Engineered Char Materials derived from Organic Waste and their Application to Remediate Water and Soil Polluted with Anionic Contaminants

Abstract

The anthropogenic and natural processes play a significant role in the massive accumulation of anionic contaminants in the environment, posing serious health effects on living organisms. Among these anionic contaminants, fluoride (F), antimony (Sb), and arsenic (As) are the most problematic because of difficulty in removal from water and soil by conventional treatment methods. Drinking F−-contaminated water (>1.5 mg L−1) causes severe dental and skeletal disorders. Antimony (Sb–V), a carcinogenic metalloid, is becoming prevalent in water and soil due to anthropogenic activities. Similarly, consuming even extremely small amounts of As(V) (10–50 µg L−1) can result in severe health disorders. Conventional remediation technique of adsorption using carbonaceous adsorbents for the removal and immobilization of anionic contaminants in water and soil, respectively, is ineffective and needs engineering to enhance adsorption efficiency. In this study, the potential of pinecone-derived biochar and its engineered biochars obtained via impregnation with aluminium and iron salts was evaluated for the adsorption and immobilization of F-, Sb(V), and As(V) in water and soil. Batch mode adsorption experiments were carried out under variable conditions of solution pH, adsorbent dose, and contact time. The engineered biochars resulted in greater adsorption than that of pristine biochar both in spiked and real water. Specifically, the AlCl3-modified biochar (A-BC) exhibited a maximum adsorption capacity of 14.07 mg g−1 in F−-contaminated water. The equilibrium isothermal and kinetic models predicted monolayer, cooperative, and chemisorption types of the F− adsorption. The chemical interaction and outer-sphere complexation of F− with Al, Na, and OH- elements were further confirmed by the post-adsorption analysis of the AlCl3-modified biochar by FTIR and XRD. The AlCl3-modified biochar resulted in 87.13% removal of F− from the in-situ F−-contaminated groundwater, even in the presence of naturally occurring competing ions (such as Cl−, HCO3−, SO42−, and NO3−). Similarly, the A-BC successfully immobilized the F− in a contaminated paddy soil as indicated from the water-soluble, exchangeable, PBET and TCLP-extraction results. Furthermore, sequential extraction and elemental dot mapping confirmed the immobilization of F− in soil by increase in the residual fraction and formation of stable F− complex, respectively. Considering Sb(V), adsorption tests in water revealed that magnetic biochar (M-BC) showed higher adsorption capacity (8.68 mg g−1) than that of the pristine biochar (P-BC) (2.49 mg g−1) XVI and A-BC (3.40 mg g−1). Isotherm and kinetic modeling of the adsorption data suggested that the adsorption process varied from monolayer to multilayer, with chemisorption as the dominant interaction mechanism between Sb(V) and the biochars. The post-adsorption study of M-BC by FTIR and XRD further supported the chemical bonding and outer-sphere complexation of Sb(V) with Fe, N–H, O–H, C–O, and C≡C components. The P-BC and M-BC also successfully immobilized Sb(V) in a shooting range soil, more so in the latter. Subsequent sequential extractions and post-analysis by SEM, EDX and elemental dot mapping revealed that Sb in the treated soil transformed to a more stable form. Similarly, As(V)-contaminated water and soil were remediated with P-BC and iron modified biochar (FES), respectively. According to adsorption studies, FES exhibited higher adsorption capacity (12.14 mg g-1) than P-BC in water. The adsorption data of isotherm and kinetic modelling revealed that the mechanism varied from monolayer to multilayer, with chemisorption operating as the primary type of interaction between As(V) and the biochars. Furthermore, post-adsorption analysis of FES through FTIR and XRD revealed that chemical bonding and complexation between As and Fe, O, N-H, and O-H are the main reasons for As(V) adsorption onto biochars. Recycling of FES showed strong adsorption of As(V) even after repeating several regeneration cycles in an aqueous solution. The P-BC and FES successfully immobilized As in the paddy soil too. Sequential extraction results indicated transformation of bioavailable As to a more stable residual form, which was further confirmed by post-analysis through SEM, EDX, and elemental dot mapping. The study concluded that engineering the pristine biochar (derived from pinecone) with simple inorganic salts significantly improved the adsorption potential for F−, Sb(V), and As(V) removal from spiked and real water, and also resulted in enhanced immobilization of these anionic contaminants in real contaminated soils. Specifically, the aluminium and iron modified biochars outperformed the pristine biochar, and could be applied as potential adsorbents for the remediation of anionic contaminated water and soil.