**1. Introduction**

Citrus is one of the most popular and largest cultivated fruit crops in the tropical and subtropical regions on the planet, with an annual turnover exceeding 110–124 million tons. The fruits are largely processed in the food processing industries, and approximately half of the fruit mass (45–55%) is discarded as waste. The discarded mass consists of peels

**Citation:** Mahato, N.; Agarwal, P.; Mohapatra, D.; Sinha, M.; Dhyani, A.; Pathak, B.; Tripathi, M.K.; Angaiah, S. Biotransformation of Citrus Waste-II: Bio-Sorbent Materials for Removal of Dyes, Heavy Metals and Toxic Chemicals from Polluted Water. *Processes* **2021**, *9*, 1544. https:// doi.org/10.3390/pr9091544

Academic Editor: Jose Enrique Torres Vaamonde

Received: 3 August 2021 Accepted: 26 August 2021 Published: 30 August 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

(flavedo and albedo), pith residue, seeds and parts inappropriate for human consumption. The waste, when directly discarded to the environment, causes huge problems in terms of pollution to the land and aquatic ecosystems, and underground as well as surface water resources. Alternately, this waste can be utilized as a sustainable and renewable natural resource and feedstock in a number of ways to obtain industrially important chemicals and valuable products by utilizing modern extraction methods, chemical processing techniques and biotransformation. On the one hand, utilization of the waste biomass provides an opportunity to produce valuable chemicals from green resources and avoid usage of harsh chemicals, and on the other hand, it helps in protecting the environment from the adverse effects of pollution. The authors have extensively reviewed the nature, properties and different technological processes of citrus waste valorization in their previously published articles [1–8].

### *1.1. Citrus Waste Pre-Treatment and Disposal*

Citrus wastes produced by food processing industries are generally sent to wastedisposal plants, which require substantial transportation costs and availability of suitable sites for waste disposal. The most common waste managemen<sup>t</sup> methods employed for the citrus wastes are composting, anaerobic digestion, incineration, thermolysis and gasification [9]. Commonly, in small processing units in the underdeveloped or developing countries, the wastewater is not believed to be toxic or harmful as sewage wastewater and is disposed of into drainage. When the dilute wastewaters, disposed of through drainage, reach streams, lakes, tidal waters, ponds or dumping wells, the organic solids carried by the wastewaters begin to decompose. During the process of decomposition, the dissolved oxygen present in the water is consumed. As a result, anaerobic or putrefaction reactions set in, and the aquatic organisms, such as insects, planktons (zooplanktons and phytoplanktons), fish, aquatic organisms, etc., die because of oxygen deficiency in the water [8]. Within the past three decades, there has been a multifold rise in the demand, and consequently, an increase in the production, supply, processing and applications of processed citrus products. At the same time, there has also been a huge rise in the quantities of dilute liquid and solid wastes, which are required to be disposed of appropriately. Large processing plants and units utilize large amounts of chlorinated waters to keep the processing tables, containers, vessels and equipment clean, and hence, the wastewater also contains chlorine. The effluent waters contain varied quantities of peel oil traces, pulp, juice sacs, follicles and organic materials [10]. Depending upon the concentration of materials and source of origin, the wastewater is divided into three categories, as follows:


When dilute wastewater is disposed into water bodies, lakes and basins located in faroff places from the residential areas, sandy beds or sandy lands, the liquid in the wastewater can either percolate down into the soil or evaporate easily, leaving behind the solids at the surface. Later, the clogged wastewaters were also suggested for spray irrigation with somewhat diluted concentration to pasture lands [11,12]. One such experiment

conducted at pasture land bearing leguminous cover crops has been reported to show good results [13]. However, spray irrigation to the wood lands, pastures and green vegetation showed negative effects. For disposal, usually, several ponds are constructed, sometimes in series, to manage the waste to flow from one pond to another. The discharge into groves or wells may result in defoliation of trees, probably because of the loss of oxygen in the soil around the roots of the trees. Wastes released into the city sewage system may contaminate the underground water sources and cause damage to pumps and piping, clogging of sand beds and foaming in primary settling tanks. Additionally, pumping the wastewaters into the wells is prohibited because of probable contamination of underground water supplies. Furthermore, fermentation causes gaseous build-up inside the dumping wells and sometimes the pressure mounts to an excessive level to blow back and cause fire outbreaks. Some of the workable solutions in such cases are treatment with nitrogen, extensive aeration and chemical flocculation followed by lagooning [14–17]. Chemical flocculation with lime combined with aeration in order to promote floating of solids has been observed to result in a 64% reduction of suspended solids and up to a 30% reduction in biological oxygen demand (BOD) [18–20]. Although this treatment does not reduce the BOD of the wastewaters, it helps in fulfilling the purpose of waste pre-treatment by regulating the pH to a less acidic consistency and clarifying the suspended solid particles. Common methods of citrus waste removal from the food processing plants and industries, and its adverse effects on the environment, are shown in Figure 1.

**Figure 1.** Conventional methods of waste removal from citrus processing plants and deteriorating effects to the environment.

The semi-solid citrus wastes have very high-water content, and they are difficult to dry through conventional methods. Furthermore, these processes consume huge energy. The most common method of solid and semi-solid biowaste managemen<sup>t</sup> methods is composting. However, in case of citrus waste, it causes additional problems. Composting or digesting the citrus waste is not a practical choice as these contain large amounts of essential oils, mainly limonene, which inhibits microbial growth and the fermentation process and affects decomposition. Therefore, extraction of oils from the peel waste is very important before disposal to the landfills. Murdock and Allen reported that the oils present in the orange peels are toxic to yeasts [19]. These have been found to exhibit inhibitory effects on the growth of several useful bacteria, yeas<sup>t</sup> and molds, e.g., *Bacillus subtilis*, *Saccharomyces cerevisiae* and *Aspergillus awamori* [20]. Removal of oils from the citrus waste enables application of decomposition methods or anaerobic digestion, incineration, thermolysis and gasification. Detailed illustrations of practical methods of systematic and environment-friendly disposal of citrus wastes and their relative merits are displayed in Figure 2.

**Figure 2.** Methods of systematic and environment-friendly disposal of citrus wastes, conversion into useful materials and their relative merits.

Thus, it appears that the extraction of oil from the peel waste streams not only provides useful by-products, but also contributes toward pollution abatement. Citrus wastes also contain large amounts of sugars, which along with moisture, invite bacteria to grow. Decaying waste causes visual displeasure, odoriferous environment and attracts flies. Furthermore, citrus wastes are required to be processed quickly before compositional changes occur. Therefore, waste disposal to the dumping grounds has additional disadvantages. The nitrogen content in the solid waste materials (<0.14%) is quite insufficient to support bacterial decomposition. Therefore, to support decomposition, it robs oxygen from the soil underneath, resulting in a deficiency of nitrogen in the soil. The effect can be witnessed in terms of de-coloration of the vegetation or grasses at or surrounding areas of the dumping ground. The problem can be overcome by adding N2 supplements in the form of chemical fertilizers. In this process, 200–400 pounds of calcium cyanide is added to each ton of ground waste, mixed thoroughly and allowed to dry until crumbly. Besides calcium cyanides, nitrates, ammonium sulfate and super phosphate are also sometimes added [10].

### *1.2. Pollutants: Dyes, Heavy Metals, Pharmaceutically Active Compounds (PACs) and Other Contaminants*

Processes and manufacturing industries, such as metal plating, metal finishing, automotive, semiconductor manufacturing, pulp and paper production, mining operations, ceramics production, tanneries, radiator manufacturing, smelting and alloy manufacturing, battery manufacturing, corrosion of pipes and infrastructures, textiles and dye industries, etc., release a number of harmful and hazardous chemicals into wastewaters and effluents [21,22]. Researchers have reported that more than 700 kinds of pollutants, both organic and inorganic chemicals, mostly toxic and non-biodegradable in nature, are regularly discarded into the water bodies. The non-biodegradable pollutants are persistent in the environment. The heavy metals in the list of pollutants are cadmium, platinum, mercury, copper, lead, chromium, arsenic, antimony, etc., which have been observed to cause adverse health effects, such as gastrointestinal disorders, stomatitis, tumors, hemoglobinuria, ataxia, paralysis, diarrhea, neurological disorders, muscular dystrophy, vomiting, convulsions and so on [23–25]. Heavy metal pollution is a serious problem because of the metals' persistent nature, and their ability to enter and accumulate in the food chain [26–28].

Dyes are organic compounds used for coloring textile materials, paper, plastic, paints and synthetic coloring materials. There are about 40,000 dyes and pigments with approximately 7000 different kinds of chemical structures known to chemists. A dye substance has two parts: chromophores and auxochromes. The chromophores impart color whereas the auxochromes impart intensity for the dye. The dyes are classified as acid, base, reactive, direct, disperse, solvent, sulfur, vat, etc., and involve a wide variety of applications as well as application methods. A vast majority of dyes and pigments are non-biodegradable and persistent in nature. Textile and fiber industries employ approximately 10,000 different kinds of dyes for dying and printing of clothes and fabrics. The concentration of the dye bath during the dying processes ranges between 10 and 200 mg <sup>L</sup>−1, which retains approximately 10–15% of its initial concentration post-dying and released into the effluent [29,30]. The dye concentration of approximately one ppm or less in the wastewaters has been considered as a potential threat to the environment and to human and aquatic lives [29]. Cyanides from industries released into the environment, particularly in wastewaters, have detrimental effects. Cyanides and their complexes have been demonstrated to have bioaccumulative properties that result in ecological deterioration [31]. Most of the industrial wastewaters contain F-CN, the simplest and most toxic form of cyanide, which is formed by dissociation of cyanide complexes during cyanide-based electroplating operations [32]. Chemical methods are usually employed for the conversion of free cyanide (F-CN) into a complex of NH4+ and CHOO<sup>−</sup>, but most of the methods are expensive. Furthermore, they also produce harmful by-products which contribute to environmental contamination as well as being detrimental to the bioremediation processes [33]. Citrus wastes have been found to be useful for the conversion of F-CN into a complex of NH4+ and CHOO<sup>−</sup>. Apart from F-CN, most of the wastewaters contain heavy metals and the presence of the metallic species (Ni, Zn, As, Cr, Hg and Cu), which have been observed to slow down the conversion of F-CN. The latter occurs as the metallic ions become attached to the hydroxyl groups of the absorbent material responsible for the removal. In addition, it has been reported that material obtained after acid hydrolysis of citrus solid wastes increases the catalytic conversion of F-CN by ~3.86-fold compared with the unhydrolyzed solid waste. The conversion has been found to increase linearly with an increase in pH and temperature [33]. Adsorption of heavy metal ions on the adsorbate material is fundamentally driven by electrostatic attraction between the oppositely charged ions. Both the heavy metal ions present in the wastewater and the functional groups (-COOH, -OH) present on the bio-sorbent material carry charges. The electrostatic attraction between the charges results in binding and ion exchange between Na+, Mg2+, K+, Ca2+ and heavy metal ions (M2+). However, the ion exchange predominates due to stronger binding forces between the charged species.

In recent years, pharmaceutically active compounds (PACs) are emerging as one of the most harmful contaminants in the natural and wastewater systems. PACs are the vast range of complex organic chemical formulations which possess a range of medicinal properties and are used in the treatment, control and eradication of diseases in both humans and animals. These include antibiotics, anti-inflammatories, cytotoxins, birth control pills, synthetic hormones and statins. The commonly encountered PACs in surface waters are erythromycin, metronidazole, sulfamethoxazole, trimethoprim, ciprafloxacin, amoxicillin, trimethoprim, tetracyclin, metformin, acetaminophen, diclofenac, ibuprofen, ketoprofen, naproxen, diazepam, fluoxetine and so on [34–37]. In the past few decades, consumption of pharmaceutical compounds has increased compared with many earlier decades in the century across the globe, resulting in a significant increase in the production of raw materials as well as the final PAC products. Manufacturing of PACs and life-saving drugs by the pharmaceutical companies and processing and packaging factories also releases huge amounts of compounds into the wastewaters. When excreted by humans or animals and disposed of inappropriately, these enter into the environment. In addition, PACs are also released by urban sewage, domestic hospital wastewaters, intensive livestock farming, liquid livestock manure production, sewage sludge from agricultural activities and effluents from sewage treatment plants [38]. The compounds undergo transformation into certain metabolites or breakdown compounds under the influence of temperature, light and vicinity of other chemical ingredients or microbes in nature under the process of biodegradation or photodegradation. Hence, the new molecules produced may sometimes be more toxic compared to their parent PACs. Furthermore, the transformation renders it difficult to monitor the presence of PACs in their original parent formulation in the environment (ground or surface waters). Presence of PACs in significant amounts in the drinking water has become an alarming concern in many countries. These compounds are capable of accumulating in the biological entities, primarily microbes, and making them drug-resistant. As a consequence, they lose their potency to act effectively against diseasecausing microorganisms. Apart from drug resistance developments in disease-causing pathogens, PACs in the environment also cause feminization of male fish and amphibian species because of enhanced amounts of natural and synthetic estrogens in the habitat waters. In addition, increased amounts of PACs in irrigation waters to the crops produce food enriched with PACs. Such foods have been found to cause femaleness in human males, multiple organ complications/failure, genetic and hereditary diseases [38,39].

Diclofenac, a non-steroidal anti-inflammatory (NSAID) drug, has been extensively employed in veterinary usage, fever, pain and injury, livestock farming, fisheries and dairy industries, and has resulted in increased amounts of this chemical in the flesh of animals. When the animals die and are fed upon by predatory birds, diclofenac enters into their bodies and causes serious health consequences, leading to death and collapse of their population. Genus *Gyps.* is most severely affected and has been included in the category of global extinction risk. Some of the highly endangered species from genus *Gyps.* are oriental white-backed vulture (*Gyps bengalensis*), long-billed vulture (*Gyps indicus*) and slender-billed vulture (*Gyps tenuirostris*), and the decline in their population has been recorded to be more than 95% since the early 1990s. The decline has been observed to continue at an annual rate of 22% to 48% [40–43]. Diclofenac causes kidney damage, increases in the concentration of uric acid and crystal formation in serum and vital tissues, visceral gout, bone tumor and death of the predatory birds. Decline of predatory birds' populations in the ecosystem has been reported to cause a severe imbalance in the food chain and an alarming threat to the survival of the human population in Europe and Asia. Absence of the predatory bird population (eagles and vultures), or scavengers of wild and domestic ungulate carcasses in the ecosystem, has led to an increase in the population of feral dogs (*Canis familiaris*), and consequently, an increased risk of rabies spread in the human population. Furthermore, there has also been an alarming increase in the population of rats (*Ratus* spp.) and increasing risk of transmission of various diseases observed, such as bubonic plague, brucellosis, tuberculosis and anthrax in humans and

livestock. Disappearing eagle and vulture populations have also created a huge challenge for the Parsee and Tibetese communities in India, Nepal and Tibet, as they find it difficult to continue their burial ritual/practices at sky burial sites and towers of silence [39].

### *1.3. Health Hazards of Pollutants*

Metal industries related to mechanical works and battery manufacturing carrying out electroplating, metal plating, etc., release substantial amounts of heavy metals into the wastewaters. Additionally, large amounts of poisonous dyes are released from pigments or printing industries [44]. The pigment and dye industries produce different kinds of dyes, such as methylene blue (MB), oxazine and xanthenes compounds, azo dyes, methyl violet, etc. Among various dyes, methylene blue and heavy metal salts are the most frequently used chemicals for dying silk, wood and cotton [45]. MB is a common cationic dye extensively used in medical, textile and printing industries. The wastewater effluents from these industries contain high amounts of dyes, which can cause severe environmental pollution. It contaminates the water bodies, such as rivers, ponds, lakes, ditches and even the ground waters. These chemicals are toxic and cause severe impacts on human health as they may cause nausea, vomiting, diarrhea, etc., when ingested. Thus, removal of MB from wastewater is of grea<sup>t</sup> concern not only from an environmental point of view, but also for the sake of human life. Various methods used for dye removal include adsorption, flocculation, precipitation, ion exchange, electro-kinetic coagulation, ozonization and so on. Among the above-mentioned processes, adsorption is one of the most efficient methods due to its simple design, ease of operation and insensitivity to toxic substances [46]. The molecular structures of some of the popular anionic and cationic dyes are shown in Figure 3. The hazardous effects of dyes and heavy metals on human health, the ecosystem and the environment, and the possible use of citrus waste-derived bio-sorbents for the removal of harmful chemicals from the wastewaters, are illustrated in Figure 4. The permissible limits and the hazardous effects of heavy metals, dyes and other contaminants on human health havebeensummarizedinTable 1.

**Figure 3.** Structures of the anionic and cationic dyes.


**Table 1.** The permissible limits and the hazardous effects of heavy metals, dyes and other contaminants on human health.

### *1.4. Citrus Peel-Derived Adsorbent Materials*

The most popular adsorbent for the adsorption process is activated carbon. However, its use is still limited because it is expensive in terms of its high operational cost [57–59]. Advanced technology enables scientists to attempt to overcome the cost of the treatment process by using inexpensive, efficient and easily available adsorbents. In the literature, numerous studies have been reported to obtain low-cost activated carbons from agricultural wastes, such as wheat shells, rice husk, tea waste, neem leaf powder, cotton waste, banana peel and orange peel for the removal of heavy metals and other hazardous compounds from wastewaters [60–65]. However, the sorption potential of most of these low-cost sorbents is limited. The surface of the bio-sorbent can be modified to enhance its activity. Among many low-cost adsorbents, one such inexpensive and economical adsorbent precursor material is citrus fruit peel [66,67]. Citrus fruit peel has no use after the extraction of essential

oils and other valuable chemicals, and thus, it is a commercially valuable and readily available resource for making adsorbent materials. Hence, conversion of citrus fruit peel to low-cost adsorbents serves multiple purposes. The unwanted waste can be converted to value-added products, such as low-cost adsorbent materials for the removal of heavy metals and dyes from aqueous solution. The latter efficiently alleviates the environmental pollution. Types of modification that can be introduced to the biomass-derived bio-sorbents are shown in Figure 5.

**Figure 4.** Hazardous effects of dyes and heavy metals released in the effluents from chemical and textile industries on humans and the environment, and the possible use of citrus waste-derived bio-sorbents for the removal of harmful chemicals from the wastewaters. Artwork developed from the information provided in [47].

**Figure 5.** Basic principle of surface modification of citrus waste-derived bio-sorbent material by chemical pre-treatment for the removal and recovery of heavy/precious metals. Artwork created from the information provided in [68].
