The bioaccessibility and fractionation of arsenic in anoxic soils as a function of stabilization using low-cost Fe/Al-based materials: A long-term experiment.

Arsenic (As)-contaminated soils occur widely worldwide. In the present study, three low-cost Fe/Al-based materials, including red soil (RS), sponge iron filter (SIF) and Al-based water treatment sludge (WTS), were applied as amendments to remediate As-contaminated soils under anoxic conditions. After 180 d of incubation, the proportion of the sum of nonspecifically absorbed As (F1) and specifically absorbed As (F2) to the total As was reduced by 6%, 52% and 13% with 5% of RS, SIF and WTS addition, respectively, compared to the control soil (31%). The results showed that among the three amendments, SIF was the most effective at decreasing As bioaccessibility in soils. Compared with RS and WTS, SIF intensified the decrease of labile fractions and the increase of unlabile fractions, and the redistribution of the amorphous oxide-bound fraction (F3) and crystalline hydrous oxide-bound fraction (F4) occurred in the SIF-amended soil. Moreover, the As stabilization processes were divided into two stages in the control and RS-amended soil, while the processes were divided into three stages in both SIF- and WTS-treated soil. The As stabilization processes in all treated soils were characterized by the transformation of labile fractions into more immobilizable fractions, except for F4 transforming into F3 in the first stage in SIF-amended soil. Correspondingly, inner-surface complexation and occlusion within Fe/Al hydroxides were the common driving mechanisms for the transformation of As fractions. Therefore, taking into consideration the results of this study, SIF could be a more promising alternative than the other two materials to passivate As in anoxic soils.

How different are the arsenic fractions inhibit alkaline phosphatases on aggregates scale?

Arsenate [As(V)], in general, is associated with various aggregates and exists as different species in soil, which in turn influences its toxicity and potential contamination. Previous studies have demonstrated the usefulness of alkaline phosphatases (ALP) to evaluate As(V) pollution. However, the effect of different arsenic fractions on ALP among soil aggregates is still unclear. Thus, the distribution of As fractions and ALP kinetics was determined in four-month As-aged paddy soil aggregates. Results revealed the two major fractions of As in aggregates were humic-bound and Fe and Mn oxides-bound [both around 30% under 800 mg kg-1 of As(V)]. Besides, it was observed that available soil phosphorus could positively affect the relative content of water-soluble, exchangeable and carbonate-bound arsenic. In the kinetics experiment, both the Michaelis-Menten constant (Km) and maximum reaction velocity (Vmax) of ALP increased with increasing As(V) concentration under four months ageing for each size aggregate. Multiple linear stepwise regression analysis between kcat and the relative content of arsenic fraction indicated that carbonate-bound arsenic is the main fraction that inhibited the kcat for macroaggregates (> 0.25 mm size). For soil aggregates of 0.1-0.25 mm size, kcat increased with an increase in arsenic residual fraction. As for aggregates <0.1 mm size, Fe and Mn oxide-bound fraction is the main fraction that inhibited the kcat. Overall, this study suggests carbonate-bound and Fe and Mn oxide-bound arsenic fractions could decrease the ALP activities via a decrease in the catalytic efficiency in macroaggregates and <0.1 mm size aggregates, respectively. Besides, available phosphorus should be considered as the main factor when assessing As biotoxicity and mobility.

The distribution of arsenic fractions and alkaline phosphatase activities in different soil aggregates following four months As(V) ageing.

Soil as a heterogeneous mass is composed of different size aggregates. The distribution of different arsenic (As) fractions in soil aggregates is vital to assess the potential risk of As pollution. In this study, soil samples were aged for 4 months with different arsenate [As(V)] concentrations. Dry sieving method was used to obtain five different size aggregates and the content of As in these fractions was determined. The results showed that P4 (0.1-0.25 mm) contained the highest organic matter (OM) than other size aggregates. After 4 months of ageing, available phosphorus (AP) content increased with the increase of As(V) concentration among 5 aggregates. The distribution of different arsenic fractions among 5 aggregates was similar. The relative contents of water-soluble (F1), exchangeable (F2) and carbonate (F3) fractions increased with the increase in As concentration, while the residual fraction (F7) decreased sharply. Humic-bound (F4), and Fe and Mn oxide bound fractions (F5) were about 35% and 20% respectively, after 4 months of As(V) ageing. Generally, the alkaline phosphatase (ALP) activities of P4 were lowest among five aggregates under each concentration of As(V). Moreover, F2 and F3 exhibited a strong inhibition of ALP activity. This study demonstrates that not only water-soluble and exchangeable arsenic but also humic-bound fraction should be considered when assessing As bioavailability and toxicity.