Silicate-regulated Iron(manganese)environmental Remediation Process And Mechanism

Developing and improving efficient,economical,and environmental-friendly groundwater remediation materials and technologies are vital to improve the quality of ecological environment.Iron,a non-toxic and the second most abundant metal,has played a fundamental role in the remediation of groundwater.However,problems such as easy deactivation,passivation,agglomeration,low electron utilization rate,and poor oxidation ability limit its further application.Permanganate（Mn（VII））based in-situ injection technology is commonly used for in-situ chemical oxidation（ISCO）remediation of groundwater,and the key is improving the oxidation efficiency of Mn（VII）and alleviating the blockage of manganese dioxide.Silicate,a background component of natural water,is reportedly to strongly interact with iron and manganese oxides/minerals,affecting surface chemistry of the processes in natural and engineered environments.However,there is little clarity regarding the effect of silicate on the redox capability of iron（manganate）and on its efficiency for remediation applications.Therefore,this thesis introduces silicate into the environmental remediation processes by iron（manganese）,aiming to expand its potential application in the field of environmental remediation.The specific research contents of this dissertation are as follows:1)Developing a synthesis method of siliconized micron zero-valent iron(Si-ZVI^BM)material by mechanical ball-milling.Using micron reduced iron powder as raw material and crystalline layered sodium disilicate（DS）as process control agent,Si-ZVI^BM was successfully prepared through high-energy mechanical ball-milling.The Si-ZVI^BM could achieve rapid and efficient removal of Cr（VI）,and its removal rate was 45-fold higher than that of non-siliconized ball-milling zero-valent iron(ZVI^BM).By analyzing its morphology,crystalline state,surface structure,and microstructure of the shell oxide and exploring its interaction mechanism with Cr（VI）,it was revealed that the efficient Cr（VI）removal performance of Si-ZVI^BM was mainly due to the following factors:The electron transfer rate improved by special siliconized shell on surface of Si-ZVI^BM,Cr（VI）interface mass transfer behavior,and the enhanced reduction and removal of chromium by various ferrous components present in the system.Finally,a strategy of ball-milling silicidation pretreatment was proposed to modify the surface interface of micron zero-valent iron,regulate the electron transport rate of zero-valent iron,and directionally improve the reducing activity of Cr（VI）.2)Further expanding oxidation performance of Si-ZVI^BM,through the persulfate（PS）-mediated method with the enhanced electron utilization rate of Si-ZVI^BM.The degradation rate of sulfamethazine（SMT）by Si-ZVI^BM/PS system was 12-fold higher than that of ZVI^BM/PS.By analyzing the surface corrosion behavior of Si-ZVI^BM in the presence of PS,it was revealed that PS as an electron acceptor significantly increased the electron transfer rate of Si-ZVI^BMand the release of ferrous iron.After receiving electrons,PS was activated to form free radicals,thereby achieving efficient removal of SMT.Free radical quenching experiments and electron spin resonance spectrum analysis demonstrated that sulfate radical and hydroxyl radicals were the dominant reactive oxygen species in the system.Controling the Fe:Si ratio in the synthstic process could change the Si-ZVI^BM electron transfer rate and surface-bond ferrous content,thereby achieving the regulation of the oxidation performance of Si-ZVI^BM/PS system.The synergistic effect of Si-ZVI^BM and oxidant has established a ZVI oxidation remediation system dominated by oxidation reaction.3)Enhancing the oxidation efficiency of ZVI/ferrous under aerobic condition by using dissolved disilicate as iron ions ligand.We propose a dissolved disilicate-assisted Fe/Al electrolysis（D-Fe Al-E）process for the degradation of p-arsanilic acid（p-ASA）and the subsequent immobilization of inorganic arsenic.Results showed that,in the iron anode electrolysis process with disilicate,p-ASA was degraded by 92%under a near-neutral condition.The efficient degradation of p-ASA could be attributed to the disilicate-coordinated electrolytic ferrous ions,which activated dioxygen to produce more reactive oxidizing species（e.g.,^·O₂^-,H₂O₂ and·OH in this study）to attack p-ASA molecules.Following the first stage,the produced inorganic arsenic and other intermediates can be further removed in the Al anode electrolysis,via the coagulation effect initiated by the electrogenerated hydroxylated aluminum species.Electrogenerated Al（III）ions hydrolyzed into positively charged monomeric/oligomeric Al species,resulting in the dissociation of disilicate-Fe（III）complexes,and the formation of hydroxides and oxo-bridging polynuclear entities for arsenic immobilization.Leaching stability tests suggested that the D-Fe Al-E process was superior to the conventional electrocoagulation method with respect to the stability of the generated arsenic-containing solid sludge.4)Dissolved silicate is an important background constituent of natural waters,which affect the redox performance of ZVI,but there is little clarity regarding the effect of silicate on the oxidizing capability of Mn（VII）and on its efficiency for remediation applications.We found that dissolved silicate,metasilicate or disilicate,could significantly promote the oxidation of 2,4-dichlorophenol（2,4-DCP）by Mn（VII）,and the extent of promoting effect was even more evident than that of pyrophosphate（PP）.Unlike PP,DS was not capable of coordinating with Mn（III）ions,and the promoting effect of DS was not due to the oxidizing capability of complexed Mn（III）.Instead,DS ions,as a weak base,could combine with the hydroxyl groups of Mn O₂ via hydrogen bonding to limit the growth of colloidal Mn O₂particles.The DS-stabilized colloidal Mn O₂ particles,with hydrodynamic diameters less than100 nm,could act as catalysts to enhance the oxidation of 2,4-DCP by Mn（VII）.The best promoting effect of DS on the performance of Mn（VII）oxidant was achieved at the initial solution p H of 7,and the coexisting bicarbonate ions further improved the oxidation of 2,4-DCP in the Mn（VII）/DS system.Sand column experiments showed that the combined use of Mn（VII）and DS additive could mitigate the problem of permeability reduction of sand associated with the retention of Mn O₂ particles.This study not only deepens our understanding on the role of dissolved silicate in a Mn（VII）oxidation process,but also provides an effective and green method to enhance the oxidizing capacity of Mn（VII）-based treatment systems.