Mobilization of Colloid- and Nanoparticle-Bound Arsenic in Contaminated Paddy Soils during Reduction and Reoxidation.

The association of arsenic (As) with colloidal particles could facilitate its transport to adjacent water systems or alter its availability in soil-rice systems. However, little is known about the size distribution and composition of particle-bound As in paddy soils, particularly under changing redox conditions. Here, we incubated four As-contaminated paddy soils with distinctive geochemical properties to study the mobilization of particle-bound As during soil reduction and subsequent reoxidation. Using transmission electron microscopy-energy dispersive spectroscopy and asymmetric flow field-flow fractionation, we identified organic matter (OM)-stabilized colloidal Fe, most likely in the form of (oxy)hydroxide-clay composite, as the main arsenic carriers. Specifically, colloidal As was mainly associated with two size fractions of 0.3-40 and >130 kDa. Soil reduction facilitated the release of As from both fractions, whereas reoxidation caused their rapid sedimentation, coinciding with solution Fe variations. Further quantitative analysis demonstrated that As concentrations positively correlated with both Fe and OM concentrations at nanometric scales (0.3-40 kDa) in all studied soils during reduction and reoxidation, yet the correlations are pH-dependent. This study provides a quantitative and size-resolved understanding of particle-bound As in paddy soils, highlighting the importance of nanometric Fe-OM-As interactions in paddy As geochemical cycling.

Dispose waste liquor of fresh biomass of a hyperaccumulator Sedum plumbizincicola in phytoextraction process.

Sedum plumbizincicola has been widely employed to remove cadmium (Cd) and zinc (Zn) from contaminated soils and harvested biomass is used to recover valuable metals. While chopping and compacting are efficient methods to rapidly reduce the volume and moisture of fresh biomass, the resulting waste liquor containing metals needs treatment. Two types of contaminated soils were cropped with S. plumbizincicola and top-dressed with this liquor to study metals migration in soil profile and their uptake by plants. There were three treatments: planting and adding liquor (PL), planting without liquor (P) and adding liquor without planting (L). The results showed that Cd and Zn from liquor were mainly retained at top soil 0-10 cm under L treatment. Compared with L treatment, soil Cd and Zn under PL treatment decreased significantly in soil profile due to the extraction of S. plumbizincicola. Moreover, the amount of Cd and Zn extracted by plants was greater than that applied in soils. The metal removal rate by S. plumbizincicola in acid clay loam soil was higher than that in neutral sandy soil. To sum up, metal retaining in soil and uptake by S. plumbizincicola can be used to treat waste liquor from its fresh biomass. Novelty StatementRapid and efficient treatment of harvested fresh biomass is still a challenge although phytoextraction using hyperaccumulator Sedum plumbizincicola has been widely employed. Chopping and compacting fresh biomass are efficient methods for rapid dehydration, however, a large amount of waste liquor that contains of Cd and Zn is produced and needs treatment. In the present study, a simple and low-cost method was tested to dispose the liquor, i. e. irrigating it onto the surface of contaminated soils where grown S. plumbizincicola. It was found that Cd and Zn applied in soils from liquor were mainly retained at top 0-10 cm soil depth where S. plumbizincicola root system was widespread, and the amount of Cd and Zn extracted by plants was greater than that applied in soils. Therefore, it is technically feasible to dispose the waste liquor dewatering from fresh biomass of S. plumbizincicola in its phytoextraction process. This study is helpful for the rapid, efficient and low-cost treatment of harvested fresh biomass in the large-scale application of phytoremediation.

Potential mobilization of water-dispersible colloidal thallium and arsenic in contaminated soils and sediments in mining areas of southwest China.

Water-dispersible colloids (WDCs) are vital for trace element migration, but there is limited information about the abundance, size distribution and elemental composition of WDC-bound thallium (Tl) and arsenic (As) in mining-contaminated soils and sediments solutions. Here, we investigated the potential mobilization of WDC-bound Tl and As in soils and sediments in a typical Tl/As-contaminated area. Ultrafiltration results revealed on average > 60% of Tl and As in soil solution (< 220 nm) coexisted in colloidal form whereas Tl and As in sediment solution primarily existed in the truly dissolved state (< 10 kDa) due to increased acidity. Using AF4-UV-ICP-MS and STEM-EDS, we identified Fe-bearing WDCs in association with aluminosilicate minerals and organic matter were main carriers of Tl and As. SAED further verified jarosite nanoparticles were important components of soil WDC, directly participating in the migration of Tl and As. Notably, high pollution levels and solution pH promoted the release of Tl/As-containing WDCs. This study provides quantitative and visual insights into the distribution of Tl and As in WDC, highlighting the important roles of Fe-bearing WDC, soil solution pH and pollution level in the potential mobilization of Tl and As in contaminated soils and sediments.

Potential high-risk release sources of thallium and arsenic from surrounding rocks of a typical thallium and arsenic mining area in southwest China.

Abundant naturally and anthropogenically exposed surrounding rocks (NESRs and AESRs) in mining areas may pose persistent threats as sources of potentially toxic elements (PTEs), but this has been historically overlooked, especially for thallium (Tl) and arsenic (As). Here, the release risks of Tl and As from both NESRs and AESRs in a typical TlAs sulfide mining area were investigated. In a single leaching process, AESRs released 10.4 % of total Tl (157 µg L-1) and 32.5 % of total As (4089 µg L-1), 2-3 orders of magnitude higher than NESRs. Prolonged multiple leaching tests revealed notable and long-term risks of release of Tl and As from AESRs, associated with oxidation and dissolution of iron/sulfur-bearing minerals. Substantial release of PTEs was linked to the transformation/degradation of the -OH functional group and extensive dissolution of secondary sulfate minerals in AESRs. Ultrafiltration and STEM-EDS indicate that 18.4 % of water-extracted As released from AESRs existed as natural nanoparticles consisting of iron/sulfur-bearing minerals. This study highlights the high risks of Tl and As release from anthropogenically exposed surrounding rocks and the importance of nanoparticles in PTE transport, and provides insights into the control of PTEs in mining areas.