Performance And Mechanism Of Nano-zero-valent Iron Supported By Attapulgite For The Removal Of Nickel And Nitrate Nitrogen Complex Pollutants In Water

With the development of today’s society,heavy metals as well as inorganic salt wastewater discharge is increasing,nickel（Ni）and nitrate nitrogen（NO₃^--N）have become an important pollutant in the process of industrial and agricultural production in China,nitrate and heavy metals are common compound pollution.Nickel is a very toxic substance,excessive intake of nickel can lead to inflammation,lead to neurological weakness,and even induce cancer,nickel on the human body is extremely harmful.Nitrate nitrogen is one of the common pollutants in groundwater.Excessive consumption of groundwater containing nitrate nitrogen can cause harm to human body.Nitrate nitrogen is one of the common contaminants in groundwater.Excessive consumption of groundwater containing nitrate nitrogen can be harmful to human body.Nitrate enters the body and is reduced to nitrite,which converts hemoglobin in the body into methemoglobin,causing the blood cells to lose their ability to transport oxygen,which can lead to death in severe cases.In this paper,to address the pollution problem of NO₃^--N and heavy metal Ni in water,nano zero-valent iron（nZVI@ATP）loaded with albite was prepared by liquid-phase reduction method using albite from Linze County,Gansu Province as raw material to improve the easy aggregation phenomenon and oxidation of nZVI and increase its performance in the removal of Ni²⁺as well as NO₃^--N.The nZVI@ATP with different Fe/ATP mass ratios were prepared by liquid-phase reduction method,and sequential batch experiments were performed to determine the optimal loading ratio and to fit the kinetic model.The effects of different influencing factors（initial concentration,dosage,p H,dissolved oxygen,coexisting ions and recyclability of the materials）on the removal performance of nZVI@ATP were investigated.The apparent morphology,size,surface structure,functional groups and chemical composition of the materials were combined with SEM-EDS,XRD,XPS,BET,FT-IR and Zeta characterization to investigate the removal mechanism of nZVI@ATP on Ni²⁺and NO₃^--N single pollutants.The removal performance of nZVI@ATP on Ni²⁺and NO₃^--N complex pollution was also explored to analyze the synergistic removal mechanism of ATP and nZVI.The evolution of nitrogen in the reaction process of NO₃^--N removal by nZVI@ATP was studied to optimize the nitrogen selectivity for NO₃^--N removal.The results of the study are intended to provide a novel and efficient composite material for the removal of nickel ion and nitrate complex pollution in water.The results obtained from the study are as follows:（1）Characterization of the original ATP stone by SEM-EDS,XRD,FT-IR and Zeta potential characterization.The surface of ATP is rough and uneven,with a massive distribution,and its surface contains C,O,Mg,Al,Si,K,Ca,etc.The main components of ATP are graptolite,Si O₂and the surface of ATP contains-OH,Si-O-Si,C=O and Si-O bonds,and its surface is electrically neutral at p H=3.The nZVI@ATP was prepared by loading nZVI onto ATP using the liquid phase reduction method,and the prepared nZVI@ATP showed a significant increase in the specific surface area relative to nZVI,and the nZVI agglomeration phenomenon was significantly weakened.（2）The adsorption performance of nZVI@ATP on Ni²⁺was significantly improved compared with both the original ATP ore and pure nZVI.The removal capacity of nZVI@ATP on Ni²⁺was the highest at 143.2 mg/g when the iron-soil ratio was 2:1.The adsorption process of nZVI@ATP on Ni²⁺was in accordance with the pseudo-second order model,and its removal rate of Ni²⁺was inversely proportional to the initial concentration and directly proportional to the dosing amount.The removal capacity of nZVI@ATP showed a trend of increasing and then decreasing with the increase of p H.The coexisting ions such as Ca²⁺,Mg²⁺and SO₄^2-inhibited the removal of Ni²⁺by nZVI@ATP,and the inhibition effect was from strong to weak in the order of Ca²⁺>Mg²⁺>SO₄^2-.The recycling experiments showed that the material still had a high removal capacity for Ni²⁺after the first cycle,and the removal capacity of the material gradually decreased with the increase of the number of cycles.nZVI@ATP and Ni²⁺removal mechanisms are mainly adsorption,reduction,precipitation and co-precipitation.（3）The Fe/ATP mass ratio of 2:1 was the best loading ratio for NO₃^--N removal by nZVI@ATP,with a removal capacity of 24.36 mg/g and a removal rate of 81.23%,including40.92%conversion of nitrogen.In the process of NO₃^--N removal by nZVI@ATP,nZVI provides electrons for the reaction and reduces NO₃^--N to NH₄⁺-N,NO₂^--N and N₂sequentially,and the content of NO₂^--N after the reaction is very low（less than 3.5%）.The removal process of NO₃^--N by nZVI@ATP was in accordance with the pseudo-second order,and its removal rate of NO₃^--N was inversely proportional to the initial concentration and directly proportional to the dosing amount.The removal efficiency of nZVI@ATP for NO₃^--N was high in the p H range of 2 to 9,with the highest selectivity for the conversion of NO₃^--N to N₂at p H 5.The coexisting ions such as Cl^-,SO₄^2-and CO₃^2-all had an inhibitory effect on the removal of NO₃^--N with SO₄^2-having the strongest inhibitory effect.The recycling experiments revealed that the nZVI@ATP still had 72.34%removal efficiency for NO₃^--N after the first recycling,and its adsorption capacity for NO₃^--N was 17.63 mg/g.The more times the nZVI@ATP was reused,the lower its removal rate for NO₃^--N.After the third recycling,the adsorption capacity of the nZVI@ATP for NO₃^--N decreased to 6.09 mg/g.nZVI@ATP’s main mechanism for NO₃^--N removal was adsorption and reduction,and its products were mainly NH₄⁺-N,NO₂^--N and N₂.（4）The removal efficiency of Ni²⁺in the composite contamination of NO₃^--N and Ni²⁺by nZVI@ATP decreased compared to the single system,while the removal efficiency of NO₃^--N increased compared to the single system because of the reduction of Ni²⁺removal capacity due to the competition between NO₃^--N and Ni²⁺for the active sites on the material surface.Part of Ni²⁺was reduced to Ni⁰,Ni⁰and Fe⁰formed a bimetal,which promoted the electron transfer on the material surface and effectively slowed down the oxidation of the material,further promoting the NO₃^--N removal performance of the composite.