Permeable Reactive Barrier For Remediation In Groundwater: Exploitation Of Media And Heavy Metal Removal Study

Groundwater,as the primary source of water in many parts of the world,is critical for economic development.However,in recent decades,groundwater pollution caused by industrial and agricultural development seriously posed threats to environmental safety and public health.Relevant studies showed that in northeast China,the areas around machinery manufacturing and non-ferrous smelting sites,groundwater has been widely contaminated with heavy metals and other pollutants including persistent organic contaminants.Heavy metals in groundwater endangers drinking water safety;and excessive intake of metals might damage the neurological system,kidney function,ossification process and various other organs/processes.There are several advantages of permeable reactive barrier（PRB）technology in comparison with other remediation methods such as reactivity,stability and safety.Zero valent iron（ZVI）,an active material widely used in PRB technology,can effectively transform,remove and reduce groundwater pollutants.However,low reactivity due to its intrinsic passive layer,narrow working p H,and lack of sufficient active sites limit its wide-scale application.To overcome these problems,immobilization should be implemented.In our study,five iron-based materials were synthesized by immobilization.These materials can expand working p H of ZVI,provide more adsorption sites and improve the adsorption efficiency of heavy metals.In this study,the preparation and characterization of iron-based material were firstly carried out as well as heavy metals adsorption efficiency analysis.In order to optimize the material,the adsorption behavior of heavy metal ions on five iron-based materials was also studied.The results indicated that chitosan-ZVI,hydroxyapatite-ZVI,β-cyclodextrin-ZVI and sodium alginate-ZVI showed excellent performance for the adsorption of Cr（VI）,As（Ⅲ）,Co（Ⅱ）and Cu（Ⅱ）,respectively.And it was also found that the removal efficiency of four heavy metals by composites could reach 98%at equilibrium.The results on recyclability indicated that the high performance of ion-based materials on the removal of heavy metals were well maintained after five regeneration cycles.The functional groups of composites played an important role in the adsorption of heavy metals;the functional groups of C-O-C,O-C=O,-OH and-NH₃participated in the removal of metal.Electrostatic adsorption contributed hugely in the adsorption of Cr（VI）by chitosan-ZVI.In the next section,iron-based materials were used as the reactive media of PRB to explore dynamic removal heavy metals.According to the experimental results,p H（at 6-9）has a limited effect on the adsorption of four metals.The average removal percentage of Co（Ⅱ）and Cu（Ⅱ）in the PRB constructed byβ-cyclodextrin-ZVI and sodium alginate-ZVI up to 96.52%and 93.64%,respectively.In the multi-metal system,there was competitive adsorption between four heavy metals.However,when chitosan-ZVI was used as the adsorbent,the competitiveness among the different metals was weak,and 95.79%,70.58%,99.80%,92.65%removal of Cu（Ⅱ）,Co（Ⅱ）,Cr（Ⅵ）,As（Ⅲ）were obtained within 8 hours,respectively.In the multi-metal PRB column experiment,the removal efficiency of Co（Ⅱ）was the first to decrease in the column 5-8;meanwhile,the column 8,which with hydroxyapatite-ZVI as media,exhibited the best performance for the adsorption of multi-metals.The removal of Cu（Ⅱ）,Co（Ⅱ）,Cr（Ⅵ）and As（Ⅲ）at 30 days was 88.96%,81.04%,73.34%and 92.37%,respectively.Assembling of media is conducive to the dynamic removal of multi-metals.The assembled PRB column improved the removal of Co（Ⅱ）.In addition,column 9 showed better metal adsorption performance than other columns including column 8.The present study demonstrated that our prepared iron-based materials have excellent removal efficiency for the target heavy metals,and possess good regeneration ability.These composites can serve as PRB media and realize the remediation of groundwater heavy metal pollution;and assembling strategy of PRB active media is beneficial to the multi-metal removal.All above results provide experimental support and theoretical basis for the remediation of heavy metal in groundwater through iron-based material,which have wide engineering practical significance.