Distribution And Production Mechanisms Of Hydrogen Peroxide In Riparian Unconfined Aquifers

Reactive oxygen species(ROS)such as superoxide(O₂^?-),hydrogen peroxide(H₂O₂)and hydroxyl radical(?OH)are the intermediates in the reduction of O₂ to H₂O.ROS are highly reactive,which can oxidize or reduce a wide range of substances in the environment,contribute to the transformation of redox-active elements and the attenuation of contaminants in different environmental domains,and affect the activity of functional microbe.In recent years,ROS in subsurface environments have attracted attention.Natural processes(e.g.,surface water and groundwater interactions and dry-wet alternation)and artificial processes(e.g.,the South-North Water Diversion Project and the Three Gorges Dam Projects)can induce fluctuations of groundwater table,which can introduce oxygen into the subsurface environment.In situ measurements demonstrated that O₂^·?and H₂O₂ appeared in soil water in the rhizosphere where O₂was generated by root.In addition,up to 54 n M H₂O₂ occurred in the groundwater of an unconfined aquifer adjacent to a river,which documented the existence of ROS in natural subsurface environments.However,the measurements was performed at a specific time,which may over or underestimate the environmental significance in the time scale of hydrological years.Therefore,it is essential to survey the spatial and temporal distribution of ROS and explore the control mechanisms.It has been confirmed that abiotic reactions between dissolved O₂ and reduced components(e.g.,Fe(II)and reduced organic carbon(DOM))can contribute to H₂O₂produciton,and microorganisms such as heterotrophic bacteria can also produce H₂O₂ through biological pathway.Whereas,the effects of natual environment conditions(e.g.,aquifer composition,lithology,and redox condition of the environment)on the mechanism of H₂O₂ production have not been reported.Therefore,this paper studied the distribution and production mechanisms of H₂O₂ in riparian unconfined aquifers.The main progress and conclusions are summarized as follows:(1)Distribution of hydrogen peroxide in riparian unconfined aquifersA floodplain adjacent to the section of Yangtze River in Xinzhou District,Wuhan,central China was selected as the research field site,and we measured temporal and spatial distributions of H₂O₂ in an unconfined aquifer which was impacted by a 14-m water level fluctuation in the adjacent river in one hydrological year.Besides,the water level of Yangtze River and groundwater was recorded simultaneously and the chemical features of groundwater,such as p H,dissolved oxygen(DO),dissolved Fe(II),dissolved organic carbon(DOC)were analyzed.The results show that the concentration of H₂O₂in an unconfined aquifer impacted by a 14-m water level fluctuation in the adjacent Yangtze River attained up to 123 n M at rising water table stage,and H₂O₂ distributed in a banded zone from the river to the aquifer,and the peak concentration appeared at the depth close and below the groundwater table.Stabilization of high water table maintained a moderate level of H₂O₂,while stabilization of low water table and falling of water table corresponded to low or even undetectable H₂O₂.In addition,a solute transport model by COMSOL Multiphysics was developed to further explore the influence of water table fluctuations on H₂O₂ distributions.The dynamic of H₂O₂productin and the spatial distribution by the reactive transport model is in good agreement with the field observations.With the increase in the speed of water table rise,H₂O₂ production flux increases,with the total amount of H₂O₂ decreases.Both field observations and reactive transport models unveile that water table fluctuations regulate the production and distribution of H₂O₂ in uncjonfined aquifers.Rising water table developes a thermodynamically unstable banded zone in the unconfined aquifer in which elevated coexisting dissolved O₂ and reduced species favored abiotic H₂O₂production.(2)Experimental simulation of hydrogen peroxide production during groundwater oxygenationTo explore the source of H₂O₂,sediments and simulated groundwater were oxygenated respectively in lab,and instantaneous concentration of H₂O₂was detected.And sterilization experiments were conducted to evaluate the relative importance of abiotic versus biotic mechanisms.The results show that abiotic reactions are mainly responsible for the production of H₂O₂ below groundwater table.And the dissolved components(i.e.,Fe(II)and DOM)mainly contributes to H₂O₂ production during oxygenation of sandy sediments,whereas the solid components(i.e.,Fe(II)-bearing minerals)mainly contributes for silty sediments.To further unveil the mechanisms of H₂O₂production due to dissolved reduced species oxygenation,Fe(II)oxygenation and H₂O₂ accumulation during oxygenation of Fe(II)in the absence and presence of HA was detected,and kinetic model was bulidt.The modeling results show that the most important reactions for H₂O₂ accumulation changes from oxidation of aqueous and adsorbed Fe(II)in the absence of HA to oxidation of Fe(II)-HA complex in the presence of HA.The kinetic models were employed to further evaluate the influences of Fe(II),DOC and DO concentrations on H₂O₂ accumulation.Modeling results show that Fe(II)and DOC along with their interactions collectively influence both the rate and amount of H₂O₂ accumulation.DO is essential for H₂O₂production and mainly influences the rate given sufficient reaction time.(3)Mechanisms of hydrogen peroxide production during sediment oxygenationTo explore the biochemical mechanisms of H₂O₂production by sediment oxygenation,ten natural sediments differing in components were collected from different sites and depths of a floodplain adjacent to the Yangtze river,and cumulative H₂O₂,surface adsorbed Fe(II),mineral structural Fe(II),and electron donation capacity(EDC)of soil organic matter(SOM)were measured during sediment oxygenation.To distinguish the relative importance of abiotic versus biotic mechanisms of H₂O₂production,sterilization experiments were performed by high temperature autoclave method.Results showed that 14.59-111.82?M H₂O₂ was produced during the ten sediments oxygenation.In the natural subsurface environment,the sediments in deep layer and close to the river are tend to experience a long flooding stage and contain much reactive reduced component(i.e.,Fe(II)and reduced organic matter),and thus probably produce H₂O₂ mainly through an abiotic pathway.However,the sediments in shallower layer and far to the river are tend to experience a long drought stage and contain little reactive reduced component but much heterotrophic bacteria,and thus probably produce H₂O₂ mainly through a biological pathway.Furthermore,the reduced substances in the sediments can generate high concentration of·OH during oxygenation,while microbes in the sediments cannot convert H₂O₂ to extracellular·OH.The novelty points of this paper are 1)to revealed that water table fluctuations regulate the production and distribution of H₂O₂ in unconfined aquifers;2)to supplemente the effect of natual environment conditions(e.g.,aquifer composition,lithology,and redox condition of the environment)on the mechanisms of H₂O₂production in subsurface environment.The scientific significance of this paper is to provide fundamentals for understanding and predicting ROS production and distribution in unconfined aquifers.