Targeted Remediation Of Groundwater Contamination Based On PH-response Nano-CaO₂

Groundwater is an important part of the hydrosphere in nature,and it is not only a precious fresh water resource for human survival,but also a fundamental material for industrial production and social development.However,the unreasonable man-made exploitation and excessive development have led to very serious pollution of the worldwide groundwater.It is well known that groundwater is buried in the underground complex geological environment medium,which has the characteristics of fluidity,fragility,concealment,dynamic variability,etc.,resulting in high professional and technical requirements,engineering difficulty and high cost of groundwater pollution remediation.In recent years,the rise of nanotechnology provides a new technical support for in-situ remediation of groundwater,and nano-scale remediation agents can migrate effectively in groundwater-soil medium.Thus,it is possible for in-situ remediation of groundwater pollution with high efficiency and low consumption.Calcium peroxide(Ca O?),as a special remediation agent with p H response,would be hydrolyzed to H?O?under acidic condition or O?and Ca(OH)?under alkaline condition.The three hydrolysates are key issues in chemical,biological,and physical remediation processes,respectively.When the particle size of Ca O?is reduced to the nanometer scale,the nano-Ca O?can quickly and efficiently remediate groundwater pollution due to its strong underground migration ability and chemical activity brought by nano-effect.More importantly,there would be no secondary pollution because the transformation products of nano-Ca O?are various calcium salts which are the main components of natural underground environment.Therefore,nano-Ca O?has better application prospect than macro-Ca O?in the field of groundwater remediation.In this paper,p-nitrophenol(PNP),tris(2-chloroethyl)phosphate(TCEP)and heavy metals were selected as typical target contaminants to study remediation of groundwater pollution based on remedial agent of nano-Ca O?.The overall technical route is as follows:Site pollution background characterization?Pollutant test method development?Nano-Ca O?technology application?Theoretical prediction and experimental verification.The"Method Development"had solved the problem of interference of nanoparticles to PNP determination in subsequent experiments.In the"Nano-Ca O?Application",the macro-Ca O?/Fe(II)was firstly used to remediate the PNP pollution,and then the nano-Ca O?was prepared and applied to remediate co-pollution of PNP and Cd under groundwater conditions.Finally,two core-shell nanocomposites,namely CS/Ca O?and EPO/Ca O?,were successfully prepared by nano encapsulation technology,and were applied to groundwater pollution remediation.In order to investigate the remediation effect under the condition of groundwater,the underground low light environment was simulated by avoiding light,and the fluctuation of groundwater was simulated by oscillating the samples in different frequencies,and the hydrogeochemical conditions of groundwater were simulated by adjusting p H and adding commonly anion and cation in the groundwater pollution removal experiments.In order to understand the internal remediation mechanism,the possible degradation mechanism of TCEP(an emerging pollutant)by nano-Ca O?was discussed by density functional theory(DFT)of quantum chemistry,and the rationality of the mechanism was verified through experiments.The main conclusions of the study were as follows:(1)The challenge of nano-Fe(OH)?interference had been solved by the improved pretreatment method of"10%methanol quenching—5%hydrochloric acid acidification—60?medium temperature heating—0.1%ascorbic acid masking",and the accurate colorimetric determination of PNP concentration at 317 nm was achieved,which provided a guarantee for the following-up experiments.(2)96%of the pollutant PNP(40 mg/L)was degraded by Ca O?/Fe(II),and the OH·played essential roles.Except for HCO₃^-,the hydrochemical conditions,such as Na⁺,K⁺,Ca²⁺,Mg²⁺,Fe³⁺,Mn²⁺,SO₄^2-,Cl^-and NO₃^-in groundwater,did not affect the degradation effect of PNP.The initial p H and reagent dosages were the key parameters,while the ossilation frequency,temperature and light conditions were secondary parameters,which indicated that Ca O?could be used for groundwater PNP pollution remediation.(3)The particle size of the nano-Ca O?was less than 50 nm,and the Ca O?content was 71%.93%of PNP(40 mg/L)and 99%of Cd²⁺(10 mg/L)were removed by combination of nano-Ca O?and Fe(II).The removal mechanisms of PNP and Cd were predicted to be OH·oxidation and ferrite coprecipitation,respectively.Low p H,high Fe(II)dosage,high oscillation frequency,and high temperature were conducive to the degradation of PNP,while high p H,low Fe2?dosage,high nano-Ca O?dosage,high oscillation frequency,and high temperature were conducive to the precipitation of Cd.Light and common groundwater ions had no effect on the remediation of co-pollution,but HCO₃^-and Mn²⁺had a negative effect on the degradation of PNP,and Fe3?,Ca2?,Mg2?were disadvantageous to the removal of Cd due to precipitation competition.(4)CS/Ca O?and EPO/Ca O?nanomaterials with nucleocapsid structure were prepared,and their particle sizes were 100?200 nm and 200?300 nm,respectively.The introduction of wrapping materials not only improved the p H response,but also reduced nanoparticles agglomeration.The two nanomaterials both had good remediation effects on Cr(VI),Cd(II),Ni(II),Cu(II),Pb(II),Zn(II),As(V),and Hg(II)pollutions(The initial concentrations were 10 mg/L).All the removal rates were more than 95%.Moreover,the nanomaterials also had a good remediation effect in real Cr(VI)contaminated groundwater.The Cr(VI)removal mechanism was predicted to be as follows:Firstly,the shell polymers CS or EPO reacted with H⁺to release the core material Ca O?.Secondly,Ca O?was acidolyzed to produce H?O?.Finally,Cr(VI)in dichromate was reduced to Cr³⁺by H?O?in acid system.p H and oscillation frequency had a great influence on Cr(VI)removal,while temperature and light conditions had no effect.In groundwater,HCO₃^-,Fe³⁺and Mn²⁺were disadvantageous to Cr(VI)remediation.The mechanisms of heavy metals remediation except Cr(VI)were precipitation removal,and Fe addition would promote their remediation efficiency due to ferrite coprecipitation.Moreover,Fe(II)would catalyze CS/Ca O?or EPO/Ca O?to produce OH·,which could be used for the removal of organic pollutants.(5)The products of nano-Ca O?degrading TCEP and their production pathways were studied by using DFT method,which reasonably explained the process of OH·oxidation degradation of TCEP,and provided evidence for the formation mechanism of the related products.Moreover,the correctness of DFT was confirmed by the experiment of capturing the degradation products,which was indicated the mutual support between theoretical calculation and experiment.