[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 (qe) 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 qe 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 qe 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 qe 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 qe values of MTA, arsenate, and arsenite on goethite increasing to 2211.030, 3444.023, and 1835.537 mg·kg-1, respectively.

[Characteristics and Mechanism of Monothioarsenate Adsorption on Sand, Sediment, and Goethite].

Batch experiments were conducted to investigate the adsorption characteristics and mechanism of monothioarsenate (MTA) (>99%) on sand, soil sediment, and goethite under different pH and solid-liquid ratio conditions. Results showed the following. ① When MTA ranged from 0.14 to 23.59, 0.19 to 41.27, and 0.27 to 32.02 mg·L-1 in solutions, its maximum equilibrium adsorption capacity (Qm) in sand, soil sediment, and goethite was 21.54, 277.98, and 2607.42 mg·kg-1, respectively. After its adsorption reached equilibrium, a small amount of the MTA in the solutions transformed into arsenite and arsenate. ② As pH increased from 4 to 10, the equilibrium adsorption capacity (Qe) of MTA on sand decreased gradually, whereas Qe first decreased and then increased for soil sediment and goethite. As the solid-liquid ratio increased, the Qe of MTA in the three media gradually decreased. ③ X-ray powder diffraction (XRD), scanning electron microscope (SEM), and BET results further showed that the major factors controlling MTA adsorption on the three media included the low pore volume of sand, the high degree of crystallization of the soil sediment, and the large number of hydroxyl functional groups (-OH) on goethite.