[Characteristics and Influencing Factors of Monothioarsenate Adsorption on Goethite]

The adsorption kinetic of monothioarsenate (MTA) on goethite was characterized in this study, and batch experiments were then designed to further explore the effects of arsenate, arsenite, humic acid (HA), nitrate, and phosphate on the adsorption of MTA on goethite, and to identify the adsorption mechanism. The results showed that:① When a single arsenic species was present in a solution, the adsorption equilibrium times of MTA, arsenate, and arsenite on goethite were 8, 2, and 4 h, respectively. The adsorption experimental data of these three arsenic species were well fitted to a pseudo-second-order kinetic model. The equilibrium adsorption capacities (q_e) of MTA, arsenate, and arsenite on goethite were 2129.851, 3291.838, and 1788.767 mg·kg^-1, respectively. When MTA coexisted with arsenate or arsenite in a solution, MTA adsorption on goethite continued to be well fitted to a pseudo-second-order kinetic model. The value of q_e for MTA was significantly reduced to 1236.941 mg·kg^-1 when MTA coexisted with arsenate, and to 1532.287 mg·kg^-1 when MTA coexisted with arsenite, due to the fact that arsenate and arsenite competed for adsorption sites with MTA. ② With an increase in HA concentration (10-50 mg·L^-1), the q_e of MTA decreased gradually, due to the fact that a large number of functional groups in HA preempted the surface adsorption sites of goethite with MTA. ③ When phosphate was added into the MTA solution, the q_e values of MTA, arsenate, and arsenite on goethite were reduced greatly, to 492.802, 815.782, and 303.714 mg·kg^-1, respectively, which was caused by the competitive adsorption of P and As. When nitrate was added into the MTA solution, the number of electron receptors and Eh of the solution increased, leading to the q_e values of MTA, arsenate, and arsenite on goethite increasing to 2211.030, 3444.023, and 1835.537 mg·kg^-1, respectively.