| It is well known that the subsurface environment is usually under reducing conditions, in which the redox-active species such as Fe, organic matters and S exist predominantly in reduced states. However, in many natural and anthropogenic processes, such as surface water and groundwater interaction, artificial groundwater recharge, groundwater extraction for usage, water table fluctuation caused by hydraulic engineering and so on, O2 in the air or surface water would interact with the subsurface environment, leading to an increase in the oxidation/reduction potential (Eh). When the Eh of subsurface environment increases, the species originally in reduced states, such as reactive Fe(II) species (soluble Fe2+, ligand complexed Fe(II), iron oxides surface bounded Fe(II), structural Fe(II) in mineral) and reduced organic matters, would be oxidized and might generate reactive oxidizing species such as hydroxyl radicals (·OH). Hydroxyl radical (’OH) is a very strong reactive oxygen species whose oxidizing ability is second only to fluorine in the environment (oxidation potential:2.8 V). Due to its strongly oxidizing ability, it can oxidize almost all the organics and redox-active elements, contributing to their transformation. For a long time, sunlight is believed to be essential for ·OH production in natural environment until 2012 when oxigenation of reduced aquatic dissolved organic matter (DOM) and iron was reported to produce ·OH. However, little is known about the production of ·OH from the interaction of O2 and the subsurface environments along with the associated environmental impact.In order to explore the possibility of ·OH production when the subsurface environment interacts with O2, we sampled a total of 33 sediments in Jianghan plain at different depths in the river/lake and groundwater interaction zone, in a farmland and a wetland and in the deep aquifer, representing different types of subsurface environments. The sediments were exposed to air to simulate the Eh increasing phenomenon in natural environment, and the amount of ·OH was detected. Results showed that the yield of·OH production reached as high as 6.45 mg/kg, which is two orders of magnitude higher than the concentration of·OH reported when arctic soil and surface waters interact with O2. Production of·OH from oxygenation of the sediment pore water and solid separately revealed that solid phase contributed predominantly (76.6-99.2%). Production of ·OH was found to be well correlated with Fe(II) oxidation in the sediment (R2=0.974), depicting that Fe(II) content in the sediments significantly contribute to the production of ·OH. Comparison experiments depicted that the mineral structural Fe(II) exhibited greater contribution to the production of·OH than extractable Fe(II).·OH produced from the one electron transfer mechanism between solid Fe(II) and O2. Furthermore, the ·OH produced from the interaction of subsurface environment with O2 could oxidize the As(III) in sediments to As(V).Inspired by the above results, we proposed a concept study of electrochemically induced oxidative precipitation of Fe(II) for in-situ remediation of high arsenic groundwater. Taking advantages of the good conductivity of groundwater, two anodes and one cathode, including a mixed metal oxides anode (MMO), an iron anode and a MMO cathode, were inserted into the subsurface. The MMO anode was used to generate O2 for oxidative precipitation of Fe(II), and the iron anode was used to produce Fe(II) under iron-deficient conditions. When As(III)-contaminated groundwater is electrolyzed in the dual anode system, As(III) can be oxidized to As(V) by the reactive oxidizing species produced from the reaction of Fe(II) with O2. Both As(III) and As(V) can be adsorbed and/or coprecipitated by the produced Fe(III) precipitates. In this new process, both O2 and Fe(II) can be produced in situ by installing electrodes in the contaminated aquifer. The production rate of O2 and Fe(II) can be quantitatively regulated by the current applied to the two anodes according to Faraday’s Law. Arsenic was finally immobilized in the subsurface, eliminating the need for hazardous waste disposal.Firstly, we justified the above concept in batch experiments. For 30 min’s treatment,500 μg/L of As(III) was completely oxidized and removed from the solution during the oxidative precipitation process when a total current of 60 mA was equally partitioned between the two anodes. The current on the MMO anode determined the rate of O2 generation and was linearly related to the rates of Fe(II) oxidation and of As(III) oxidation and removal, suggesting that the process could be manipulated electrochemically. The composition of Fe precipitates transformed from carbonate green rust to amorphous iron oxyhydroxide as the MMO anode current increased. Mechanism studies showed that As(III) was mainly oxidized to As(V) by the reactive intermediates (about 86%), rather than ΛOH in solution phase, and was partly oxidized by the anode (about 14%).Humic substances are abundant in most of high arsenic groundwater and are reported to impact Fe(II) oxidation and precipitation as well as As(III) oxidation and removal through ligand complexation, serving as electron shuttles and competing for the adsorption sites on iron hydroxides. Therefore, the effect of humic substances on the Fe(II) oxidation and precipitation as well as As(III) oxidation and removal in dual anode system was further studied by using humic acid (HA) as a represention of humic substances. Results depicted that HA promoted the oxidative precipitation of Fe(II) as well as As(III) oxidation and removal in dual anode system, especially at higher HA concentrations. The effect of HA on oxidative precipitation of Fe(II) and As(Ⅲ) varied with different pH conditions (6,6.5,7,7.5), the promotion was more notable at lower pH. HA could only improve the oxidative precipitation of aqueous Fe(II), but has no impact on the oxidative precipitation of Fe(II) in solid phase. In addition, the effect of HA on the oxidative precipitation of Fe(II) as well as As(III) oxidation and removal varied with different electrolyte conditions.On the basis of the above batch experiments results, the concept of electrochemically induced oxidative precipitation of Fe(II) for remediating high arsenic groundwater was further justified in a flow-through sand column which mimics the aquifer conditions, and the effects of operation parameters and humic substance were investigated. An inert anode, an inert cathode and an iron anode were arrayed in an upward mode in the column to regulate the oxidative precipitation of Fe(II) by 0%. As(Ⅲ) at 500 ug/L was completely oxidized to As(V), which was then removed by the newly formed amorphous ferric oxyhydroxides. Quantitative models for the dependence of As(III) oxidation, total As removal and Fe(II) oxidative precipitation on the flow rate and the current applied to Fe anode were developed. The presence of humic substance promoted the oxidation of As(III) on the inert anode but inhibited the oxidation and removal induced by Fe(II) oxidative precipitation. A 10-day continuous operation showed that the system effectiveness can be sustained at a stable level for a relatively long time.Overall, our study found that oxygenation of subsurface environment could produce abundant ·OH. The large amount of ·OH produced could oxidize As(III) in sediment to As(V). The results depicted a new geochemical process to substances cycling in the oxic/anoxic interface. Considering Fe(II) and O2 were found to predominantly contribute to ·OH production, we proposed and justified a concept study of electrochemically induced oxidative precipitation of Fe(II) for in-situ remediation of high arsenic groundwater. This study also provides a new option of in-situ remediation technologies for high-arsenic groundwater. |
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