Removal Of Nitrate And Phosphate By Strengthening Technology Of Iron-Based Materials:Efficiency And Mechanism

In recent years,the pollution of nitrate and phosphate in water has become serious which would induce eutrophication of water and threat the environment as well as human health.Therefore,the pollution of nitrate and phosphate needs to be solved urgently.Iron is widely distributed in nature and is the fourth most abundant element in the earth's crust.Iron-based materials,such as zero-valent iron?ZVI?and feroxyhyte??-FeOOH?,have attracted attention because of their good affinity to pollutants,abundance,cheapness and reproducibility.This paper mainly includes the following two parts:first,removal of nitrate from water by acid-washed zero-valent iron/ferrous ion/hydrogen peroxide;second,synergistic effects of mesoporous feroxyhyte nanoparticles and Fe?II?on phosphate immobilization.Meanwhile,the mechanisms of nitrate and phosphate removal were studied by Brunauer-Emmett-Teller method?BET?,X-ray diffraction?XRD?,Scanning electron microscope?SEM?,Transmission electron microscopy?TEM?,Fourier transform infrared spectroscopy?FTIR?and X-ray photoelectron spectroscopy?XPS?.The main conclusions of this paper are as follows.?1?Under different initial nitrate concentration?0.7,0.9,1.2 mmol/L?,nitrate removal efficiency achieved more than 99.0%within 6 h using 89.3 mM acid-washed ZVI,4.0 mM Fe?II?,and 10.0 mM H₂O₂.Nitrite was the intermediate product of nitrate reduction while ammonium and nitrogen gas were the end product.?2?Hydroxyl radical has strong oxidation ability which can facilitate the corrosion of zero-valent iron.In addition,trivalent iron ions produced by Fenton reaction and excessive hydrogen peroxide in the reaction will also promote the corrosion of iron.At last,the oxide layer mainly consisting of Fe₃O₄ formed on the surface of zero-valent iron.?3?The prepared?-FeOOH is a mesoporous nanomaterial with an average pore size of 9.5 nm.Its BET specific surface area is 171.5 m²/g and zero potential charge is7.9.There are a large number of hydroxyl groups on its surface,which are beneficial to phosphate adsorption.?4?When initial pH was 3.0,the dosage of?-FeOOH was 2.0 g/L,and the dosage of Fe?II?was 0.5 mmol/L,the removal rate of phosphate by?-FeOOH/Fe?II?system reached 94.1%after 2 h reaction?initial P concentration was 40.0 mg/L?.The Freundlich isotherm model and pseudo-second-order model were more appropriate to describe the reaction process.The reaction process was found to be endothermic,spontaneous and entropy-increasing.?5?The mechanism of phosphate immobilization by?-FeOOH/Fe?II?system mainly includes two aspects:on the one hand,the phosphate was attracted to the?-FeOOH surface because of the electrostatic attraction force between phosphate and positively charged functional groups on the?-FeOOH surface.Then the hydroxyl groups on the?-FeOOH surface were superseded by phosphate through ligand exchange.On the other hand,the Fe?II?in solution was quickly oxidized into Fe?III?in presence of?-FeOOH.The in-situ generated Fe?III?in solution combined with phosphate easily and formed precipitates in aqueous solution.