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

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 (AsV) 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 AsV using a range of techniques. X-ray photoelectron spectroscopy results revealed the partial reduction of pentavalent AsV to the more toxic trivalent AsIII form by FeCl3BSBC and Zr-FeCl3BSBC, which was not observed with FeBSBC. The Langmuir maximum AsV 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 AsV removal efficiency in the order PO43- > CO32- > SO42- > Cl- > NO3-, although no significant inhibitory effects were observed with cations like Na+, K+, Mg2+, Ca2+, and Al3+. The positive correlation of AsV 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 AsV up to six-repeated cycles.