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

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 (FeCl₃BSBC), and Zr-Fe-salt (Zr-FeCl₃BSBC) 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 FeCl₃BSBC and Zr-FeCl₃BSBC, 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), FeCl₃BSBC (at pH 5), and Zr-FeCl₃BSBC (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 PO₄ ^3- > CO₃ ^2- > SO₄ ^2- > Cl^- > NO₃ ^-, although no significant inhibitory effects were observed with cations like Na⁺, K⁺, Mg²⁺, Ca²⁺, and Al³⁺. 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.