A review of researches on bioavailability and interfacial processes of arsenic based on passive sampling techniques: Progress and prospect

Geological and human activities in quite a number of regions of the world are found to have brought about serious arsenic (As) pollution in soil and groundwater, gravely threatening the ecosystems and human health in those regions. In order to effectively control As pollution risk at large scales, it is necessary to accurately evaluate interfacial behaviors of As in different media. Being regulated by chemical and microbiological factors migration and transformation of the element in certain typical environmental interfaces, like that of soil-water and rhizosphere, exhibit the characteristics of drastic changes in species at μm-To-mm-scales. Conventional active sampling techniques, which mostly consist of destructive field sampling and afterwards sample analysis in lab, have proved to be not good enough to meet the demands of the study on interfacial process of the element, such as handling an element varying drastically in species, quantifying the element at trace levels, and time-and labor-saving. In recent years, passive sampling technology, represented by diffusive gradients in thin-films (DGT), diffusive equilibrium in thin-films (DET), in-situ porewater iterative sampler (IPI) and dialysis sampler (Peeper), has emerged, displaying great advantages over the conventional ones in the research. The DGT device is composed of filter membrane, diffusion gel, binding gel and plastic bases/caps used to fix the three layers of membrane/gel. The filter membrane is mainly used to prevent particles in the environment to be tested from entering the device; the diffusion gel to facilitate free diffusion of ions and formation of a diffusion gradient; and the binding gel, chosen according to the purpose of the experiment, to absorb the pollutants to be tested. DET is a sister technique of DGT, omitting the binding gel phase. The IPI sampler consists of hollow fiber membrane sampling tubes and catheters. For sampling, the sampling tube is filled with deionized water in advance, and ions and small molecules in the environment diffuse into the tube. After the diffusion reaches equilibrium, the solution in the sampling tube is directly pumped out for measurement of concentrations of the ions tested. In principle, Peeper is similar to DET and IPI, but lower in spatial resolution for measurement of porewater concentration. These passive sampling techniques have been used to determine in situ of total As and As speciations in water and soil porewater, and their one-dimensional distribution profiles. DGT-measured As concentration in soil has a good correlation with its content in plants, showing that DGT is suitable for the evaluation of As phytoavailability. It turns out in recent years to be an important trend to use these passive samplers to study two-dimensional spatio-Temporal distribution of As at the soil/sediment-water interface. DGT has been used to characterize the two-dimensional distribution of As at soil/sediment-water interface and plant rhizosphere in submillimeter high-resolution, so it cherishes great advantages in the study on spatial distribution of As, whereas IPI can sample iteratively with low disturbance, thus being one of the few tools that can be used to study dynamic distribution of As relative to species. These studies elucidate biogeochemical behaviors of As from a microscale perspective. In the end, the paper describes a prospect of the research in future, including: 1) taking advantage of the merits of the passive sampling techniques in future studies on dynamic-controlled processes of As uptake by plants; 2) developing novel passive sampling techniques with both the spatial resolution and the temporal resolution of As concentration taken into account; 3) combining the passive sampling techniques with other 2D sampling techniques, such as planar optodes and soil zymography, in comprehensive studies on biogeochemical process of As in soils and sediments; 4) extending the use of passive sampling techniques to the study on processes of As uptake by fauna living in soils and sediments; and 5) building models of As transporting across interfaces based on data of changes in spatiotemporal concentration of As at the interfaces in complex environmental matrix.