Na Liu,Ruili Gao,Jingjing Guo,Longwei Fu,Bin Xue,Kun Ma,Chen Lin
The environmental impact of arsenic (As) pollution has been a focal point within environmental science. In arsenic-polluted saline-alkali environment, the addition of exogenous biochar can affect the morphological transformation of As both through direct and indirect mechanisms, with calcium ions (Ca(II)) playing a crucial role. This study investigates the immobilization mechanisms of undissolved biochar (UOB) and dissolved biochar (DOB) on As(V) in the absence and presence of Ca(II) under alkaline conditions and aerobic atmosphere. While UOB and DOB alone are insufficient for As(V) immobilization, their combined action in the presence of Ca(II) achieves remarkable immobilization rates of 91.9% and 98.1%, respectively. Precipitation of calcium arsenate is identified as the primary immobilization pathway in both the UOB-Ca(II)-As(V) and DOB-Ca(II)-As(V) systems. Furthermore, Ca(II) acts as a mediator for As(V) immobilization through the formation of ternary UOB/DOB-Ca-As complexes, which are corroborated by Density Functional Theory (DFT) analysis from a microscopic perspective. Notably, the synergistic immobilization of As by DOB and newly generated CaCO 3 in DOB-Ca(II)-As(V) system is highlighted. Additionally, the increase in Ca(II) concentration (0–100 mM) and solution pH (9.0–12.0) both significantly enhance the immobilization of As(V). An increase in the dosage of UOB (0.4–4 g/L) reduces the immobilization of As(V), while effect of the DOB concentration is insignificant. This study provides new insights into how the release of two biochar fractions into a typical Ca(II)-rich saline-alkali environment may alter the fate and transport of As species.
Production of reactive oxygen species (ROS), such as hydrogen peroxide (H 2 O 2 ) and hydroxyl radical (•OH), has been increasingly discovered upon oxygenation of reduced species in anoxic soil and sediment pore waters. We therefore hypothesize that H 2 O 2 and •OH can be produced during groundwater abstraction and discharging due to oxygenation. To evaluate the capacities of H 2 O 2 and •OH accumulation during groundwater oxygenation, here we measured H 2 O 2 and •OH accumulations during dark oxygenation of groundwater abstracted from different depths of 11 wells adjacent to the Han River. This abstraction could simulate the scenarios of groundwater abstraction by riverside pumping and also of groundwater discharging in hyporheic zones. Results showed that H 2 O 2 and •OH formed during oxygenation of the groundwater with low Eh values (< 63 mV), and only H 2 O 2 (< 3.37 μM) formed for the groundwater with high Eh values (63–206.5 mV). Statistical analysis indicated that dissolved Fe(II) was mainly accountable for the production of measurable •OH in the low Eh locations. The aquifer conditions (i.e., aquifer lithology) and external environmental factors (i.e. pumping disturbance) within short-term abstraction process had negligible influence on ROS production. The relationship between ROS production and groundwater chemistry was further explored by multiple linear regression, which deduced the quantitative models for estimating ROS production upon oxygenation of groundwater from different sources with different chemistry. Using the abstracted groundwater in western Bengal basin as an example, the estimated results indicated that ROS production had a great potential to oxidize the typical contaminant of As(III). As ROS, particularly •OH, represents strong oxidants, our findings implicate an overlooked oxidative capacity during groundwater abstraction and discharging in hyporheic zones or other areas, which could lead to the oxidative transformation of substances and the oxidative stress of microbial metabolism and associated biogeochemical processes in the environments suffering from groundwater oxygenation. This oxidative capacity is particularly important when the groundwater with low Eh values, i.e., under strongly reductive conditions, were abstracted and oxygenated in the surface.
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