Adsorption–Desorption Behavior of Arsenate Using Single and Binary Iron-Modified Biochars: Thermodynamics and Redox Transformation

[Image: see text] Arsenic (As) is a dangerous contaminant in drinking water which displays cogent health risks to humans. Effective clean-up approaches must be developed. However, the knowledge of adsorption–desorption behavior of As on modified biochars is limited. In this study, the adsorption–desorption behavior of arsenate (As(V)) by single iron (Fe) and binary zirconium–iron (Zr–Fe)-modified biosolid biochars (BSBC) was investigated. For this purpose, BSBC was modified using Fe-chips (FeBSBC), Fe-salt (FeCl(3)BSBC), and Zr–Fe-salt (Zr–FeCl(3)BSBC) to determine the adsorption–desorption behavior of As(V) using a range of techniques. X-ray photoelectron spectroscopy results revealed the partial reduction of pentavalent As(V) to the more toxic trivalent As(III) form by FeCl(3)BSBC and Zr–FeCl(3)BSBC, which was not observed with FeBSBC. The Langmuir maximum As(V) adsorption capacities were achieved as 27.4, 29.77, and 67.28 mg/g when treated with FeBSBC (at pH 5), FeCl(3)BSBC (at pH 5), and Zr–FeCl(3)BSBC (at pH 6), respectively, using 2 g/L biochar density and 22 ± 0.5 °C. Co-existing anions reduced the As(V) removal efficiency in the order PO(4)(3–) > CO(3)(2–) > SO(4)(2–) > Cl(–) > NO(3)(–), although no significant inhibitory effects were observed with cations like Na(+), K(+), Mg(2+), Ca(2+), and Al(3+). The positive correlation of As(V) adsorption capacity with temperature demonstrated that the endothermic process and the negative value of Gibbs free energy increased (−14.95 to −12.47 kJ/mol) with increasing temperature (277 to 313 K), indicating spontaneous reactions. Desorption and regeneration showed that recycled Fe-chips, Fe-salt, and Zr–Fe-salt-coated biochars can be utilized for the effective removal of As(V) up to six-repeated cycles.