 14–90 mg/g	[104]
Corn cob biochar/No activation used	Fixed bed	Chiang Mai (Thailand)	Wastewater from a pig farm	Ammonium ion and phosphates	72–76	Co = 0.7–15 mg/L. bh = 1 m	[136]
Corn stover biochar modified with calcium/Biosorbent used after chemical activation	Batch	China	Synthetic water	Phosphates	30–86	Co = 30 mg/L. Da = 1 g/L. q = 34 mg/g	[133]
Corn straw biochar, modified with (1) potassium hydroxide (KOH) and (2) ferric chloride (FeCl3)/Biosorbent used after chemical activation	Batch	China	Synthetic water	Ammonium ion	5–38	Co = 100 mg/L. Da = 2 g/L. q = 5–22 mg/g	[139]

Note: [Co] = Initial concentration of the adsorbate (nutrient) in mg/L. [Da] = Biosorbents dosage in g/L. [bh] = bed height in m. [q] = biosorption capacity in mg/g.

Table 3. Characteristic of different biosorbents obtained from rice husks and corn residues.

Biosorbent/Classification	Biosorption Method (Operation Mode)	Geographic Location	Type of Water Loaded with Nutrients	Pollutant	Removal Efficiency (%)	Data for Biosorption Capacity	Reference
Rice husk biochar/No activation used	Batch	Khordha (India)	Synthetic water solutions	Nitrates and phosphates	65–75	Co = 0.5–10 mg/L. Da = 200 g/L. q = 0.07–0.5 mg/g	[16]
Rice husk: 1) ground, 2) chemically modified with (3-chloro-2-hydroxypropyl)-trimethylammonium/Biosorbent used after chemical activation	Batch	Chonburi (Thailand)	Wastewater from swine processes	Nitrates and phosphates	65–84	Co = 0.5–10 mg/L. Da = 5–30 g/L. q = 2–12 mg/g	[117]
Rice husk/No activation used	Fixed bed	Iraq	Synthetic water	Total phosphorus	95	Co = 1 mg/L. bh = 1 m	[78]
A layer mix of recycled concrete aggregate (RCA), crushed glass, and rice husk/No activation used	Fixed bed	Rio Grande Valley of Texas (United States)	Synthetic waters to simulate storm runoff	Nitrates and phosphates	88–99	Co = 6–75 mg/L. bh = 0.5 m	[125]
NaOH-modified rice husk biochar/Biosorbent used after chemical activation	Batch	Besut, Terengganu (Malaysia)	Synthetic water	Phosphates	97	Co = 2–10 mg/L. Da = 0.2–1 g/L. q = 1.8 mg/g	[26]
Rice husk biochar/No activation used	Batch	Pakistan	Synthetic water	Nitrates and phosphates	40–95	Co = 50–100 mg/L. Da = 1 g/L. q = 47–95 mg/g	[16]
Rice husk biochar/No activation used	Batch	Beijing (China)	Residual water from a pig-manure digester plant	Nitrates and nitrites	50–80	Co = 35–60 mg/L. Da = 1–50 g/L. q = 40–45 mg/g	[17]
HCl-modified rice husk biochar/Biosorbent used after chemical activation	Batch	Panipat (India)	Synthetic water	Phosphates	89	Co = 10 mg/L. Da= 0.4–4 g/L. q= 0.8–1.4 mg/g	[10]
Rice husk/No activation used	Fixed bed	Iraq	Synthetic water	Total nitrogen	97	Co = 1 mg/L. bh = 0.4 m	[5]
Rice husk biochar/No activation used	Batch	Huzhou (China)	Synthetic water	Ammonium Ion	10–50	Co = 40 mg/L. Da = 0.8–8 g/L. q = 0.6–2 mg/g	[18]
Rice husk biochar treated with chemical solutions, such as NaOH and H2SO4/Biosorbent used after chemical activation	Batch	Benarés (India)	Synthetic water	Phosphates	60–97	Co = 10–30 mg/L. Da = 2–7 g/L. q = 9–13 mg/g	[82]
Rice husk biochar/No activation used	Batch	Juja (Kenya)	Wastewater from an animal slaughterhouse	Nitrates and nitrites	35–65	Co = 13–130 mg/L. Da = 0.8–8 g/L. q = 0.3–13 mg/g	[39]
Silica compound MCM-41 synthesized from rice husk/Biosorbent used after chemical activation	Batch	Egypt	Synthetic water	Phosphates	36–76	Co = 0.5–2.5 mM of Na2HPO4·2H2O. Da = 2–8 g/L. q = 11–16 mg/g	[142]
Rice husk biochar mixed with calcite/Biosorbent used after chemical activation	Batch	Jiaxing (China)	Synthetic water	Phosphates	54–87	Co = 25–125 mg/L. Da = 0.25–0,35 g/L. q = 11 mg/g	[123]
Rice husk biochar activated with MgO/Biosorbent used after chemical activation	Batch	Thai Nguyen (Vietnam)	Synthetic water	Ammonium ion and phosphates	44–71	Co = 100 mg/L. Da = 1 g/L. q = 17–118 mg/g	[74]
Rice husk biochar with and without a mixture of sludge/No activation used	Batch	Ho Chi Minh City (Vietnam)	Synthetic water	Phosphate and ammonium	46–74	Co = 50–100 mg/L. Da = 1–1.2 g/L. q = 61–67 mg/g	[143]
Mixture of rice husk biochar and oyster shell/No activation used	Batch	Jiaxing (China)	Synthetic water and domestic sewage	Total phosphorus	93–99	Co = 3–100 mg/L. Da = 0.2 g/L. q = 150–200 mg/g	[144]
Rice husk biochar activated with Ca/Biosorbent used after chemical activation	Batch	Taiwan	Synthetic water	Nitrates	20–55	Co = 100 mg/L. Da = 0.1 g/L. q = 2.4–32 mg/g	[145]
Corn straw biochar chemically modified with ferrous sulfate/Biosorbent used after chemical activation	Fixed bed	Henan (China)	Synthetic water to simulate runoff	Total phosphorus	80–99	Co = 1.9–2.5 mg/L. bh = 0.5 m. q = 0.7 mg/g	[99]
Corn cob biochar/No activation used	Batch	Beijing (China)	Synthetic water	Ammonium Ion	7–15	Co = 100 mg/L. Da = 10 g/L. q = 0.6–1.1 mg/g	[76]
Corn stalk biochar chemically modified with Mg/Biosorbent used after chemical activation	Batch	Beijing (China)	Swine plant wastewater	Total phosphorus	83–95	Co = 84–2600 mg/L. Da = 10 g/L. q = 7–18 mg/g	[134]
Cellulose extracted from corn stalks chemically modified with dimethylformamide, pyridine, and diethylamine/Biosorbent used after chemical activation	Batch	Shaanxi (China)	Synthetic water	Nitrates and phosphates	10–60	Co = 0.5–100 mg/L. Da = 2 g/L. q = 14–23 mg/g	[19]
Corn stover biochar/No activation used	Batch	New York (United States)	Synthetic water	Nitrates and phosphates	98–99	Co = 0.1–10 mg/L. Da = 10 g/L	[20]
(1) Corn cobs modified with graft amines. (2) Unmodified corn cob./Biosorbent used after chemical activation	Batch	Sydney (Australia)	Synthetic water	Nitrates	10–50	Co = 20 mg/L. Da = 0.1–1 g/L. q = 50 mg/g	[75]
Raw granular corn cob (GCC)/No activation used	Batch	Baghdad (Iraq)	Domestic wastewater	Ammonium Ion	56	Co = 5–100 mg/L. Da = 3 g/L. q = 2–10 mg/g	[146]
Corn cob biochar/No activation used	Batch	Bogor (Indonesia)	Synthetic water	Ammonium ion, nitrate, and phosphate	90	Co = 0.1–50 mg/L. Da = 8 g/L. q = 0.18 mg/g	[107]
Corn cob biochar/No activation used	Batch	Bhubaneswar (India)	Synthetic gray water	Phosphates	39–63	Co = 16–22 mg/L. Da = 0.1–0.55 g/L. q = 4–13 mg/g	[135]
Corn straw biochar/No activation used	Batch	China	Synthetic water	Ammoniacal nitrogen	45–89	Co = 30 mg/L. Da = 33 g/L. q = 0.8–1 mg/g	[147]
Corn straw biochar modified with magnesium chloride (MgCl2) /Biosorbent used after chemical activation	Batch	Guilin (China)	Synthetic water	Ammonium ion and total phosphorus	30–80	Co = 20–350 mg/L. Da = 5 g/L. q = 10–25 mg/g	[128]
Corn stalk biochar assembled with double-layer hydroxyls (Ni–Fe, Mg–Al, and Zn–Al)/Biosorbent used after chemical activation	Batch	Harbin (China)	Synthetic water	Phosphates	30–93	Co = 50 mg/L. Da = 0.25 g/L. q = 152 mg/g	[52]
Corn stalk biochar, previously modifying the stalk with FeCl3/Biosorbent used after chemical activation	Batch	China	Real eutrophic water	Total nitrogen and phosphorus	35–85	Co = 2.5–50 mg/L. Da = 1.25 g/L. q = 14–90 mg/g	[104]
Corn cob biochar/No activation used	Fixed bed	Chiang Mai (Thailand)	Wastewater from a pig farm	Ammonium ion and phosphates	72–76	Co = 0.7–15 mg/L. bh = 1 m	[136]
Corn stover biochar modified with calcium/Biosorbent used after chemical activation	Batch	China	Synthetic water	Phosphates	30–86	Co = 30 mg/L. Da = 1 g/L. q = 34 mg/g	[133]
Corn straw biochar, modified with (1) potassium hydroxide (KOH) and (2) ferric chloride (FeCl3)/Biosorbent used after chemical activation	Batch	China	Synthetic water	Ammonium ion	5–38	Co = 100 mg/L. Da = 2 g/L. q = 5–22 mg/g	[139]

Note: [Co] = Initial concentration of the adsorbate (nutrient) in mg/L. [Da] = Biosorbents dosage in g/L. [bh] = bed height in m. [q] = biosorption capacity in mg/g.

7. Selection Criteria and Application of Biosorbents for Treating Wastewater Nutrients

Figure 3 presents the criteria for the selection and application of biosorbents in nutrient removal from water bodies. Sustainable solutions were proposed to minimize environmental impacts on wastewater treatment projects involving biosorption from agro-industrial residues [7,65,97].

However, certain characteristics must be ensured for sustainable application of a biosorbent and its modification through an activating agent, as follows:

• Low cost. Agro-industrial residues are low-cost when their biosorption is simple or when they require low energy content to obtain them, such as when only applying washing and drying to the material or biomass (raw material without additional treatment), such as husks of rice, cob, stalk, and corn straw. However, physical or chemical modifications, such as pyrolysis (to obtain biochar) or chemical activation (through impregnation of precursor substances to increase the biosorption capacity of biosorbents), on agro-industrial waste increases their cost because of the energy required for heating or the use of substances as activating agents, among other aspects.

Other factors governing the cost of biosorption processes must be considered, such as pH adjustment, practical loading, decontamination method (batch or column), and the regeneration of the biosorbent [148,149]. The latter factor is particularly crucial, as it requires a significant investment due to the costs associated with energy consumption, as seen in the case of activated carbon (which may not be attractive for industrial-scale applications). In contrast, biosorbents based on agro-industrial residues, as discussed earlier, are less impacted by this aspect, making them more economical compared to commercial biosorbents [150]. Therefore, an evaluation of the production and operating costs associated with biosorbents is crucial for their selection in nutrient removal and recovery from wastewater.

• Innocuous (nontoxic for the environment).
• Abundant (such as agro-industrial waste), and the use of nonrenewable materials should be avoided.
• Readily available in various territories.
• Generate less negative environmental impacts. For example, prioritizing biodegradable materials or materials that have an affinity with the environment for reuse or exploitation.
• Potential for reuse or recovery of materials, promoting the circular economy. For example, biosorption residue can be used in the production chain, as is the case for improving soil properties and power generation.
• Easy to prepare, operate, and maintain.
• Should not inhibit or reduce the efficiency of another wastewater treatment process (such as a biological process for wastewater treatment) if it is required to be coupled with other wastewater treatment technologies. In case of negatively affecting another process, the characteristics or variables related to it should be studied to guarantee the effectiveness or proper functioning of the sequential set of operations and unitary processes for eliminating contaminants from residual water according to the water quality requirement of the treated effluent.
• Does not require expensive equipment or supplies to obtain an activating agent or biosorbent to be applied in biosorption.

8. Current Challenges in Studying Biosorbents with Regard to the Treatment of Wastewater Nutrients

The following research opportunities have been found based on the gaps identified herein:

Limited studies have been conducted on the evaluation of biosorbents from agro-industrial residues derived from rice and corn husk residues in nutrient removal from municipal wastewater. Most studies have focused on distilled water loaded with nutrients in the batch mode, which does not represent the treatment of wastewater performed in WWTPs operating in a fixed-bed mode and with real wastewater having a varied composition. This process differs from the synthetic process. Studies with wastewater from municipal WWTPs should focus on realistic decisions for adjusting variables that control biosorption, such as pH, temperature, and the concentration of nutrients in the influent (initial concentration).

Mixtures of agro-industrial residues (such as those consulted in this investigation) are yet to be evaluated for application as biosorbents in the removal of nutrients from wastewater. This process is neither completed on a laboratory scale nor in WWTPs. Reasons to study mixtures from agro-industrial residues for nutrient removal from aqueous solutions are (1) to avoid relying on residues from a single crop, thus reducing the proliferation of monocultures and, consequently, minimizing negative environmental impacts; and (2) the integration of a mixture of biosorbents derived from agro-industrial residues could have a synergistic effect on biosorption efficiency by providing more functional groups to the surface of the resulting mixed biosorbent for the biosorption process [151,152].

An economic analysis with real data is yet to be conducted to estimate the cost of wastewater treatment with an implementation of biosorbents for nutrient removal and assess its execution competitiveness compared with other technologies in the world. The results are to be compared with the efficacy of using conventional activated carbon.

The production of biosorbents for eliminating nutrients from aqueous solutions should be investigated for the recovery of these substances to incorporate them in the production chain and for environmental restoration (e.g., application of recycled nutrients in improving the physicochemical properties of soil). Given the environmental and economic importance of these porous materials, producing them on a large scale is critical for developing patents providing solutions according to the requirements of communities in various contexts.

Another relevant aspect of biosorbents that is to be studied is the determination of their shelf life, which is crucial when evaluating their environmental and economic benefits and for decision-making in WWTPs. In addition, the shelf life of biosorbents should be investigated to establish strategies to achieve zero residues, providing other environmental benefits aimed at sustainable development (strong association with sustainable development goals), constituting a crucial tool for economic growth, especially in developing countries.

Few studies have compared the use of biosorbents derived from agro-industrial residues with other low-cost technologies for pollutant elimination from wastewater. Therefore, studies should investigate low-cost technologies for the purification of residual water such that they can be implemented around the world, including in developing countries, as such polluted waters require several processes and unitary operations to guarantee their effective purification.

A methodology is yet to be proposed for an application of biosorbents to nutrient removal from wastewater that can be scaled to WWTPs. Therefore, this limits its applicability in decontaminating wastewater in various communities globally.

9. Conclusions

Herein, the application of biosorbents derived from rice husks and corn residues in eliminating nutrients from aqueous solutions was reviewed. To this end, a multicriteria analysis was applied to select articles related to the biosorption of nutrients from these two agro-industrial wastes. Various methods of eliminating and recovering such contaminants were investigated from technical, economic, and environmental sustainability perspectives. Significant removals of nitrates and phosphates in water masses were found from biosorbents derived from rice husks and corn residues, of up to 99% of these two contaminants. Nevertheless, research opportunities were identified to cover gaps in knowledge to propose projects for low-cost WWTPs that include biosorption with agro-industrial residues for the purification and recovery of nutrients from wastewater, which could be applied in all countries to sustainably manage global water resources. However, several challenges and benefits require technological and scientific development for an effective application of the use of natural biosorbents (e.g., rice and corn waste), such as obtaining low-cost biosorbents, taking advantage of the abundance of these materials, easy preparation, reuse in water treatment and other media (such as soil), useful life, and recovery of wastewater materials, to maximize the environmental and social benefits of the application of these novel materials (biosorbents) for water decontamination.

Finally, it is recommended to carry out studies on nutrient biosorption with the proposed materials (rice husk and corn residue) applied to industrial wastewater, in order to cover knowledge gaps and scale the process to industrial wastewater treatment plants. To do this, the variables that affect the nutrient biosorption process must be studied, carrying out optimization experiments in both the preparation and production of biosorbents, as well as the determination of process operational variables. Also, it is important to consider the coupling of other water treatment processes with biosorption, so that the removal of nutrients in aqueous solution can be improved.