Adsorption and desorption of phenylarsonic acid compounds on metal oxide and hydroxide, and clay minerals

Adsorption and desorption of p-arsanilic acid (p-ASA) and roxarsone (ROX) on six soil minerals, including hematite (α-Fe2O3), goethite (α-FeOOH), ferrihydrite (Fe(OH)3), aluminum oxide (α-Al2O3), manganese oxide (γ-MnO2), and kaolinite, were studied, and the impact of solution matrices on their adsorption was systematically evaluated. Adsorption of p-ASA/ROX on the metal (hydro)oxide and clay minerals occurred quickly (mostly within 2 h), and could be well described by the pseudo second-order kinetic model. The apparent maximum adsorption capacities of α-Fe2O3, α-FeOOH, Fe(OH)3, α-Al2O3, γ-MnO2, and kaolinite (at an initial pH of 7.0) for p-ASA were 1.7, 0.9, 2.5, 0.08, 1.1, and 0.02 μmol/m2, while those for ROX were 1.6, 0.7, 2.4, 0.1, 0.5, and 0.05 μmol/m2, respectively. Besides adsorbing p-ASA/ROX, γ-MnO2 also caused their oxidation. Experimental results suggest that formation of inner-sphere complexes through the arsonic acid group is the primary mechanism for adsorption of p-ASA/ROX on iron (hydro)oxides and γ-MnO2, while outer-sphere complexation plays a critical role in their adsorption on α-Al2O3 and kaolinite. Adsorption of p-ASA/ROX on the metal (hydro)oxide and clay minerals was affected by solution pH, co-existing metal ions (Ca2+, Mg2+, Al3+, Cu2+, Fe3+, and Zn2+), oxyanions (H2PO4−, HCO3−, and SO42−), and humic acid. The solid-to-liquid partition coefficients of p-ASA during the desorption from α-Fe2O3, α-FeOOH, Fe(OH)3, α-Al2O3, γ-MnO2, and kaolinite were 0.47, 2.69, 4.38, 0.03, 30.4, and 0.1 L/g, while those of ROX were 0.28, 1.68, 3.48, 0.02, 4.0, and 0.02 L/g, respectively. Agricultural soils with lower contents of organic carbon exhibited higher adsorption capacities towards p-ASA/ROX, which indicates that soil minerals play a key role in the adsorption of phenylarsonic acid compounds while organic matter could have strong inhibitory effect. These findings could help better understand and predict the transport and fate of p-ASA/ROX in surface soils with low contents of organic matter. [Display omitted] •Adsorption of p-ASA/ROX could be described by the pseudo second-order kinetic model•Metal (hydro)oxides had higher adsorption affinity towards p-ASA/ROX than kaolinite•p-ASA/ROX adsorbed on iron (hydro)oxides and γ-MnO2 through inner-sphere complexation•Outer-sphere complexes formed during adsorption of p-ASA/ROX on α-Al2O3 and kaolinite•Agricultural soils with lower contents of organic carbon could adsorb more p-ASA/ROX