Research On Permeable Reaction Wall Materials Used For Prevention And Control Of Groundwater Contaminated By Cr(Ⅵ)and Cd

Groundwater,as an indispensable source of drinking water,due to the mining,metallurgical and other human activities after making excessive heavy metals,can accumulate and cause many diseases in organisms.The permeable reactive barrier（PRB）technology in in-situ repair has the advantages of low investment,low energy consumption and good treatment effect compared with the traditional extraction treatment method.It is more and more used in the prevention and control of heavy metals in groundwater.PRB system consists of a composition filled with reactive material,Therefore,in this paper,the commonly used zero-valent iron and maifanite are used as raw materials,and the organic compound is used to modify the anhydride-modified Fe@Si O₂@PEI particles and the anionic surfactant sodium dodecyl benzene sulfonate（SDBS）modified medical stone.Taking Cr（Ⅵ）in the form of oxygen-containing anions and cationic Cd²⁺as representative pollutants,the effects of temperature,p H,heavy metal ion concentration,and adsorbent dosage on the adsorption performance of the modified adsorbent were discussed.Then mix the two with a suitable mass ratio as a composite adsorbent as a filler for PRB,study the effect of influent concentration and flow rate on dynamic column experiments,and finally use the model to verify the feasibility of PRB.Experimental results show that:（1）The anhydride-modified Fe@SiO₂@PEI particles have an adsorption capacity of 47.638 mg/g and 62.047 mg/g for Cr（Ⅵ）and Cd,which are increased by159.95%and 76.60%respectively compared with the unmodified zero-valent iron when the initial concentration is 300 mg/L and the adsorbent dosage is 0.10 g.Studies have shown that the introduction of a large number of carboxyl groups,amino groups,and hydroxyl groups makes heavy metal ions chelate to these groups,enhancing the removal efficiency of zero-valent iron.The adsorption capacity of the adsorbent for Cr（Ⅵ）and Cd is still above 30 mg/g and 50 mg/g after 4 adsorption-desorption cycles.（2）SDBS-modified medical stone increased the surface electronegativity.Scanning electron microscopy showed that the surface had a layered dispersion structure and the specific surface area increased.XRD and FTIR analysis showed that the introduction of anionic surfactant did not change the material structure and surface groups.The adsorption capacities of Cr（Ⅵ）and Cd on SDBS modified medical stone were 27.776 mg/g and 35.858 mg/g respectively at the initial concentration of 300 mg/L,the dosage of adsorbent of 0.10g and the reaction temperature of 298K.The difference in surface charge makes the adsorbent adsorb Cd higher than Cr（Ⅵ）in the presence of oxygen-containing anions.The adsorption process of Cr（Ⅵ）and Cd by the adsorbent conforms to the quasi-second-order kinetic model and the Langmuir adsorption isotherm model.（3）The composite of SDBS modified medical stone and anhydride modified Fe@Si O₂@PEI with a mass ratio of 5:1 is used as a dynamic test and PRB filler.The removal rates of Cr（Ⅵ）,total chromium,and Cd by the system are 66.51%,61.43,and 68.16%,when the filling height is 90 cm,the influent concentration is 60 mg/L,and the flow velocity is 2 m/d,respectively.The larger particle size of medical stone in the mixed system of the two can increase the effective porosity of the filler and increase the permeability coefficient,delaying the blockage caused by precipitation and the modified zero-valent iron can reduce Cr（Ⅵ）to Cr（III）,which is more conducive to the adsorption of negatively charged SDBS modified medical stone.In the simulated PRB test with initial conditions of influent concentration of 5 mg/L,reaction wall thickness of 40 cm,and influent flow rate of 1 m/d,Cr（Ⅵ）and Cd exceeded groundwater class III water standards at 33 d and 24 d,respectively.But at the end of the test after 60 days,the adsorbent has not completely failed,and the removal rate of the two types of heavy metals still reached 85.94%and 83.45%,with strong removal ability.