Adsorption of arsenic–fluorine in groundwater by Fe/La-modified wheat straw–derived biochar

Abstract

The groundwater in the Kuitun region of Xinjiang is contaminated by a complex mixture of As–F and other contaminants, posing significant challenges for irrigation and other applications. An adsorbent that enables the simultaneous adsorption of arsenic and fluorine (As–F) must be urgently developed. To address these issues, wheat straw–derived biochar (XM) was optimised herein by loading Fe and La for obtaining Fe/La–XM composites. The influence of factors such as As–F initial concentration, solution pH and coexisting ions on As–F adsorption by Fe/La–XM was analysed. Results revealed that Fe/La–XM was mesoporous with excellent surface structure and could rapidly adsorb As and F simultaneously in just 5 h. Its adsorption capacities for As and F were 128 and 33 times higher, respectively, compared with those of original biochar. It achieved a remarkable As–F removal rate of 98 % at a pH of 5.0–10.0. The adsorption model strongly conformed to the Langmuir isotherm model and was consistent with the pseudo-second-order kinetic equation. The adsorption process was governed by intra-particle diffusion, surface adsorption and other mechanisms. During this process, As and F exhibited antagonistic effects and underwent competitive adsorption. Fe/La–XM primarily relied on electrostatic and chemical adsorption processes at a pH of <6.43. At a pH of >6.43, chemical adsorption was dominant. In addition, As exhibited strong tendency to form stable Fe–O–As ligand complexes. Meanwhile, La readily formed high-strength La–F coordination bonds with F, ultimately leading to the adsorption of LaF₃ on the surface of Fe/La–XM. Fe/La–XM also exhibited an ion-exchange effect during F⁻ adsorption, with CO₃²⁻ emerging as the primary competing anion. Even after four adsorption–desorption cycles, Fe/La–XM retained >62 % and >65 % of its removal efficiency for As and F, respectively. Tests on actual groundwater samples yielded exceptional results,The equilibrium concentration of As was lower than the agricultural irrigation water standard (C_As ≤ 0.05 mg/L) and that of F was lower than the drinking water standard (C_F ≤ 1.0 mg/L) after adding only 0.06 g of Fe/La–XM in the groundwater sample. demonstrating the material's potential for practical applications.