Study On Arsenate Adsorption-desorption By Different Minerals And Soils

Adsorption and desorption are major processes to affect arsenic mobility and transformation in soils. Arsenic in soils can form outer- or inner-sphere complexes by electrostatic adsorption and/or specific adsorption with clay minerals and metal oxides/hydroxides, which is influenced by mineral types, pH, temperature and so on. In this paper, kaolinite, montmorillonite, and synthetic iron and aluminum oxides/hydroxides were collected to investigate arsenate adsorption-desorption as a function of pH, ionic strength, phosphate, and organic matter. Furthermore, seven Chinese soils with different physic-chemical properties, including black soil, red soil developed from the Quaternary red clay soil (red soil-clay), red soil developed from granite (red soil-granite), red soil developed from purple sandy shale (red soil-purple), yellow soil, purple soil, and fluvo-aquic soil, were studied as arsenate adsorbents to determine the relationship between soil mineral composition and arsenate adsorption in soils. The main results are as follows:1. As (V) adsorption by kaolinite and montmorillonite were much lower than that by synthetic iron and aluminum oxides/hydroxides. As (V) adsorption by four iron and aluminum oxides/hydroxides increased with the increase of initial As (V) concentrations (0.1¹00 mg·L^-1), in which ferrihydrite showed a rising trend in the whole concentration range, while the rapid increase in lower initial concentration and slow change in higher initial concentrations for the adsorption capacities of goethite, gibbsite, and hematite were obtained. The adsorption capacity of minerals from high to low is in the order of ferrihydrite, goethite or/and gibbsite, hematite, kaolinite and montmorillonite. With the increase of adsorption time, As (V) adsorption by kaolinite, montmorillonite, and four synthetic iron and aluminum oxides/hydroxides increased gradually. The adsorption amount of ferrihydrite roses most quickly, followed by goethite and gibbsite, while hematite required more time to reach the equilibrium.2. The results of As (V) desorption from minerals showed that As (V) adsorbed by kaolinite and montmorillonite desorbed in a large percent by all extractants. NaOH and phosphate extracted As (V) is adsorbed more by iron and aluminum minerals than other extractants, followed by NaHCO₃ and oxalic acid, and desorption amount by citric acid and NaCl was lower. As (V) desorption amount of four synthetic iron and aluminum oxides/hydroxides increased sharply within 1h, and changed slightly after 8h, when As (V) desorption reaches approximately the equilibrium. The desorption amount of gibbsite was higher than others.3. With the increase of pH, As (V) adsorption by kaolinite and montmorillonite decreased, while the effect of pH on As (V) adsorption by iron and aluminum oxides/hydroxides is associated to As (V) initial concentrations. In lower initial As (V) concentrations, As (V) adsorption by four synthetic iron and aluminum oxides/hydroxides decreased only under extremely alkaline conditions (pH>10), and when the initial concentrations are higher, adsorption amount dropped sharply with pH increasing. With the increase of ionic strength, As (V) adsorption by ferrihydrite, gibbisite and goethite shows no significant changes, while As (V) adsorption by hematite, kaolinite and montmorillonite increases. On gibbistie and ferrihydrite, As (V) adsorption among three addition orders shows no significant differences, and only when P/As molar ratio is higher. As (V) adsorption decreased when P was added before As, while on goethite and hematite, As (V) adsorption amount is the highest when As is added before P. With the increase of P/As molar ratio, As (V) adsorption decreased gradually when P was added before As or P and As are added at the same time.4. Within the initial concentration of 1¹00 mg·L^-1, As (V) adsorption by seven soils increases dramatically with As (V) concentration, and changes slightly under higher concentration. The maximum As (V) adsorption from high to low is in the order of black soil, red soil-clay, yellow soil, red soil-granite, red soil-purple or purple soil, and fluvo-aquic soil. With the increase of adsorption time, As (V) adsorption by seven soils roses significantly at the beginning, followed by slow increase in adsorption. Black soil and red soil-clay reach adsorption equilibrium faster than other soils.5. NaOH and phosphate are most effective to extract As (V) adsorbed by soils. Except for black soil, the extractability of the citric acid and oxalic acid are moderate. For the solutions of NaHCO₃ and NaCl, As (V) adsorbed by soils is extracted less, especially for NaCl. With the increase of initial As (V) concentration, As (V) adsorption and desorption amount by seven soils increases, and a linear relationship between them is found. With the increase of desorption time, As (V) adsorbed by seven soils releases rapidly within 5min, and approximately reaches the equilibrium after 2 h.6. When 10 mg·L^-1 As (V) are added, the adsorption amount by seven soils changes slightly over a pH range of 3¹0, while it decreases drastically in red soil-clay, black soil, red soil-granite and yellow soil when pH over 10. But it decrease slowly in other soils. The ionic strength affected As (V) adsorption by soils insignificantly. The removal of organic matter could increase As (V) sorption in red soil-clay, red soil-purple and yellow soil, while As (V) adsorption decreases in red soil-granite and purple soil. The effect of phosphate on As (V) adsorption by soils depends on P/As molar ratio and addition sequence. As (V) adsorption amount is the highest when As was added before P. With the increase of P/As molar ratio, As (V) adsorption decreases gradually.