Keywords

11.1 Introduction

Industrialization and rapid population growth have polluted water resources globally and caused a challenge of freshwater availability. This is particularly important when the demand of freshwater doubles every two decades. Approximately 40% of the global population (from 80 countries) is facing water deficiency. About 500 billion m3 fresh water is used by industries every year (Ali et al. 2021). Industries such as fertilizer, paper and pulp, steel, sugar, textile, thermal power plant, etc. contribute majorly towards water pollution. Water resources are being polluted due to unwanted entry of various pollutants.

Pollutants may basically be classified as conventional and/or emerging pollutants. Conventional pollutants include fluoride, nitrate, and trace metals while emerging pollutants comprise but not limited to steroid, hormones, pharmaceuticals and personal care products, artificial sweeteners, surfactants (Ahamad et al. 2020), antibiotics, sunscreens (Caliman and Gavrilescu 2009), naturally occurring algal toxins, steroids, endocrine disruptors and their degradation intermediates (Preda et al. 2012), gasoline additives, fire retardants, plasticizers, and microplastics (Browne et al. 2015). Emerging pollutants are present in trace amount (ng/L–μg/L), but their long persistence in the environment significantly affects water quality, health of human and animals and ecosystem. Further, pollutants may be classified as inorganic, organic and biological in nature. A constant flux of heavy metals and other inorganics originated from industrial and municipal wastewater, mine drainage is contaminating surface waters and sediments (Hoque and Philip 2011). Organic pollutants include fertilizers, plasticizers, polybrominated biphenyls, phenols, detergents, greases, formaldehydes, oils, hydrocarbons, pharmaceuticals, and pesticides (fungicides, herbicides, insecticides) (Ali et al. 2012). Major sources of organic pollutants are chemical and agricultural industries. Bacteria, viruses, fungi, algae, and amoeba are the examples of biological pollutants which affect human health and may cause nausea, rheumatoid arthritis, kidney damage, chronic diseases, circulatory system, and nervous disorders (Ali 2012).

Monitoring of emerging pollutants is difficult due to their low concentration but advanced separation techniques (liquid-liquid extraction, polymer grafted matrix, magnetic nanoparticles based solid phase extraction) (Moliner-Martínez et al. 2011) and analytical techniques (GS-MS, LC-MS, LC-MS-MS) (Hernández et al. 2007), capillary electrophoresis (Moliner-Martínez et al. 2011) are capable of detecting emerging pollutants even at trace level. Efficient, selective and cost-effective wastewater treatment techniques are necessary to develop in the above scenario. Available wastewater treatment methods are filtration (Barakat 2011), precipitation (Fu and Wang 2011), ion exchange (Kurniawan et al. 2006; Peng and Guo 2020), advanced oxidation processes (García-Montaño et al. 2006; Dhaka et al. 2017), biological treatment (Aksu 2005), reverse osmosis, distillation, electrochemical dialysis (Gunatilake 2015), and adsorption (Kwon et al. 2016; Patel et al. 2019, 2021; Kumar et al. 2020). The process of wastewater treatment is becoming expensive regularly as the prescribed standards for discharge are getting more and more stringent.

Adsorption is one of the prominent wastewater treatment techniques due to low operational and maintenance cost and ease of operation (Gupta et al. 2012; Singh et al. 2018). The efficiency of adsorbents depends upon its specific surface area, pore volume, and available binding sites (Hassan and Carr 2021). A number of conventional adsorbents, namely activated carbon, silica gel, clays, limestone, chitosan, and zeolites are used for wastewater treatment (Krstić et al. 2018). Different pollutants like dyes (Muhd Julkapli et al. 2014), heavy metals, metalloids, pesticides, and pharmaceuticals (Patel et al. 2019) have been removed using adsorption process (Dawood and Sen 2014; Singh et al. 2018; Rathi and Kumar 2021).

Conventional adsorbents have issues like their high cost, low removal efficiency, fast exhaustion, and poor regeneration capacity. The performance of adsorbent may be improved via surface modification, composite preparation among the several other routes. The complex and multistep processes for modifications are either expensive or less feasible for large-scale production (Bhatnagar et al. 2013). To overcome these limitations, nanoadsorbents can be used for wastewater treatment (El-Sayed 2020). Nanoadsorbents have relatively high chemical reactivity, conductivity, large surface area, and specificity making them a preferred choice over the conventional adsorbents (El-Sayed 2020). Nanoadsorbents may be prepared through sol-gel, sono-chemical, mechanical, microwave, and chemical process (Rangari et al. 2017; Biswas et al. 2019). The use of nanoadsorbents for wastewater treatment is a win-win strategy as they can be developed using agricultural residue. Further, agricultural residue burning is a serious problem in certain areas of India; it causes loss to soil health, unbearable air pollution and the health affects. Therefore, converting this agri-waste into nano-adsorbents is a “double-edged sword” with benefits of solid waste management and wastewater treatment.  

This chapter gives an overview of various wastewater treatment techniques and emphasizes over sorptive removal of pollutants from wastewater using agriculture residue-based nanoadsorbents. Efficiency of nanoadsorbents over conventional adsorbents has been discussed. Silica, cellulose, lignin, and biochar-based nanoadsorbents are being included in the discussion.

11.2 Available Wastewater Treatment Techniques

Wastewater treatment includes primary, secondary, and tertiary stages (Fig. 11.1). Primary stage includes physical processes such as screening, grit removal, and sedimentation. Secondary stage comprises biological treatment, namely aerobic, anaerobic digestion, activated sludge treatment, trickling filtration among the other process. The tertiary stage mostly includes chemical treatment of wastewater. This stage is the most important as, at this stage, toxic and harmful pollutants are converted into less toxic forms or eliminated to meet the accepted standards. These wastewater treatment stages should be collectively used to achieve better results (Gupta et al. 2012). A brief overview of the wastewater treatment techniques has been provided in the following paragraph and summarized in Table 11.1.

Fig. 11.1
figure 1

Wastewater treatment stages

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Table 11.1 A comparative evaluation of wastewater treatment processes
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The screening includes filtration (i.e., removal of coarse solids) via slow sand filtration (Verma et al. 2017) is a primary wastewater treatment technique which eliminates commonly present conventional suspended pollutants from wastewater (Barakat 2011). Biological filtration (Aziz and Ali 2016), a secondary stage process, is the bacteria-derived degradation of ammonia to nitrites and eventually to less toxic nitrates. Membrane filtration techniques such as microfiltration, nanofiltration, ultrafiltration and reverse osmosis, fall under tertiary stage of wastewater treatment, are used to remove pollutants according to their particle size (Zazouli and Kalankesh 2017). Microfiltration and ultrafiltration using ceramic membranes have been reported to remove chemical oxygen demand (COD) (>87%), color (>96%), total suspended solids (TSS) (~100%), and turbidity from anaerobically treated dairy wastewater (Zielińska and Galik 2017).

In case of chemical precipitation, precipitant reacts with dissolved metal ions in wastewater and converts into insoluble sludge followed by its removal via sedimentation or filtration (Zamboulis et al. 2004; Fu and Wang 2011). Precipitant like lime, limestone, alum, sodium hydrogen carbonate, and ferric chloride are used for wastewater treatment (Gupta et al. 2012). The precipitant (for example, limestone) has been reported to remove more than 90% of Cd2+ and Cu2+ from water (pH 8.5) (Aziz et al. 2008).

$$ {\mathrm{M}}^{n+}\left(\mathrm{aq}\right)+n{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)\leftrightarrow \mathrm{M}{\left(\mathrm{OH}\right)}_n\left(\mathrm{s}\right)\downarrow $$

Ion exchange process uses an ion exchange membrane which potentially eliminates colloidal and soluble ionic contaminants from wastewater (Kurniawan et al. 2006). The anion exchange membrane retains the anions and rejects cations. However, the cation exchange membrane retains cations and rejects anions (Barakat 2011). A combined anion and cation exchange membrane efficiently removes dissolved organic carbon (76%) and total hardness (97%) using NaCl regeneration solution (20%) (Comstock and Boyer 2014).

In case of biological methods of wastewater treatment, microbes are used to decompose organic pollutants especially emerging pollutants from water into simpler non-toxic compounds under controlled temperature and physicochemical conditions (Aksu 2005). Microbial action in the absence of oxygen to degrade organic matter from wastewater is known as anaerobic biological wastewater treatment (Eq. 11.1) (Mohan et al. 2007). Aerobic microbes (bacteria and fungi) decompose organic contaminants in wastewater in the presence of oxygen (Eq. 11.2). Aerobic digestion of enzyme pre-treated biosolids has been reported to remove emerging pollutants from wastewater collected from local municipal wastewater treatment plant (up to 90%) (Vaithyanathan et al. 2021). Cassava starch wastewater was treated using an up-flow multistage anaerobic reactor and 87.9% COD was removed within 6 h of hydraulic retention time (Sun et al. 2012).

$$ \mathrm{Organic}\ \mathrm{matter}+\mathrm{Anaerobic}\ \mathrm{bacteria}\to {\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}+\mathrm{Co}\hbox{-} \mathrm{products}+\mathrm{Anaerobic}\ \mathrm{bacteria} $$
(11.1)
$$ \mathrm{Organic}\ \mathrm{matter}+{\mathrm{O}}_2+\mathrm{Aerobic}\ \mathrm{bacteria}\to {\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}+\mathrm{Co}\hbox{-} \mathrm{products}+\mathrm{Aerobic}\ \mathrm{bacteria} $$
(11.2)

The wastewater treatment using oxidation process involves decomposition of organic contaminants into simpler compounds like aldehydes, carboxylates, and sulfates (Garcia-Segura et al. 2013). Numerous oxidants like chlorine (Cl2), hydrogen peroxide (H2O2), Fenton catalyst (H2O2 + Fe(II)), Fenton like catalyst and ozone (O3) may be used as oxidants in wastewater treatment (Saputra et al. 2011). However, when single oxidation technique is not sufficient for complete decomposition of organic pollutants, two or more oxidation techniques might be used simultaneously to generate reactive radical species such as hydroxyl free radical and sulfate radical anion which degrade organic pollutants in wastewater. This technique is known as advanced oxidation process (AOP) for wastewater treatment (García-Montaño et al. 2006). Nanoscale zerovalent iron (nZVI) impregnated biochar (BC) composite (a Fenton like heterogeneous catalyst) was prepared. The nanobiochar (nZVI to BC (1:5)) successfully degraded trichloroethylene via Fe(II)/Fe(III) redox and electron-transfer reaction pathways in aqueous solutions (Yan et al. 2015a). Advanced oxidation processes such as photo-Fenton oxidation process (Moncayo-Lasso et al. 2009), ultraviolet (UV) assisted sono-Fenton (Kakavandi et al. 2019), and ultrasonic irradiation assisted Fenton or Fenton like processes (Mahamuni and Adewuyi 2010) have been reported frequently in the literature.

Wastewater treatment using electrodialysis technique removes ionic pollutants as high voltage across ion exchange membrane is applied. When polluted water passes through the electrodialysis cell, anionic pollutants migrate toward the anode and the cationic one toward the cathode (Gunatilake 2015). The cation exchange membrane made up of poly vinyl chloride and 2-acrylamido-2-methylpropane sulfonic acid based hydrogel was prepared. The membrane had a low electrical resistance and showed high removal efficiency for K+ (99.9%), Pb2+ (99.9%), and Ni2+ (96.9%) (Nemati et al. 2017).

11.3 Wastewater Treatment Through Adsorption Method

Adsorption is an efficient and cost-effective wastewater treatment method which involves retention of pollutants onto adsorbent’s surface through forces such as electrostatic attraction, hydrophobic interaction, π–π interactions, hydrogen bonding, precipitation, ion exchange, and surface complexation (Tan et al. 2015; Kumar et al. 2018; Patel et al. 2019). Adsorption can potentially be used to remove conventional as well as emerging pollutants from wastewater (Ali 2012; Patel et al. 2019). Adsorption process can remove inorganic and organic pollutants efficiently even up to 99% from water (Ali et al. 2012). Working efficiency of adsorption process depends upon concentration and type of pollutant, nature and particle size of adsorbent, structure, properties, and morphology of adsorbent like pore size, specific surface area, surface functional group, working pH, temperature, adsorbent dose, and contact time (Kumar et al. 2014; Patel et al. 2019). Adsorption process is preferred over the other conventional methods of wastewater treatment due to following reasons.

  • Cheap adsorbents can be prepared from locally available agricultural residues and waste biomass.

  • Selective adsorbents can be prepared by choosing appropriate biomass and their modification for intended application.

  • Adsorption, unlike conventional wastewater treatments, generates no secondary pollutant or sludge.

  • Adsorbent can be regenerated using physicochemical approaches.

11.3.1 Nanoadsorbents for Wastewater Treatment

The commonly used conventional adsorbents for wastewater treatment are clay, lime stone, activated carbon, silica gel, activated alumina, and zeolite. These adsorbents have numerous challenges associated with them (i.e., expensiveness, rapid exhaustion, surface modification, and low removal efficiency). Adsorption efficiency of conventional adsorbents, like activated carbon, decreases rapidly in wastewater treatment process. Regeneration of these adsorbents requires thermal, chemical, oxidative treatments which increase the operational cost and thus limit adsorbent’s application for pollutant removal (Crini 2005; Zhou and Lei 2006). Adsorption capacity of conventional adsorbents is affected by the presence of co-pollutants in wastewater (Makeswari and Santhi 2013). Moreover, relatively low removal capacities of conventional adsorbents limit their application in wastewater treatment. This is due to the poor morphological and structural properties. Thus, it is advisable to use smart, efficient, low-cost adsorbents in wastewater treatment process.

Nanoadsorbents which are cost-effective and efficient may be used in place of conventional adsorbents. Nanoadsorbents have large specific surface area, pore size, rich surface chemistry and density of active sites (El-Sayed 2020). Nanoadsorbents have widely been used for mitigation of conventional and emerging pollutants from (waste)water (Belessi et al. 2009; Joseph et al. 2019). A variety of nanomaterials, namely metallic nanoparticles, magnetic nanoparticles, nanostructured mixed oxides, carbonaceous nanomaterials (carbon nanosheets, carbon nanotubes, carbon nanoparticles), silicon nanomaterials (silicon nanotubes, silicon nanoparticles, and silicon nanosheets), nanoclays, nanofibers, xerogels, aerogels, and polymer-based nanomaterials have been used as adsorbents in wastewater remediation (Khajeh et al. 2013; El-Sayed 2020). Synthesis procedure and operating conditions influence the structural (specific surface area, particle size, pore size, and pore volume), chemical (functional group, acidic/basic character, surface potential, water solubility, stability) and thermal (conductivity, stability, heat capacity, volatile content) properties of nanomaterials. Different nanoadsorbents (graphene, Fe3O4, alumina, magnetite, carbon nanotube, TiO2, Al2O3, mesoporous carbon, MnFe2O4, and nano-zerovalent iron) have been synthesized by chemical vapor deposition, spray pyrolysis, thermal evaporation, vapor phase transport, electrodeposition, laser ablation processes, ion sputtering, coprecipitation using surfactants, chemical solution decomposition, and sol-gel method (Sharma et al. 2009; El-Sayed 2020). Sol-gel method is widely used for the synthesis of nanoparticles due to low cost, eco-friendliness, and homogenous product formation (Li et al. 2011).

Adsorption capacity of nanomaterials can be enhanced by modification such as composites formation and coating with other materials. For example, the composite of TiO2 and carbon nanotubes has large surface area and active sites than the individuals, therefore having high removal capacity for pollutant removal (Ilisz et al. 2003; Hurt et al. 2006). Nanoparticles have been modified to increase the adsorption capability of the respective materials as the adsorption capacity is directly related to surface properties (Afkhami et al. 2010). Neat nanoparticles generally have a tendency to agglomerate in the absence of colloidal stability. So, the surface modification provides colloidal stability to nanoparticles (Crane and Scott 2012). Surface of nanomaterials can be modified through different mechanisms, namely ligand exchange, hydrothermal reduction, co-condensation, surface coating, covalent binding, grafting, and coprecipitation (Manyangadze et al. 2020). Modification of silica and iron nanoparticles has been sucessfully performed by hydrothermal method (Manyangadze et al. 2020). Nano size alumina, synthesized by chemical immobilization and modified with 2,4-dinitrophenylhydrazine was successfully removed Cr3+ (97.4%), Cd2+ (80.4%), and Pb2+ (97.0%) from wastewater (Afkhami et al. 2010).

11.3.2 Agricultural Residue-Derived Nanoadsorbents for Wastewater Treatment

Agricultural residue-derived nanoadsorbents may remove pollutants efficiently from (waste)water. The use of agricultural residue-derived nanoadsorbents is a win-win strategy for wastewater treatment and its management simultaneously. The use of such nanoadsorbents will not only help in wastewater remediation but also serve as a smart remedy for agricultural wastes management which is otherwise a big challenge in itself. Herein, we have presented an overview of selected studies of agricultural residue-derived nanoadsorbents used for wastewater treatment. A summary of such nanoadsorbents has been provided in Tables 11.2 and 11.3.

Table 11.2 Inorganic pollutants removal in water using agricultural residue-derived nanoadsorbents
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Table 11.3 Organic pollutants removal in wastewater using agricultural residue-derived nanoadsorbents
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11.3.2.1 Silica-Based Nanoadsorbents

An amorphous silica based bionanoadsorbent (particle size: 10–50 nm, yield: 81%) was developed using rice husk via hydrothermal technique. The adsorbent successfully removed methylene blue (65% within a minute) from aqueous solution. Thermodynamic studies indicated the spontaneous and endothermic nature of adsorption process (Tolba et al. 2015). Rice husk residue-derived nanosilica (SiO2) modified with polyelectrolyte polydiallyldimethylammonium chloride (PDADMAC) polymer was prepared by Pham and others. The modified SiO2 with PDADMAC removed 92.3% amoxicillin antibiotic from aqueous medium. The mechanism involved was electrostatic attraction between cationic (PDADMAC polymer) and anionic surface (SiO2). Non-electrostatic hydrophobic and lateral interactions were mainly responsible for amoxicillin (negatively charged) sorption onto modified nanosilica through induced electrostatic attractions (Pham et al. 2018).

Silica based bionanoadsorbents were prepared using barley and wheat grass with 92.4 and 93.0% of silica content, respectively. The barley and wheat grass based adsorbents successfully removed Ni2+ (95% and 90% removal, respectively). The mechanism involved was electrostatic interaction between negatively charged surface of bionanoadsorbent and Ni2+. Pseudo-first-order and Freundlich model described the adsorption kinetics and isotherm results, respectively (Akhayere et al. 2019). Further, a green bionanoadsorbent (99.6% pure nanosilica, 20 nm size) was prepared using rice husk and modified with polydiallyldimethylammonium chloride. The adsorbent removed beta-lactam cefixime efficiently (93.5% within 90 min) from aqueous solution. The electrostatic attraction between positively charged modified nanosilica and negatively charged cefixime was the major adsorption mechanism operated in removal process (Pham et al. 2020).

A thermally stable, efficient, eco-friendly, cost-effective, recyclable Fe2O3-enriched nanosilica (SiO2-Fe2O3) was prepared from rice straw to remove Penicillin G (95%) and Amlodipine Besylate (65%) from aqueous solution. Langmuir adsorption isotherm and pseudo-first-order kinetic model indicated multilayer adsorption of pollutants on the heterogeneous adsorbent surface. The removal efficiency of SiO2-Fe2O3 was 30% greater than pure Fe2O3 which indicates its effectiveness as an adsorbent for antibiotics removal (Fakhrian and Baseri 2020). Another cost-effective green bioadsorbent (silica nanoparticles) was synthesized using Saccharum ravannae, Saccharum officinarum, and Oryza sativa leaf biomasses through chemical method. Synthesized silica nanoparticles quantitatively removed Pb2+ and Cu2+ (>95%) from wastewater (Sachan et al. 2021).

11.3.2.2 Cellulose-Based Nanoadsorbents

Cellulose nanofibrils were prepared using wood pulp treated with glycidyltrimethylammonium chloride via mechanical disintegration. The adsorbent efficiently retained congo red (0.95 mol/kg) and acid green (1.10 mol/kg) from wastewater (Pei et al. 2013). A green, recyclable cellulose-derived nanoadsorbent was prepared using sugarcane bagasse via bleaching treatment followed by acid hydrolysis. The developed adsorbent removed methylene blue efficiently (35 mg/g within 60 min) from wastewater. Langmuir and Freundlich adsorption isotherms indicated the possibility of both mono- and multilayer adsorption. The adsorbent could be used successively up to six cycles in both adsorption and desorption processes (Kardam et al. 2017). An efficient, carboxylcellulose nanofiber bioadsorbent was prepared using Australian spinifex grass via nitro-oxidation treatment. The adsorbent successfully removed Cd2+ (84%) from aqueous solution. At low concentrations of Cd2+ (<500 ppm), electrostatic interaction was prominent between the carboxylate groups of the adsorbent and Cd2+, while at high Cd2+ concentrations (>1000 ppm) dominant operating mechanism was precipitation followed by flocculation or coagulation (Sharma et al. 2018).

Banana peel-derived Cellulose/SiO2 nanocomposite (particle size: 20 nm) was prepared using template-assistant method. Nanoadsorbent efficiently removed methylene blue (58.8 mg/g) from wastewater. The removal of pollutant followed Langmuir adsorption isotherm and pseudo-second-order kinetic model (Ali 2018). A starch-derived magnetic hydrogel nanocellulose crystal was prepared using sugar-beet pulp via acid hydrolysis method. The nanocomposites were used to remove methylene blue and crystal violet dyes from wastewater. Electrostatic force of attraction between anionic surface of modified nanocellulose and cationic dyes was main sorption mechanism (Moharrami and Motamedi 2020). Rice husk-derived magnetic carboxylcellulose nanofiber adsorbents (suspension, freeze-dried and nanocomposites) were prepared using TEMPO-oxidation method and used to remove Pb2+ and Ln3+ from wastewater. Among these, the adsorption capacity of nanocellulose suspension was maximum for Pb2+ (193.2 mg/g) and Ln3+ (100.7 mg/g). Electrostatic attraction between the negatively charged surface of adsorbent and metal ions was the main removal mechanism (Zhan et al. 2020).

11.3.2.3 Lignin-Based Nanoadsorbents

An organosolv lignin nanobioadsorbent was prepared using steam-exploded rice straw biomass. The adsorbent removed methylene blue from wastewater (40.0 mg/g). The adsorption kinetics data were simulated by pseudo-first-order and pseudo-second-order models. Langmuir adsorption isotherm indicated the monolayer adsorption of methylene blue (Zhang et al. 2016 b). Lignin supported carbon nanotubes efficiently removed Pb2+ (235 mg/g) and oil droplet (98.3%) from aqueous solution. Tridimensional structure, presence of ample oxygen functional groups, large specific surface area, water dispersibility, and mechanical stability of adsorbent made it an effective adsorbent for Pb2+ removal. Pore diffusion and interaction between oxygen functional groups on the adsorbent surface and Pb2+ were the responsible removal mechanisms (Li et al. 2017).

A cost-effective, recyclable, poly(ethyleneimine)-grafted-alkali lignin nanobioadsorbent fabricated with lanthanum hydroxide nanoparticles was prepared. The adsorbent successfully removed phosphate at high concentration (50 ppm, 94%, 60 min) and low concentration (2 ppm, 99 %, 15 min) under wide pH range (3.0-9.0). Surface precipitation and ligand exchange were the main operating mechanisms for adsorption (Zong et al. 2018).

Lignin-based nanoadsorbents (alkali lignin and lignin nanoparticles) were prepared and used successfully to remove Basic Red 2 (alkali lignin: 55.2 mg/g and lignin nanoparticle: 81.9 mg/g) from aqueous solution. The adsorbents were prepared via acidification of black liquor from paper factory waste. Alkali lignin was further modified by ethylene glycol treatment. Surface morphology and chemistry were confirmed by SEM and FTIR studies, respectively (Azimvand et al. 2018). An efficient, green, cost-effective, chitosan composite with nanolignin was developed and used to remove methylene blue (83%) from wastewater. The lignin was extracted from palm kernel shell using acid and tetrahydrofuran (Sohni et al. 2019). Pseudo-second-order kinetic model and Langmuir adsorption isotherm described the monolayer adsorption. Thermodynamic studies indicated the spontaneous endothermic adsorption of pollutant onto nanocomposite surface (Sohni et al. 2019).

Alkali lignin-based Pd nanoparticles were prepared via two different methods, namely loading followed by regeneration and regeneration followed by loading. The developed adsorbents removed toxic Cr6+ by reducing it into less toxic Cr3+. The adsorbent formed via regeneration followed by loading reduced Cr6+ completely within 5 min (Chu et al. 2021). Lignin-based nano-trap multifunctional adsorbent was prepared via inverse-emulsion copolymerization method. The adsorbent removed soft metals (Ag+, Hg2+, Cd2+) and borderline metals (Pb2+, Cu2+, Zn2+) quantitatively (>99%) from aqueous solution to the level of permissible limit in drinking water. The silver-laden nanocomposite demonstrated antimicrobial activity towards Escherichia coli (99.68%) and Staphylococcus aureus (99.76%) (Xiao et al. 2019).

11.3.2.4 Biochar-Based Nanoadsorbents

An engineered graphene-coated nanobiochar was prepared using cotton wood and used to remove methylene blue efficiently (174 mg/g) from aqueous solution. The synthesized nanobiochar was 20 times more efficient than unmodified biochar. The adsorption mechanism involved in removal process was strong π–π interactions between fused aromatic rings of dye and the graphitic sheets of biochar (Zhang et al. 2012). A zinc modified novel nanobiochar was prepared using sugarcane bagasse and removed Cr6+ from wastewater (~102 mg/g). The adsorbent worked even after sixth cycle of regeneration with a gradual decrease in performance (84.16–59.75 mg/g only). The working efficiency, regeneration capability, and economic feasibility made it an effective adsorbent (Gan et al. 2015). A nano zerovalent iron supported biochar was prepared using rice husk for industrial wastewater treatment through batch adsorption studies to remove methyl orange (98% within 10 min) (Han et al. 2015). Structural and physicochemical properties of biochar were confirmed by TEM, BET, and XRD techniques (Han et al. 2015).

A cost-effective, easily separable graphene modified nanobiochar composite was prepared using wheat straw. The adsorbent effectively removed phenanthrene and Hg2+ from aqueous solution. Structure and morphology studies indicated the higher removal capacity of nanobiochar as compared to pristine biochar. The operating mechanism was π–π interactions. The larger surface area, greater thermal stability, and presence of ample functional groups on nanobiochar improved its adsorption capacity. Surface sorption and surface complexation were the pathways for Phenanthrene and Hg2+ removal (Tang et al. 2015). A pine derived nano zerovalent iron modified biochar was prepared and the adsorbent used to remove As5+ (124.5 g/kg within 1 h) from aqueous solution. The anoxic condition was more effective (about 8%) than the oxic conditions for removal of As5+. Surface complexation and the reduction reactions were responsible removal mechanisms. Regeneration and high adsorption capacity of biochar makes it an effective adsorbent (Wang et al. 2017).

A pinewood derived nanobiochar (particle size: 60 nm) removed carbamazepine from aqueous solution (95% within 3 h) following pseudo-second-order kinetic and Freundlich adsorption isotherm model. The kinetic studies showed that the addition of surfactant enhanced the adsorption efficiency by 57% and increase in pH (from 3 to 8) improved the adsorption efficiency by 2.3-fold (Naghdi et al. 2019). A magnetic nanobiochar was prepared using wheat straw via ball milling. The adsorbent removed tetracycline and Hg2+ from aqueous solution (>99%). The electrostatic interactions, hydrogen bonding, and π–π interaction between the surface of adsorbent and tetracycline were operating in case of tetracycline removal, while electrostatic attractions, Hg–π bond formation, and surface complexation were reported for Hg2+ removal (Li et al. 2020).

11.3.3 Mechanism Involved in Adsorptive Removal of Inorganic and Organic Pollutants

Adsorption is a surface phenomenon wherein adsorbate molecules accumulate onto the surface of adsorbent. The presence of various minerals and functional groups remain present on the adsorbent surface. Various physical and chemical forces (mechanism) operate during sorptive removal of pollutants from wastewater using agricultural residue-derived nanoadsorbents. These forces allow the adsorption of different contaminants on adsorbent surface. Adsorption mechanism and efficiency are influenced by precursor materials of adsorbent and the characteristics of contaminants.

The physical properties of adsorbents such as surface area and pore volume and distribution play a vital role in adsorption of pollutants. Different mechanisms reported for sorptive removal of inorganic and organic pollutants from aqueous phase using agricultural residue-derived nanoadsorbents are depicted in Fig. 11.2a, b. The operative mechanisms in adsorption processes mainly included electrostatic attraction, hydrophobic interaction, π–π interactions, hydrogen bonding, precipitation, ion exchange, and surface complexation (Tan et al. 2015). Interaction between adsorbate and adsorbent also depends upon operational parameters like pH, matrix of the solution, contact time, pollutant concentration, adsorbent dose, temperature among the others.

Fig. 11.2
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Mechanisms involved in sorptive removal of (a) inorganic and (b) organic pollutants. (Reproduced with permission from Tan et al. 2015)

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If the solution pH is less than pHpzc, then the functional groups present on the adsorbent surface will get protonated and adsorb negatively charged pollutant species from the solution; on the other hand if pH is more than pHpzc then positive charged species adsorb onto the surface. Wang et al. (2018) described various possible mechanisms between functionalized magnetic nanoparticles and metals, antibiotics, pesticides. In case of pesticides, mechanisms included hydrophobic interaction, electrostatic interaction, π–π interaction, π–π electron donor–acceptor interaction, π-stacking interaction, and physisorption. For antibiotic removal hydrogen bonding, π–π interaction, cation–π interaction, electrostatic attraction, electron-π donor–acceptor interaction, and amidation reactions were the possible mechanisms.

Coating on nanoparticles surface could improve the properties of adsorbents like increased active sites, large surface area, and improved removal efficiency. It has been observed that the coating of graphene on biochar attributed to the adsorption of organic pollutants through π–π interactions (Tang et al. 2015; Zhang et al. 2012). Both hydrophobic and π–π interaction may operate between the carbon nanotubes loaded bagasse biomass derived biochar and organic contaminant sulfapyridine (Inyang et al. 2015). Redox reaction and surface complexation were involved mechanisms for As3+ adsorption onto cerium modified chitosan ultrafine nanobiosorbent (Zhang et al. 2016a). The monodentate and bidentate complexes formation mechanism involved between the hydroxyl groups of nanoadsorbent and As3+ followed by oxidation of As3+ to As6+. Other mechanisms of adsorption like pore filling, partitioning, and electron donor-acceptor were also studied for the interaction of water contaminants with biochar-based materials. Zheng et al. (2013) found redox reaction between electron donor biochar and electron acceptor sulfamethoxazole. The functional groups like carboxylate and hydroxyl groups present onto the surface nanoadsorbents are responsible for interactions (ion exchange and electrostatic attraction) with inorganic contaminants like metal ions. Incorporation of nanosized material into adsorbent like biochar increases the surface active sites and surface area. For example, a nanocomposite biochar/AlOOH adsorbed arsenic and phosphate ions from the solution more effectively than pristine biochar (Zhang and Gao 2013).

11.3.4 Adsorbent Selection and Regeneration

The selection of a treatment method involves economic, technical, and social considerations (Kumar et al. 2019). In terms of socio-economic aspect, an adsorbent should be cost-effective, readily available, developed from locally available materials, requires least technical expertise and should be easily applicable in large scale. For an adsorbent to be successful and sustainable in technical aspects include high sorption properties, high selectivity, high applicability in a wide range of contaminants concentration, easy applicability in vast ranges of matrices, and high reusability. Technical aspect of adsorbent selection can be evaluated through a variety of experimental assessment (Kumar et al. 2019; Patel et al. 2019; Ateia et al. 2020; Mudhoo et al. 2021). For the better selection of an adsorbent, its physicochemical, morphological, and structural properties as well as contaminants removal capacity must be properly characterized (Patel et al. 2019). Both batch and dynamic sorption studies must be conducted followed by adsorbent regeneration studies. Batch sorption studies help in the optimization of adsorbent properties during its developmental stage (Choudhary et al. 2020; Patel et al. 2021), while dynamic studies simulate the real-world scenarios of the adsorbent applicability. Thus, agricultural residue-derived nanoadsorbent with minimum chemical utilization, low cost, easy preparation and wide applicability would be the adsorbent of choice.

Regeneration of nanoadsorbents leads to the applications such as reusability of nanoadsorbent and recovery of valuable metal species. Regeneration of exhausted adsorbent is essential to revive its adsorption capacity. It involved the stripping off the adsorbed metals or other contaminants so that the adsorbent can be used again and again. Due to this stripped metals can be recycled back to the process of origin, it makes the adsorption method economically viable. In the several regeneration cycles, the adsorption capacity decreased to some extent due to decrease in surface area and active sites. For the recovery of metal ions mainly dilute acids (HNO3, H2SO4, and HCl) are used. Hu et al. (2020) synthesized a cheap adsorbent (Zr@MCS) for phosphate removal by incorporation of nanosized Zr4+ oxide onto amino modified corn straw. Zr@MCS successfully adsorbed the phosphate ions and this loaded Zr@MCS regenerated using 5% NaOH–NaCl solution. After the first regeneration it was found that the removal efficiency decreased from 70% to 65% and after 2–8 runs the efficiency decreased only up to 63%. Further, phosphate was regenerated from phosphate-rich desorption solution by struvite crystallization method and used as phosphate fertilizer in soil. 0.5 M NaOH solution was used for Cr6+ desorption from β-cyclodextrin–chitosan modified biochar by Huang et al. (2016) and it was analyzed that after the five adsorption-desorption cycles the removal capacity decreased up to 56%. Nanobiochar from artichoke leaves was synthesized and modified with NaOH to adsorb antidiabetic drug Metformin hydrochloride (MFH) from the solution (Mahmoud et al. 2020). MFH loaded Artich-Bch-NaOH then regenerated using 0.1 M HCl. For this, first of all Artich-Bch-NaOH-bounded-MFH dried at 70 °C and treated with 0.1 M HCl for 40–45 min, filtered and dried at 70 °C. The regenerated adsorbent was then used about 4–5 times and it was estimated that even after 5 cycles the adsorption capability was in the range 78.5–82.0%. So, for the economic reasons it should be ensured that prepared adsorbent can be used several times.

11.4 Conclusion and Recommendations

Adsorption is an easy to use and economic wastewater treatment unit process. However, the design and development of an effective, cheap and specific adsorbent is a challenge. The conventional adsorbents (for example, clay, lime stone, silica, soil, zeolite, activated charcoal, activated alumina) have certain challenges associated with them like high cost, difficulty in surface modification, regeneration, exhaustion, and low pollutants removal efficiency. Nanoadsorbents prepared using low-cost and abundantly available agricultural residue may help to deal with the above said constraints associated with the conventional adsorbents. Agricultural residue-based nanoadsorbents (silica, cellulose, lignin, biochar) might be ideal candidates being cost-effective, eco-friendly and effective for conventional emerging pollutants removal from (waste)water. These adsorbents must be made more smart through surface functionalization, composite formation and improving their reusability for emerging pollutants removal in aqueous phase. Future studies must focus on exploring mechanistic aspects of wastewater remediation using novel agri-residue-based nanoadsorbent. Agricultural residue-derived nanoadsorbents are expected to have a great future in the wastewater purification sector.