**1. Introduction**

Water pollution is a serious world environmental problem mainly caused by the climate change, rapid urbanization and advance of industrialization [1,2]. Heavy metals are among most released pollutants or contaminants into the water and are not biodegradable; therefore, they accumulate in living organisms entering in the food chain, also through the consumption of water and other contaminated products, producing corresponding pollution biomagnification [3,4]. These metals are an environmental and public health concern, not only because of their persistence and concentration that influence exposure, but also because of their toxicity, and their mobility in the environment that determines their bioavailability, which is given by the type of compound or metabolite that each metal can form, and also by the characteristics of each the specific environment [5,6].

Nickel, zinc and cadmium are common and relevant heavy metals in the environment [4,7]. Electroplating, metallurgical and batteries industries are some of the anthropogenic sources of nickel, zinc and cadmium contamination [8,9]. Also, nickel and zinc can easily leach due to mineral weathering [10]. Ni(II) and Zn(II) are essential elements and, in low concentrations, they are necessary for the metabolic development of humans, plants or animals. However, these elements can be toxic and harmful to health effects when exposure/assimilation exceeds the upper limit of the physiologically required range [11–13].

**Citation:** Simón, D.; Palet, C.; Costas, A.; Cristóbal, A. Agro-Industrial Waste as Potential Heavy Metal Adsorbents and Subsequent Safe Disposal of Spent Adsorbents. *Water* **2022**, *14*, 3298. https://doi.org/ 10.3390/w14203298

Academic Editor: Laura Bulgariu

Received: 23 September 2022 Accepted: 14 October 2022 Published: 19 October 2022

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**Copyright:** © 2022 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/).

For example, high exposure to nickel can cause cancer, dry cough and lung problems, dermatitis, nausea, gastrointestinal and kidney problems in humans, and high exposure to zinc can cause fever, vomiting, anemia, and skin problems in humans [7,8]. Cd(II) is a highly toxic metal, even at very low concentrations, and is a non-essential element because it has no known benefit to human health or other living beings [6,10]. Cadmium is a human carcinogen as established by The International Agency for Research on Cancer (IARC) and can cause kidney problems, hypertension, stomach irritation, among others, and its chronic exposure can lead to the development of "Itai-Itai" disease [11,14].

In the province of Buenos Aires, Argentina, the permitted maximum discharge limits of these heavy metals in sewers, surface water or stormwater conduits and the open sea vary from 2 to 3 mg/L for Ni, 2 to 5 mg/L for Zn and 0.1 to 0.5 mg/L for Cd [15]. Industrial effluents generally can contain concentrations of heavy metals above the maximum permissible limits; therefore, industries must treat their effluents before discharging them into the environment [13,14]. There are conventional technologies for treating wastewater and minimizing heavy metal pollution (e.g., chemical precipitation, coagulation/flocculation, membrane filtration, electrochemical technologies, ion exchange), however they can be expensive, and can generate by-products or sludge, involving complicated procedures [8,16]. Adsorption is considered one of the best options for heavy metal removal due to its flexibility in operation and design, low energy consumption, minimization of sludge and by-products, possibility of regenerating adsorbents, and high removal efficiency even at a very low metal concentrations [17,18]. Activated carbon (AC) is the most used and recognized heavy metal adsorbent but it is expensive due to its preparation process and the impossibility of its regeneration, which limits its use at large-scale application [19,20].

For developing countries, the application and development of heavy metal removal technologies represents a challenge [20]. In recent years, agro-industrial residues have emerged as low-cost adsorbents, also for heavy metals, due to its availability and abundance, allowing to apply processes under the bases of the Circular Economy (which corresponds to the recovery and reuse of wastes) [7,10]. Every year, worldwide, tons of waste are produced from the agro-industrial sector that are stored in the open air and disposal in landfills, causing negative environmental impacts due to leachates and gases, following with the CO<sup>2</sup> generation with their burning [21]. Literature examples of agro-industrial residues used as heavy metal adsorbents are: cow dung [5], potato peel [22], cucumber peel [23], groundnut husk [24], eggshells [25], pine and modified pine [26], rice and rapeseed [27], coffee husk and lignin [28], among others. All the plant-based wastes are made up of hemicellulose, cellulose and lignin, and has a wide variety of functional groups (e.g., aldehydes and ketones, carboxyl groups, phenolics, hydroxyls, methyls, ethers, amides, aminos, etc.) that can interact with pollutants through various mechanisms [7,8].

As reported by the national government in 2020, in Argentina agro-industrial is really important and constitutes the 25% of the manufacturing industry and represents the 40% of exports. Among the agro-industrial residues, sawdust constitutes from 9 to 15% of the forest biomass discarded by sawmills and comes mainly from pine and eucalyptus plantations [29]. Sunflower crop is one of the most important in Argentina with a production of 3.5 million tons of seeds per year, obtaining 50% by weight of discarded hulls per seed [30]. On another hand, the corn production extends over a large area of the country and generates a great volume of biomass when compared to others such as wheat or barley [31].

This paper focuses the study of adsorption processes of Ni(II), Zn(II) and Cd(II) by using pine sawdust, sunflower seeds hulls and corn residues mix as adsorbents. A comparison of these three agro-industrial wastes as adsorbents is here presented, by checking firstly each individual heavy metal adsorption process, and secondly the influence of the mixture of these three heavy metals on their adsorption (as they are present in real sewage all together), by corresponding adsorption experiments. Later, such biomass residues containing heavy metals are immobilized in clay ceramics (as brick's precursors), to here propose an environmentally safe way to dispose the spent adsorbents together

with adsorbates (heavy metals). This procedure would help to minimize the secondary contamination that could be generated by the disposal of spent adsorbents, which is rarely explored in the adsorption literature and is fundamental for the real application of the adsorption from low-cost materials. sorbates (heavy metals). This procedure would help to minimize the secondary contamination that could be generated by the disposal of spent adsorbents, which is rarely explored in the adsorption literature and is fundamental for the real application of the adsorption from low-cost materials.

propose an environmentally safe way to dispose the spent adsorbents together with ad-

*Water* **2022**, *14*, x FOR PEER REVIEW 3 of 20

#### **2. Materials and Methods 2. Materials and Methods**

#### *2.1. Chemicals and Reagents 2.1. Chemicals and Reagents*

All reagents used were of analytical grade. Stock solutions of concentration 1000 mg/L of individual heavy metals ions, Ni(II), Zn(II) and Cd(II), were prepared dissolving adequate amounts of Ni(NO3)2.6H2O, Zn(NO3)2.6H2O and Cd(NO3)2.4H2O (all from Panreac, Castellar del Vallès, Spain), respectively. From these solutions the corresponding dilutions used in the adsorption experiments were prepared and the pH (Omega 300 pH meter, Crison Instruments, S.A, Barcelona, Spain) was adjusted with HNO<sup>3</sup> 70% (Panreac, Spain). Solutions were prepared with Milli-Q water. All reagents used were of analytical grade. Stock solutions of concentration 1000 mg/L of individual heavy metals ions, Ni(II), Zn(II) and Cd(II), were prepared dissolving adequate amounts of Ni(NO3)2.6H2O, Zn(NO3)2.6H2O and Cd(NO3)2.4H2O (all from Panreac, Spain), respectively. From these solutions the corresponding dilutions used in the adsorption experiments were prepared and the pH (Omega 300 pH meter, Crison Instruments, S.A) was adjusted with HNO3 70% (Panreac, Spain). Solutions were prepared with Milli-Q water.

#### *2.2. Biomass 2.2. Biomass*

The biomasses of sawdust, sunflower and corn were selected according to the reasons already mentioned. The pine (*Pinus elliottii*) sawdust residues were provided by a sawmill in the province of Corrientes, Argentina, and corresponded to the main cutting process of the wood, before any addition. Sunflower seed hulls (*Helianthus annuus*) were provided by a company located in the province of Santa Fe, Argentina, dedicated to the oilseed market, and were obtained from the processing of sunflower grains. The corn residues mix (*Zea mays var. saccharata*) were kindly provided by the National Institute of Agricultural Technology (INTA), and corresponded to the harvest stage. The biomasses of sawdust, sunflower and corn were selected according to the reasons already mentioned. The pine (*Pinus elliottii*) sawdust residues were provided by a sawmill in the province of Corrientes, Argentina, and corresponded to the main cutting process of the wood, before any addition. Sunflower seed hulls (*Helianthus annuus*) were provided by a company located in the province of Santa Fe, Argentina, dedicated to the oilseed market, and were obtained from the processing of sunflower grains. The corn residues mix (*Zea mays var. saccharata*) were kindly provided by the National Institute of Agricultural Technology (INTA), and corresponded to the harvest stage.

The development of the adsorbents included the collection of the biomass, grinding with a knife mill (IKA A10) and sieving to a particle size of less than 1 mm to promote adsorption. The waste did not receive additional processing (chemical or thermal treatment) to make it as friendly as possible to the environment and reduce costs. Figure 1 shows the macroscopic appearance of the used waste, after grinding and sieving. The development of the adsorbents included the collection of the biomass, grinding with a knife mill (IKA A10) and sieving to a particle size of less than 1 mm to promote adsorption. The waste did not receive additional processing (chemical or thermal treatment) to make it as friendly as possible to the environment and reduce costs. Figure 1 shows the macroscopic appearance of the used waste, after grinding and sieving.

**Figure 1.** Ground and sieved residues (**A**) pine sawdust, (**B**) sunflower seed hulls and (**C**) corn residues mix. **Figure 1.** Ground and sieved residues (**A**) pine sawdust, (**B**) sunflower seed hulls and (**C**) corn residues mix.

#### *2.3. Biomass Characterization 2.3. Biomass Characterization*

Physicochemical properties of adsorbents contribute to the process of adsorption of contaminants. Characteristics of potential adsorbents were determined from a number of techniques, that included the Brunauer-Emmett-Teller (BET) (Micromeritics Accusorb, model 2100), Scanning Electron Microscopy (SEM) (ZEISS EVO® MA 10 at the UAB Microscopy Service and FEI ESEM Quanta 200), Energy-Dispersive X-ray Spectroscopy (EDS) (Oxford SDD X-Act, software: AZTecOne), Attenuated Total Reflectance–Fourier Transform Infrared Spectroscopy (ATR-FTIR) (Nicolet 6700, Thermo Electron Corp. equipment), X-ray Fluorescence (XRF) (PW4024 Minipal2 PANalytical X-ray spectrometer Physicochemical properties of adsorbents contribute to the process of adsorption of contaminants. Characteristics of potential adsorbents were determined from a number of techniques, that included the Brunauer-Emmett-Teller (BET) (Micromeritics Accusorb, model 2100), Scanning Electron Microscopy (SEM) (ZEISS EVO® MA 10 at the UAB Microscopy Service and FEI ESEM Quanta 200), Energy-Dispersive X-ray Spectroscopy (EDS) (Oxford SDD X-Act, software: AZTecOne), Attenuated Total Reflectance–Fourier Transform Infrared Spectroscopy (ATR-FTIR) (Nicolet 6700, Thermo Electron Corp. equipment, Waltham, MA, USA), X-ray Fluorescence (XRF) (PW4024 Minipal2 PANalytical X-ray spectrometer with copper anode and operating conditions nitrogen flow, voltage 20 kV, current 5 mA and time 100 s), and Differential Thermal Analysis (DTA) and Thermogravimetric

Analysis (TGA) (Shimadzu TGA-50 and Shimadzu DTA-50 instruments, with TA-50 WSI analyzer and operating conditions air, heating rate of 10 ◦C/min to 1000 ◦C and approximately 20 mg of mass). The mineral content of biomass it was determined following the guidelines of the ASTM E1755-01 standard [32]. In addition, the possible changes produced in the biomasses after the adsorption experiments were analyzed.

#### *2.4. Batch Adsorption Experiments*

The removal of heavy metals by means of the aforementioned agro-industrial residues was carried out under batch adsorption experiments at room temperature. A volume of 10 mL of a mono-metal solution, of Ni(II), Zn(II) or Cd(II) of 0.18 mmol/L, or a multi-metal solution of all together with a concentration of 0.18 mmol/L for each heavy metal was placed in contact with 0.1 g of adsorbent in tubes. The pH of the solutions was initially adjusted to 4–5 following previous literature [26,33]. The system was stirred at 40 rpm in a rotary mixer (CE 2000 ABT-4, SBS Instruments SA) for 24 h to ensure that equilibrium was reached. The liquid phase was filtered through 0.22 µm filters (Millex-GS, Millipore, Burlington, MA, USA). The metal concentration in the aqueous solution (not adsorbed) was determined by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) (XSERIES 2 ICP-MS, Thermo Scientific, Waltham, MA, USA) from the Autonomous University of Barcelona. The adsorption of each heavy metal was expressed as adsorption percentage (A%), calculated from Equation (1), and the adsorption capacity of each adsorbent (qe) was calculated from Equation (2):

$$A\% \text{ (\%)} = \frac{(\text{C}\_0 - \text{C}\_e)}{\text{C}\_0} \times 100 \tag{1}$$

$$q\_{\varepsilon} \left( \frac{mmol}{g} \right) = \frac{(\mathbb{C}\_0 - \mathbb{C}\_{\varepsilon}) \times V}{m} \tag{2}$$

where C<sup>0</sup> and C<sup>e</sup> (mmol/L) are the initial and equilibrium concentrations of heavy metal in solution, respectively, V (L) is the volume of the heavy metal solution, and m (g) is the mass of the adsorbent [34,35]. Adsorption experiments are prepared by duplicate and the average results are reported.

#### *2.5. Spent Adsorbents Disposal in Clay Ceramics*

Safe disposal of spent adsorbents is necessary to minimize secondary contamination, especially if large-scale adsorption technology implementation is considered. Pine sawdust, sunflower seed hulls and corn residues mix, after being used as adsorbents, were added to the clay, and clay ceramics were prepared with the aim of immobilizing adsorbed heavy metals.

The amount of residue contaminated with Ni(II), Zn(II) and Cd(II) that was used in the clay ceramics corresponded to 20% in volume with respect to the volume of clay, in accordance with what was observed by the authors in previous studies [36]. For this reason, adsorption experiments were previously carried out scaling 20 times the amount of adsorbent (2 g) and 20 times the moles of metal ion (3.6 <sup>×</sup> <sup>10</sup>−<sup>5</sup> ). As a consequence of the increase in adsorbent mass, the solution volume had to be increased to 40 mL so that the liquid covers the entire surface of the biomass.

The methodology used in the preparation process of the clay ceramics is detailed in Figure 2 and it was designed considering the characteristics of the raw materials, the pressure and firing conditions used in the brick factory. The clay was provided by a local brick factory and is the same that the manufacturer uses in the hollow brick production process. Firing of clay ceramic was carried out in an electric oven INDEF 332.

initial mass of heavy metals in the clay ceramics. In this way, the retention efficiency was

Clay ceramics prepared from spent adsorbents were crushed and sieved to a particle size of less than 9.5 mm. In beakers, crushed clay ceramics were mixed with leaching solution in a 1:20 solid-liquid ratio. According to the alkalinity of the ceramic, an extraction fluid of pH 4.93 ± 0.05 was prepared from 5.7 mL of acetic acid, 64.3 mL of sodium hydroxide 1 mol/L and completing with distilled water up to 1 L. The covered beakers were shaken at 100 rpm in an orbital shaker (SK-0330-Pro) for 22 h. The mixtures were then filtered through filter paper washed with 1 mol/L nitric acid and rinsed with distilled water. The TCLP extracts of the solid phases were acidified with concentrated nitric acid until pH < 2 and stored refrigerated at 4 °C. Finally, the extracts were analyzed by Atomic Absorption Spectrophotometry (AAS) (Shimadzu 6800 with flame) from the Fares Taie Bio-

**Figure 2.** Clay ceramics preparation process. **Figure 2.** Clay ceramics preparation process.

calculated according to Yilmaz et al. [38].

**3. Results and Discussion**  *3.1. Biomass Characterization*  The phenomena that occur in an adsorbent are related to its specific surface and, therefore, to the total volume of pores and their dimensions, that influence the interaction It is essential to evaluate the mobility of heavy metals present in manufactured clay ceramics to determine the feasibility of immobilize these contaminants in the ceramic structure. Leaching tests were carried out based on EPA method 1311 [37], which is a method accepted by Argentine laws for hazardous waste.

with the adsorbate and the obtained adsorption efficiency. Table 1 shows the results obtained from the BET analysis for pine sawdust, sunflower seed hulls and corn residues mix. All biomasses showed the presence of mesopores and the surface area values are in agreement with those reported in the literature for adsorbents of lignocellulosic origin [34,39]. Corn biomass presented a higher surface area, total pore volume, and mean pore size comparing with the other two biomass residues (pine sawdust and sunflower seed hulls). Bilal et al. [7] reported that the adsorption of contaminants increases with the increase in the surface area of the adsorbent, since the adsorption process is a surface phe-**Table 1.** BET analysis results of surface area, total pore volume and mean pore size for biomass Clay ceramics prepared from spent adsorbents were crushed and sieved to a particle size of less than 9.5 mm. In beakers, crushed clay ceramics were mixed with leaching solution in a 1:20 solid-liquid ratio. According to the alkalinity of the ceramic, an extraction fluid of pH 4.93 ± 0.05 was prepared from 5.7 mL of acetic acid, 64.3 mL of sodium hydroxide 1 mol/L and completing with distilled water up to 1 L. The covered beakers were shaken at 100 rpm in an orbital shaker (SK-0330-Pro) for 22 h. The mixtures were then filtered through filter paper washed with 1 mol/L nitric acid and rinsed with distilled water. The TCLP extracts of the solid phases were acidified with concentrated nitric acid until pH < 2 and stored refrigerated at 4 ◦C. Finally, the extracts were analyzed by Atomic Absorption Spectrophotometry (AAS) (Shimadzu 6800 with flame) from the Fares Taie Biotechnological Center. The results of the leaching tests were compared with the estimated initial mass of heavy metals in the clay ceramics. In this way, the retention efficiency was calculated according to Yilmaz et al. [38].
