Effects Of Colloids On Arsenic Transport And Transformation In Water-Bearing Media

Arsenic contamination in the soil and groundwater is widespread throughout the world, posing a serious health hazards to ecological environment and human health. Colloids acts as a carrier to adsorb the pollutant in the soil and groundwater and thus impact distribution and transport of As. Unfortunately, little attention has been paid to systematically study this issue. The purpose of this study is to systematically investigate effects of colloids on As transport and transformation in water-bearing media. Batch and column experiments combined with arsenic speciations and frcations analysis were employed. In addition, the adsorption kinetics, adsorption isotherm, solute transport model, colloid transport model and(X)DLVO theory model was used to evaluate adsorption, simulate As and colloid transport, reveal colloid behaviors mechanisms, respectively. The main findings in this study are shown as follows.(1) In neutral and alkaline conditions, As adsorption quantity onto soil colloid was roughly 100 mg/kg. The water-soluble As, non-specifically-sorbed As and specificallysorbed As were the major fractions of adsorbed As on soil colloid. As adsorption quantity onto ferrihydrite colloid was roughly 100-200 g/kg. The specifically-sorbed As and As associated with amorphous and crystallized(hydr)oxides were the major fractions of adsorbed As on ferrihydrite colloid. However, most adsorbed As readily desorbed during reductive dissolution of ferrihydrite colloid.(2) Besides in acidic conditions, humic acid colloid facilitated ferrihydrite colloid transport. In neutral conditions, small(filtrated) humic acid colloid with chain structure facilitated ferrihydrite colloid transport was more prominent than large humic acid colloid facilitated ferrihydrite colloid transport. The chain structure caused permeability repulsion and space repulsion between the colloid and sand leading to strong repulsion. In alkaline conditions, the large humic acid colloid with long chain structure acid was result in strong repulsion between the colloid and sand leading to that facilitated ferrihydrite colloid transport was more significant compared with small humic acid colloid facilitated ferrihydrite colloid transport. Because ferrihydrite colloid absorb As in large quantities, when the humic acid colloid facilitated ferrihydrite colloid more than 50%, As transport was facilitated by humic-ferrihydrite colloid in water-bearing media.(3) Rapid transport of soil colloids, regulated by p H and ionic strength, promoted As(V) transport by blocking As(V) adsorption onto sand, although soil colloids had low adsorption for As(V). The promoted transport was more significant at higher concentrations of soil colloids due to greater blocking effect on As(V) adsorption onto the sand surfaces. The blocking effect of colloids was explained by the decreases in both instantaneous(equilibrium) As adsorption and first-order kinetic As adsorption on the sand surface sites. The discovery of this blocking effect improves our understanding of colloid-promoted As transport in saturated porous media, which provides new insights into role of colloids, especially colloids with low As adsorption capacity, in As transport and mobilization in soil-groundwater systems.(4) Soil colloid are composed of Fe and Mn oxides and amorphous minerals, organic matter, and microorganism. Under the influence of the soil colloid, change of As speciation has different characteristics compared with the soil. On soil colloid surface, As(III) was readily oxidized, meanwhile, As(V) was first reduced to As(III) and then oxidized to As(V). A large number of microorganisms and organic matter may promote the As redox. Under aerobic conditions, the As(III) oxidation rate accelerated. Under anaerobic conditions, the time of As(V) reduction was extension. During cotransport of As(III) and soil colloid, soil colloid inhibit oxidation of As(III) by waterbearing media and thus soil colloid promoted the As(III) transport.