The presence of toxic metals in industrial wastewater, marine water, and freshwater has become a global environmental issue (Ajmal et al., 2003; Kargar et al., 2011; Mzoughi and Chouba, 2011; Zou et al., 2019). Heavy metals poses serious threats to streams and lakes (Ghaderi et al., 2012; Alimohammad Kalhori et al., 2011; Ashraf et al., 2011; Hashem et al., 2017; Tripathi et al., 2024). Among the various heavy metals, Cr is often released into water bodies in the form of industrial wastewater. Over the past few decades, large quantities of wastes containing Cr have been directly discharged into the environment without any treatments (Qiang et al., 2011). The toxicity of Cr varies with its oxidation state. In natural water, Cr usually exists in two main oxidation states, namely, trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)) (Sharma et al., 2022). Compared to Cr(III), Cr(VI) is 100–1000 times more toxic to organisms (Roşca et al., 2023; Chanda et al., 2024) because it has strong oxidizing ability and mobility; it is absorbed through the skin and also readily transported into the soil (Karthikeyan et al., 2005; Demirbas et al., 2008).

Cr(VI) is included in the priority list of hazardous substances of the Comprehensive Environmental Response Compensation and Liability Act (CERCLA) (Moussavi and Barikbin, 2010; Peng et al., 2019). Cr(VI) is used in various industries, and the major industrial sources of Cr(VI) in wastewater are tanning, metal processing, electroplating, wood preservative, pigment, paint, steel fabrication, textile, dyeing, and canning industries, as shown in Figure 1 (Demirbas et al., 2008; Moussavi and Barikbin, 2010; Singh et al., 2022). According to the US Environmental Protection Agency, the permissible limits for Cr(VI) are 0.1 and 0.05 mg L⁻¹ in inland surface water and drinking water, respectively (Qiang et al., 2011; Brusseau and Artiola, 2019).

It has been shown that excess intake of Cr by humans results in many diseases (Figure 2), such as damage of the digestive tract, lung cancer, gastrointestinal issues, central nervous system irritation, capillary damage, and hepatic and renal damage (Martins et al., 2006; Kiran et al., 2007; Wan Ngah and Hanafiah, 2008; Boyle et al., 2025). Different methods are available for the treatment of Cr(VI), such as ion exchange (Wang and Lin 2008; Yuan et al., 2008; Kim et al., 2008; Li et al., 2017), ultrafiltration (Muthumareeswaran et al., 2017), nanofiltration, microfiltration (Chen et al., 2010; Zolfaghari and Kargar, 2019), chemical precipitation (Peng et al., 2018), electrochemical reduction (Peng et al., 2019), solvent extraction, reverse osmosis (Çimen, 2015), cementation, electrodialysis, electrocoagulation (Muhammad et al., 2006; Ahmet and Tuzen, 2008; Tarun et al., 2008; Cristina et al., 2009; Dang et al., 2009; Sibel et al., 2009; Zainul et al., 2009; Suresh et al., 2010), and adsorption (Monser and Adhoum, 2002; Erdem et al., 2004; Cherdchoo et al., 2019; Sirusbakht et al., 2018; Haroon et al., 2017). These methods are effective for concentrations of 1–100 mg L⁻¹; however, they involve high operating and capital costs (Liang et al., 2009). The waste generated after chemical precipitation also pose disposal pollution risks and hazards to the environment. Hence, efficient and cost-effective alternative methods need to be developed for scale-up applications (Oilgae, 2010).

Metal removal using adsorbents has distinct advantages over other conventional methods. Adsorbents are usually inexpensive, efficient (Sheraz et al., 2024), and prone to producing lower quantities of sludge after treatment (Sud et al., 2008; Satyam and Patra, 2024). Moreover, another advantage of adsorbent-based methods is that the statutory discharge limits for industrial wastewater, including enhancement of their environmental responsibility and corporate social profiles, are relatively easier to meet. Various adsorbents can be used for metal removal, such as those based on nanoparticles (Rong et al., 2024; Wenjie et al., 2022; Tesnim et al., 2024), nanocomposites (Katiyar and Katiyar, 2024), chitosan-based composites (Zhou et al., 2025), CeO₂-based functional materials (Zhang et al., 2025), and deep-eutectic-solvent-assisted functional materials (Nie et al., 2025). Many researchers have used agriculture feedstock for the treatment of heavy metal pollutants (Odoemelam et al., 2011; Gupta et al., 2009; Subbaiah et al., 2011; Opeolu, 2009), in addition to abundantly available organic materials. Agricultural or lignocellulosic products (Figure 3) have unique chemical compositions (Lee et al., 2014; Baruah et al., 2018) and relatively high numbers of surface functional groups (i.e., phenolic and carboxylic groups) that enable successful uptake of metals from wastes and are also useful for removing Cr(VI) from wastewater (Harmon et al., 2007; Opeolu et al., 2010). Chemical binding sites comprising carboxyl (COO), carbonyl (CO), hydroxyl (OH), amine (NH₂), or amide (RC (O) NH₂) groups are mostly present on biomass surfaces (Barriada et al., 2009). Polar-functional-group-containing compounds are also available in agricultural byproducts, such as alcohols (R-OH), aldehydes (HCOH), ketones (RCOR), carboxylates (RCOO), phenols (C₆H₅-OH), and ethers (R-O-R); these bind heavy metals through hydrogen replacement or complex formation by electron pair donation (Ofomaja and Ho, 2007). Different activated and non-activated bioadsorbents, including pine wood sawdust (Sirusbakht et al., 2018), Indian jujube (Ahmad et al., 2017), coffee pulp (Gómez Aguilar et al., 2019), coffee grounds (Cherdchoo et al., 2019), almond green hulls (Nasseh et al., 2016), groundnut shells (Bayuo et al., 2019), palm kernel shells (Razavi Mehr et al., 2019), Arachis hypogea (peanut) leaves (Ahmad et al., 2017), green moringa tea leaves (Timbo et al., 2017), Cassia fistula (Amaltas) leaves (Ahmad et al., 2017), Euclea schimperi leaves (Gebrehawaria et al., 2015), Acacia albida bark (Gebrehawaria et al., 2015), Eucalyptus camaldulensis sawdust (Haroon et al., 2016), sugarcane bagasse (Homagaia et al., 2010), white cedar sawdust (Haroon et al., 2017), and pea seed shells (Kebede et al., 2022), have been used as economical alternatives to expensive adsorbents.

Several agricultural byproducts and natural materials have been explored for Cr(VI) adsorption, but a critical gap remains in systematically comparing their effectiveness and evaluating the impacts of chemical modifications. The present review addresses this gap by offering detailed comparisons of a wide range of unmodified and chemically modified natural adsorbents for Cr(VI) removal. As a pioneering effort, we compile and analyze the Cr(VI) removal potentials of various natural biomasses in this study, including leaves, bark, shells, and agricultural residues, as well as rank them based on performance. Moreover, this review uniquely emphasizes the roles of chemical modifications, including acids, bases, and salts, in enhancing the adsorption capacities of various natural organic (lignocellulosic) materials. In highlighting these novel features, we not only combine various sources available in literature but also propose a framework for future directions for developing cost-effective, scalable, and sustainable treatment technologies using natural organic adsorbents.

Pal and Kaur (2013) investigated the effects of different particle sizes (0.3–1.0 mm) of physically activated sawdust of Dalbergia sissoo (ASD) for Cr(VI) adsorption in a batch experiment. They noted that the Cr(VI) removal improved from 66% to 85.4% with increase in adsorbent amount from 0.2 to 1.2 g per 100 mL for ASD particles of size 0.3 mm. Moreover, improved adsorption from 61% to 81.2% was observed with a particle size of 1.0 mm. However, when the metal concentration was increased over 5–50 mg L⁻¹, the efficiency of Cr removal reduced from 87% to 73% for ASD of 0.3 mm size and from 83% to 70% for ASD of 1.0 mm size. Maximum adsorption was achieved at a pH of 2.0, at which point the adsorptive surface became more positively charged and showed greater attraction to anionic species of Cr(VI). At high pH, lower adsorption was observed owing to reduced attractive forces between the anionic species of Cr and adsorbent surface. Equilibrium was established within 60 min, and there was no further adsorption thereafter because of non-availability of vacant sites. The data were best fitted to the Freundlich and Langmuir isotherms (Pal and Kaur, 2013). Physically activated Ziziphus spina-christi leaves showed a maximum Cr(VI) removal efficiency of 97.22% at optimum time, pH, initial metal concentration, adsorbent dose, and temperature conditions. The adsorption data were fitted well to the Langmuir isotherm, reflecting a maximum adsorption capacity of 13.81 mg g⁻¹. The thermodynamic parameters also revealed the endothermic nature of Cr(VI) adsorption (Abshirini et al., 2018). Physically AC of rice straw showed a maximum Cr(VI) uptake of 90% at pH = 8. At optimum time, pH, and temperature, the rice straw AC displayed a maximum adsorption capacity of 3.5 mg g⁻¹ (Kumar et al., 2017). Teak wood sawdust modified using HCl was also shown to enable maximum Cr(VI) adsorption (Jha, 2016).

H₂SO₄-activated cashew nut was used for Cr(VI) removal in a column study, which showed the best results at a bed height of 10 cm (Yahya et al., 2020). HCl-activated M. indica leaves showed a maximum adsorption capacity of 78.96 mg g⁻¹ (Pathania et al., 2020). Vikal et al. (2013) investigated activated Ziziphus jujuba leaf powder (local name Bordi), which showed maximum Cr(VI) removal at an adsorbent dose of 15 g within 105 min. Physical activation of the adsorbent was performed at 80°C for 48 h, and FTIR spectroscopy confirmed that the hydroxyl, aliphatic alkane, and O-C stretching of the ether groups were responsible for adsorption. The experimental data were fitted well to both the Langmuir and Freundlich models (Vikal et al., 2013). Sawdust modified with diethylenetriamine also revealed 75% adsorption of Cr(VI) at an optimum adsorbent dosage of 2 g L⁻¹ (Elhami and Bahadori, 2015). Chemically (phosphoric acid) activated ECS also showed 87% removal of Cr(VI) at a pH of 3 (Haroon et al., 2020). Spruce sawdust treated with diethylene glycol and sulfuric acid revealed a maximum adsorption capacity of 318.3 mg g⁻¹ for Cr(VI) (Politi and Sidiras, 2020). Teakwood sawdust modified with zinc chloride showed an adsorption capacity of 72.46 mg g⁻¹ for Cr(VI) (Ramirez et al., 2020). Leucaena leucocephala activated with orthophosphoric acid showed a maximum Cr(VI) uptake of 13.85 mg g⁻¹ at pH = 4; its PZC was revealed to be 5.42 (Malwade et al., 2016). Chemically activated almond shell (with 40% H₃PO₄) was also found to have a high adsorption capacity of 202.34 mg g⁻¹ under optimum batch experiments; the BET surface area of the activated almond shells was reported as 1,223.4 m² g⁻¹ (Rai et al., 2018). Nut shells activated with ZnCl₂ were found to have a BET surface area of 2,869 m² g⁻¹, and the maximum adsorption was observed at pH = 2; the experimental data were fitted well to the Langmuir model with an adsorption capacity of 46.21 mg g⁻¹ (Kumar and Jena, 2017). The adsorption potentials of acid-activated Juniperus procera leaves for Pb(II) and Cr(VI) were investigated in a batch experiment, which showed maximum removal efficiencies of 98% and 96% at optimum pH values of 4.6 and 4, respectively. The activated J. procera leaves could be reused up to three times, and the data were best fitted to pseudo-first-order kinetics (Ali et al., 2019). Corn cob is an abundant agricultural waste that is mostly burned or discarded. Corn cob activated with a mixture of ZnCl₂ and NH₄Cl solutions was explored for Cr(VI) adsorption in a batch experiment, which revealed a BET surface area of 924.9 m² g⁻¹; however, the iodine adsorption value of corn cob AC was 1,188 mg g⁻¹, and a lower/acidic pH was found to enhance adsorption owing to protonation of the adsorbent surface (Tang et al., 2016).

Sulfuric-acid-activated rice husk showed maximum removal of the adsorbate at a contact time of 120 min and pH of 2. Low desorption results, i.e., 0.1%–9%, revealed strong bonding between the adsorbent and adsorbate as well as the chemisorption nature of the activated adsorbent; the BET surface area was reported to be 58.54 m² g⁻¹ (Khan et al., 2016). Sodium-hydroxide-activated carbon prepared from L. leucocephala seed pod showed a Langmuir adsorption capacity of 26.94 mg g⁻¹ under optimum conditions. The data were best fitted to pseudo-second-order kinetics, indicating chemical adsorption. Furthermore, the desorption efficiency decreased from 63.42% to 47.56% from the first to third cycles as the metal uptake capacity decreased with adsorbent reuse (Yusuff, 2019). Cicer arietinum (chickpea) husk was chemically activated using KOH and K₂CO₃ and tested for removal of Pb, Cu, and hexavalent Cr; the results showed better porosity formation upon impregnation with KOH (50 wt%), with a BET surface area and total pore volume of 2,082 m² g⁻¹ and 1.07 cm³ g⁻¹, respectively. The maximum adsorption capacities were in the order of Pb(II) > Cr(VI) > Cu(II), i.e., corresponding to 135.8, 59.6, and 56.2 mg g⁻¹, and the thermodynamic parameters indicate an endothermic adsorption process (Özsin et al., 2019). Great millet husk was also shown to have an adsorption capacity of 22.21 mg g⁻¹ for Cr(VI) removal after chemical modification with sulfuric acid (Prajapati et al., 2022); the metal uptake potential of sulfuric-acid-activated neem bark was tested in a column experiment for hexavalent Cr. When the adsorbent mass was increased from 25 to 175 g at a constant metal ion concentration and flow rate, the breakthrough time was noted to increase from 9.25 to 111.66 h. Activated neem adsorbent has been shown to have excellent effects on a treated mixture of metals (hexavalent Cr, Cu, and Zn) from wastewater; it was revealed that 40% Cr(VI) could be recovered by shaking the exhausted adsorbent with distilled water at 90°C for 2 h, and the data were best fitted to the Yoon–Nelson kinetic model (Maheshwari and Gupta, 2016). Lantana camara plant activated with nitric acid displayed a maximum adsorption capacity of 26.25 mg g⁻¹ at optimum time, initial concentration, temperature, and pH conditions; the adsorption of Cr(VI) was tested in the presence of interference ions to assess the utility of the activated adsorbent in real industrial effluent applications, and it was observed that the adsorption of Cr(VI) was slightly affected by tenfold excess of co-anions like NO₃ ⁻, Cl⁻, and CO₃ ^2–; however, very little effect was noted in the case of SO₄ ^2– and PO₄ ^3– (89.0%), while no interference was observed in the case of co-cations like Zn⁺² (Ravulapalli and Kunta, 2018).

Sulfuric-acid-modified holly sawdust was studied for Cr(VI) treatment in a batch experiment (Siboni et al., 2011); it was observed that the percentage removal of Cr(VI) increased from 34.65% to 99.99% with increasing adsorbent amounts from 2 to 10 g L^–1. Increasing the initial concentration of the metal ions in the range of 20–100 mg L^–1 reduced the efficiency from 99.37% to 40.24%. Moreover, the Cr(VI) removal efficiency decreased from 99.67% to 29.78% with increase in pH from 2 to 12. At low or acidic pH, the adsorbent surface acquires positive charges and Cr(VI) generally exists in the form of HCrO₄ ⁻. However, the HCrO₄ ⁻ changes to CrO₄ ²⁻ and Cr₂O₇ ²⁻ at high pH, where the competition between CrO₄ ²⁻ and OH⁻ anions also decreases. Equilibrium was attained within 90 min, and the Langmuir model was observed to define the experimental results better than the Freundlich isotherm, with the adsorbent following pseudo-second-order kinetics. The maximum uptake was reported as 18.86 mg g⁻¹ at a high pH of 7.0. Two different forms of sawdust, namely, powder sawdust activated with acid (ASDP) and bead-form sawdust with surface modification (SDCCB), were investigated for the treatment of Cr(VI) in a batch study (Begum and Alhaji, 2013). Here, oxidizing agents like H₂SO₄, HNO₃, and H₂O₂ were successively used for chemical activation of the sawdust. The highest Cr(VI) uptake capacities for ASDP and SDCCB were 45.5 and 125 mg g⁻¹, respectively. The maximum contact times were approximately 120 min for the ASDP and 150 min for the SDCCB. The order of isotherm fitness was Freundlich > Langmuir > Temkin (based on R ² values), while pseudo-second-order kinetics was found to best fit the data. It was concluded through the thermodynamic parameters that the adsorption was endothermic, spontaneous, and non-specific chemisorption for both adsorbents (Begum and Alhaji, 2013). The Cr(VI) removal efficiencies of boiled sawdust, formaldehyde-treated sawdust, and sawdust carbon were examined in a batch reactor (Subhan and Pardeep, 2011), which showed that the adsorption process was favored at low a pH of 2.0 when using biomass doses of approximately 4 g L⁻¹ and 25°C temperature. The percentage removal efficiency of Cr(VI) was in the following order: sawdust carbon > formaldehyde-treated sawdust > boiled sawdust.

Altun and Pehlivan (2012) investigated the efficiency of chemically modified walnut shell (WNS) (Juglans regia) for removal of Cr(VI) from water in a batch experiment; here, approximately 10 g of WNS was chemically modified with 10 g of citric acid per 50 mL of water and heated for 4 h at 120°C, which showed that functional groups like alcohols, carbonyls, carboxyls, and phenols were associated with Cr(VI) adsorption. The equilibrium was attained at 120 min and the optimum pH range was between 2 and 3. The adsorption increased directly with temperature because of more chemical interactions between the adsorbent and adsorbate. The maximum Cr(VI) uptake capacities were 0.596 and 0.154 mmol g⁻¹ for CA-WNS and untreated WNS, respectively. The process of adsorption was endothermic and spontaneous (Altun and Pehlivan, 2012). Demirbas et al. (2004) revealed the Cr(VI) removal potential of sulfuric-acid-modified cornelian cherry, apricot, and almond shell from solution. Various environmental parameters like the particle size (0.63–1.60 mm), pH (1–4), and initial Cr(VI) concentration (20–300 mg L⁻¹) were optimized in a batch experiment, and the adsorption process was dependent on pH. The equilibrium contact time was 72 h, and the optimum pH was 1. The experimental data were fitted better to second-order kinetics than first-order kinetics in terms of the diffusion model (Demirbas et al., 2004). Attia et al. (2010) conducted a comparative study on sulfuric-acid-modified olive stones and AC to assess their removal potentials for solutions containing various Cr(VI) concentrations (4–50 mg L⁻¹) in a batch experiment. The highest removal was reported at the lowest pH of 1.5, and the FTIR spectrum revealed that the negatively charged groups were neutralized to form binding sites for Cr. The adsorption data were suitably fitted to the first-order kinetic and Langmuir models. The AC from olive stones was more efficient than acid-treated commercial AC for Cr(VI) removal (Attia et al., 2010).

Swietenia mahagoni shell modified by sulfuric acid and orthophosphoric acid was found to be an excellent adsorbent for the treatment of Cr(VI) in a column. The Cr(VI) adsorption capacities of sulfuric-acid- and orthophosphoric-acid-modified S. mahagoni shells from the BDST model were found to be 1,989.4 mg L⁻¹ and 2,785.2 mg L⁻¹, respectively (Rangabhashiyam et al., 2016). Mane (2010) revealed the potentials of chemically (sulfuric acid) activated tendu (Diospyros melanoxylon) leaf refuse (TLR) and commercial activated carbon (CA-CAC) for the treatment of Cr(VI) from solution. The optimum pH was reported as 2, at which point neutralization of negative charges occurred on the adsorbent surface due to excess hydrogen ions, resulting in diffusion or adsorption of the hydrogen chromate ions (HCrO₄ ⁻). At pH between 1.0 and 4.0, hydrogen chromate ions are the dominant form of Cr(VI). Another reason for maximum removal at low/acidic pH is the oxidation of Cr₂O₇ ^2– to Cr³⁺, which is easily replaced by the positive species owing to its small size. The equilibrium time attained for CA-CAC was 120 min and that for CA-TLR was 30 min. Thus, it was found that CA-TLR is more efficient for Cr(VI) removal than CA-CAC (Mane, 2010). Pandhram and Nimbalkar (2013) explored the efficiency of low-cost activated neem leaves for Cr(VI) removal from industrial effluents; here, the adsorbent was physically activated for 3 h in a furnace at 250°C. In a batch experiment at a pH of 4, the maximum removal observed was 67.5%. At a solution concentration of 30 mg/100 mL, the maximum percentage removal efficiency of Cr(VI) was 98%. At an adsorbent dose of 8 mg/100 mL, the removal efficiency was observed to be maximum at 85% (Pandhram and Nimbalkar, 2013).

The Cr(VI) percentage removal efficiencies of boiled rice husk, formaldehyde-treated rice husk, and rice husk carbon from wastewater were examined in a batch study. The adsorption process was favored at pH = 2 when using an adsorbent dose of 4 g L⁻¹ at 25°C. The Cr(VI) removal efficiency was in the order of rice husk carbon > formaldehyde-treated rice husk > boiled rice husk (Subhan and Pardeep, 2011). Ahmed et al. (2012) investigated physically activated rice husk (at 700°C for 1 h) for sequestering Cr from a synthetic aqueous solution using particles of sizes 50 and 100 mesh (150 µm) in batch experiments. The maximum adsorption of 95.2% was attained at an optimum time of 150 min, a Cr(VI) concentration of 20 mg L⁻¹, a low pH of 2, and an adsorbent dose of 5 g L⁻¹ (Ahmed et al., 2012). Strychnos potatorum seeds were modified using sulfuric acid to investigate their efficiency of Cr(VI) removal in both batch and column reactors; here, the maximum Langmuir uptake capacity was reported as 202.7 mg g⁻¹, and the column data best fitted the Thomas model (Anbalagan et al., 2016).

Grafted peels of banana displayed 96% Cr(VI) removal at optimum pH and dose of 3 and 4 g L⁻¹, respectively (Ali et al., 2016). Pomegranate seeds were also used for Cr(VI) removal (Ghaneian et al., 2017). At an acidic pH of 2, tea husk showed a maximum Cr(VI) removal of 98.5% (Saed, 2016). Sulfuric-acid-activated neem bark was assessed in a column study for Cr(VI) removal, and the result showed increase in breakthrough time from 9.25 to 111.66 h with increase in adsorbent dose (25–175 g). However, the breakthrough time is achieved earlier in multisolutes containing Cu and Zn. The experimental data were explained well by the Yoon–Nelson model. The saturation loading capacity of the column was reported to be 27.5 mg g^-1, which is very close to the Cr(VI) uptake capacity of 26.95 mg g^-1 in the batch study (Maheshwari and Gupta, 2016). Stems of the plant Ocimum tenuiflorum were activated using sulfuric acid and investigated for Cr(VI) removal in a batch system, where the maximum uptake capacity was found to be 95.9 mg g^-1. The adsorption mechanism was also evaluated, which showed interparticle diffusion at higher Cr(VI) concentrations and film diffusion at lower Cr(VI) concentrations (Suganya et al., 2017). Sulfuric-acid-modified S. platensis with a BET surface area of 765.1 m² g^–1 was proven to be a good adsorbent and remove up to 55.252% of Cr(VI) even in the presence of sulfate ions; the used adsorbent could be recovered up to 94.48% with sodium hydroxide (Gunasundari and Kumar, 2016).

Previous reports in literature have revealed the following order for average Cr(VI) removal (Figure 7) when using different biomasses without any modifications or activation: leaves > bark > agricultural waste > dry shells > tea = fungi > yeast > algae > sawdust > bacteria. The adsorption data of different types of leaves are best fitted to both the Langmuir and Freundlich isotherm models, reflecting single-layer physisorption as well as multilayer chemisorption. Leaf modifications do not increase the maximum adsorption efficiency, so it is economical to use leaves that are available in bulk refuse without prior treatment with chemicals in various industries, especially in developing countries. Studies also show that adsorption depends on the solution pH, functional groups on the adsorbent surface, particle size of the adsorbent, and metal-ion concentration.

The physical and/or chemical modifications enhanced Cr(VI) removal with almost all natural lignocellulosic (organic) adsorbents. The major mechanisms driving metal-ion adsorption are electrostatic interactions, surface complexation, and ion exchange. Modification of the adsorbents with acids produced the best results for Cr(VI) removal, followed by modifications with salts and bases (Figure 8). Modification with an acid results in positive charges on the adsorbent surface (Abegunde et al., 2020). Modification of agricultural or organic materials with acids through the wet oxidation process causes dissolution of the constituents or removal of mineral impurities along with an increase in oxygen content and a decrease in pore tortuosity. Modification of organic adsorbents with various acids increases the reactivity of the functional groups and different surface properties of the adsorbents, such as the surface area and porosity, thereby increasing adsorption. Acid modification can also increase the hydrolysis of cellulose, which results in a more reactive adsorbent compared to the untreated adsorbent (Liu et al., 2020). Modification of adsorbents with acids also reduces the mineral content of the modified adsorbent, which in turn results in increased percentage removal of Cr(VI) from water because of the decrease in competition between the active sites of the adsorbent and cations. Sulfuric acid is the most widely used substance for modification and increases the removal efficiency by 80%–100% in all the reviewed adsorbents. This is because of the presence of -SO₃ groups on the adsorbent that increases the percentage removal of Cr(VI) through covalent bonding along with increases in the surface area and porosity of the adsorbent (Goswami and Phukan, 2017). Phosphoric-acid- and tartaric-acid-based modifications are also reported in literature, and these enhance the porosities and surface areas of natural adsorbents. Modifications of adsorbents with sulfuric acid, phosphoric acid, and nitric acid increase both the surface areas and surface functional groups (Lesaoana et al., 2019). Modification with phosphoric acid has been reported to form different phosphorus groups on the adsorbent surface (PO and POOH), which then react with Cr(VI) and result in the formation of metal complexes (Kainth et al., 2024). However, in the case of modification using nitric acid, there is enhancement of the nitrate groups along with increased pore volume on the adsorbent surface, which results in more adsorption of Cr(VI). Sodium hydroxide as a modification source for biomass showed poor results because of fewer positive charges, as in the case of acid modification. The optimum pH was found to be 2 in most of the studies with both raw and modified adsorbents. In the case of treated biomass, the Langmuir model was found to be the best fit most often, indicating the availability of more surface sites for Cr(VI) removal. It is important to note that the different chemical modifications summarized in Figure 8 are generally applicable for all lignocellulosic (organic) biomasses because of their similar structural compositions; further, the best modification (chemical) may differ based on specific biomass properties along with the experimental conditions noted in the various studies.

In the box-and-whisker plots shown in Figures 7 and 8, the rectangles (boxes) represent the interquartile range (IQR; range between the 25th and 75th percentiles) while the lines (whiskers) represent the spread of the data, which extend to the minimum and maximum values (excluding outliers). The central line within each box indicates the median value.

In most of the reported works, researchers have focused on batch experimentation; however, it is important to work on column designs using natural adsorbents for field applications. Hence, further exploration is needed for the development of novel and efficient natural adsorbents from natural materials, such as functionalized adsorbents, nanocomposites, engineered biochars, and hybrid materials. More studies are needed on the treatment of real industrial wastewater and effluents containing multiple pollutants as there is less available work on this area given that most of the literature is on synthetic solutions of single adsorbates. In most cases, the Freundlich and Langmuir isotherms were used for the analyses while there are very few reported cases of using other models like the Elovich, D-R, and Temkin isotherms; this area also needs to be explored further for better understanding of the nature of adsorption processes. Exploration of the microscopic mechanisms of adsorption is also needed using advanced techniques like X-ray photoelectron spectroscopy and density functional theory simulations. The development of cost-effective methods to produce AC from locally available biomasses is the most desired need for field applications. Hence, more research efforts are required for optimizing methods for economical development of AC from various available waste biomasses. Mixtures or compositions with varying ratios of the activated and non-activated adsorbents should also be evaluated for pollutant removal. Low regeneration has been observed with most of the adsorbents; therefore, further desorption/regeneration processes should be studied using different variables like pH, temperature, rotation speed, chemicals, etc. for renewal of exhausted adsorbents. Another area for future development is the exploration of hybrid or synergetic treatment approaches in which adsorption is combined with other technologies like biological treatments, membranes, electrochemical reduction, photocatalysis, and advanced oxidation processes. There is also a need to carry out lifecycle assessments of the Cr adsorption processes. Lastly, we note the importance of conducting cost-benefit analyses to better understand the practical applications of the adsorbents.

The bulk of organic biomass is often treated as waste and refuse all around the world. The present review on these waste biomasses reveals their effectiveness as low-cost adsorbents for the removal of Cr(VI) from wastewater. Our survey indicates the following order of utility of various biomasses for Cr(VI) removal: leaves > bark > agricultural waste > dry shells > tea = fungi > yeast > algae > sawdust > bacteria. Among the various types of bulk biomasses available, leaves have been proven to be the most efficient and economical sources for Cr(VI) removal that do not require any prior treatment. The optimum pH for the treatment was found to be 2 for both raw and modified adsorbents. The major mechanisms driving metal ion adsorption are electrostatic interactions, surface complexation, and ion exchange. Both the Langmuir and Freundlich isotherms show best fits to the adsorption data in most cases, which is indicative of both physisorption and chemisorption mechanisms. Previous literature also indicates that the adsorption depends on the solution pH, surface functional groups on the adsorbent, particle size of the adsorbent, and metal ion concentration in the solution. Lastly, we recommend that the commercial application of these adsorbents be enforced and a standardized practice be established for the treatment of wastewater at the industrial level.