Study On The Remediation Of Hexavalent Chromium Contaminated Groundwater With In-situ Reaction Zone Of Carboxymethyl Cellulose Modified Sulfidated Nano Zerovalent Iron

As one of the main heavy metals,chromium is widely used in electroplating,metallurgy,pigment and leather production,and can cause groundwater pollution.With the rapid development of chromium industry,plenty of industrial wastes containing chromium are released into environment,thus resulting in serious pollution of soil,surface water,and groundwater.Being highly toxic to organisms,Cr（VI）is a recognized human carcinogen.Therefore,the remediation of Cr（VI）contaminated groundwater is very necessary.In recent years,nanoscale zerovalent iron（nZVI）is emerging as a promising option for the in-situ remediation of Cr（VI）contaminated groundwater for its high reactivity and relatively low cost.However,the application of nZVI is still problematic in some cases.Firstly,the agglomeration of particles limits their mobility in porous media.Secondly,nZVI could interact with various naturally chemical constituents of groundwater,which were ineffective reactions,thus reducing the reaction efficiency between nZVI and target pollutants.Therefore,the modification of nZVI to improve its stability,reactivity and mobility could lay a foundation for the promotion of its in-site application.Studies have shown that sulfidation can significantly inhibit the reaction of nZVI with water and extend its life,but due to the agglomeration among particles,the migration of sulfidated nZVI in the underground porous media is limited.Carboxymethyl cellulose（CMC）can effectively decrease the agglomeration and enhance the reactivity of nZVI,but its life in water is short.In this paper,sulfidation and CMC stabilization techniques were combined to synthesize carboxymethyl cellulose–stabilized sulfidated nano zerovalent iron（CMC-S-nZVI）with high reactivity,anti-aging and good dispersion.Effects and mechanisms on the removal of Cr（VI）by CMC-S-nZVI were clarified.The influence of groundwater chemical composition on the reaction process and material aging process was explored.A method of estimating the natural reductant demand was proposed.CMC-S-nZVI in-situ reaction zone was simulated in one-dimensional pillar and two-dimensional trough,and meanwhile the effects of media particle size,groundwater velocity,CMC-S-nZVI injection concentration and initial Cr（VI）concentration on the formation of the reaction zone and the remediation effect of Cr（VI）were studied.The variation of hydrochemical parameters during the operation of the reaction zone was also investigated.Specific research contents and results are as follows:（1）Study on preparation of CMC-S-nZVI and mechanism of Cr（VI）removalThe effect of synthesis approaches on morphology and properties of CMC-S-nZVI was investigated.The results revealed that CMC-S-nZVI prepared by the surface corrosion method has favorable dispersion and sulfidated uniformity.The CMC modification-sulfidation preparation process of nZVI was proposed,and the preparation and characterization were carried out.Compared with CMC-nZVI,CMC-S-nZVI had better resistance to corrosion,while compared with S-nZVI,it had better dispersion,stability and reactivity.Studies have shown that the removal efficiency of Cr（VI）by CMC-S-nZVI is influenced by S/Fe molar ratio,initial p H,initial Cr（VI）concentration and reaction temperature.The reactivity of CMC-S-nZVI first increased and then decreased with the increase of S/Fe ratio,reaching the highest value when S/Fe=0.35.Reducing p H and improving reaction temperature had positive impacts on Cr（VI）removal.The reaction followed a pseudo-second-order adsorption model,indicating that the chemical adsorption was the rate-limiting step of the reaction.（2）Study on the natural reductant demand of CMC-S-nZVI and the influence factorsThe influence of groundwater chemical composition on the reaction process and the aging process of CMC-S-nZVI was explored.The common cations in the aquifer had a significant effect on the removal of Cr（VI）by CMC-S-nZVI,in which Ca²⁺,Mg²⁺,Mn²⁺had an obvious adverse effect on the reaction process,while Fe³⁺had a certain promoting effect.Anion had no obvious effect on Cr（VI）removal by CMC-S-nZVI,and the presence of humic acid inhibited the reaction.The aging process of CMC-S-nZVI in anaerobic water followed the first-order kinetics,and cations had greater enhancing effects on the aging process than anions.The half-life of CMC-S-nZVI was shortened with the increase of ion concentration.The presence of humic acid slowed down the aging rate of CMC-S-nZVI to some extent.According to the aging kinetics regular pattern of CMC-S-nZVI and combined with the dynamic change of the reaction zone in the groundwater pollution remediation process,a method for estimating natural reductant demand was put forward,and it provided a basis for the calculation of amount of remediation agent.（3）Study on the influence factors of the mobility of CMC-S-nZVI in aquifer and the remediation effect of the reaction zoneThe particle size of aquifer media,groundwater flow rate,CMC-S-nZVI concentration and initial pollutant concentration had great influences on the migration performance of CMC-S-nZVI.Irrigation and dispersion under hydrodynamic action were the main migration mechanisms.The results showed that the migration ability of CMC-S-nZVI increased with the increase of particle size and groundwater velocity,and decreased with the increase of injection concentration.The CMC-S-nZVI reaction with Cr（VI）led to the reduction of migration ability.CMC-S-nZVI was able to maintain a stable remediation capacity in fine sand.The effective repair period was long,and the removal capacity during the effective operation period was about 307mg/g_（TFe）,which was significantly higher than that of coarse sand and medium sand.In the process of operation,the permeability of the reaction zone decreased slightly,and the hydraulic conductivity was in the same order of magnitude as the initial value.The p H of the system increased slightly,while ORP remained at a low level.Most stabilized products exist in the form of Fe-Mn binding state.The concentration of total chromium in the leach solution decreased first and then increased with the increase of p H.The amount of total chromium was lower under near-neutral conditions,indicating that the site should be kept in a near-neutral environment as far as possible after the remediation with nZVI-based materials to reduced the release of chromium.（4）Simulation experimental study on the in-situ remediation of Cr（VI）contaminated aquifer by CMC-S-nZVICMC-S-nZVI formed a stable reaction zone in the downstream of the injection well,which realized the effective removal of Cr（VI）in aquifer in fine sand media.The removal capacity of Cr（VI）was 200 mg/g_（TFe）at 120 h of operation.After CMC-S-nZVI suspension was injected,DO and ORP near the downstream of the injection well decreased rapidly,which provided favorable conditions for the reduction of Cr（VI）.The p H value increased rapidly and the highest value was less than 9,suggesting that the injection of the suspension did not significantly increase the p H of the system.During the stable operation of the reaction zone,the concentration of SO₄^2-in the downstream of the injection well was slightly higher than the inflow,Cl^–concentration returned to the inflow level,and NO₃^–concentration remained at a lower concentration than the inflow.The operation of the reaction zone had no obvious adverse effects on the hydrochemical environment of the aquifer.The innovations of this paper are summarized as follows.As a kind of material in-situ remediation,CMC-S-nZVI was developed and applied to in-situ remediation of Cr（VI）contaminated groundwater.This remediation material has various advantages,including long life,anti-aging and strong reactivity,and its comprehensive performance is better than existing Cr（VI）pollution remediation agents such as CMC-nZVI and S-nZVI.The influencing factors of the reductant natural demand were expounded,and a method for the estimation of the reductant natural demand was proposed,which provided a basis for the quality calculation of the injection agents and the estimation of the stable operation time of the in-situ chemical reduction zone.CMC-S-nZVI remediation agent also has a good migration ability in fine sand media with low permeability,and can form a stable reaction zone,referring to that the remediation agent had a better migration ability and a wider range of application compared with the existing nZVI remediation agent.