Enhanced Removal Of Nitrate And Phosphate From Water Using Biochar-supported Nano Zero-valent Iron

The excessive discharge of nutrients,especially nitrate and phosphate,has posed a serious threat to human health and the environment.Remediation of nitrate and phosphate polluted water has become a problem that people pay attention to,especially in reducing eutrophication of water bodies and protecting the environment.As a promising technology for removing nitrogen and phosphorus,adsorption technology has received extensive attention.Nano zero-valent iron（nZVI）has been widely used to remove pollutants such as nitrogen and phosphorus in the environment due to its environmental friendliness,simple use and low cost.Biochar（BC）is a carbon-rich particle prepared by pyrolysis of biomass under oxygen-limited conditions.Biochar has attracted much attention in the field of pollution remediation because of its developed pore structure,large specific surface area,and rich surface functional groups.Due to the porous structure of biochar,the biochar can be used as a carrier to disperse and stabilize nZVI particles,while slowing down the formation of an oxide film on the surface of the nZVI loaded in the pores to enhance the reactivity of nZVI.This research work covers the synthesis and characterization of nZVI/BC with the aim being to study the simultaneous removal of nitrate and phosphate form aqueous solution by using nZVI/BC.Different laboratory scale experimental works including preparation of adsorbent,adsorption isotherm,adsorption kinetic and effect due to adsorption parameters（initial temperature,contact time,p H value,adsorbent dosage）were carried out to study the feasibility of nZVI/BC for eutrophic water remediation.The three nZVI/BCs samples with nZVI to BC mass ratios of 1:8,1:4,1:2 by weight,were prepared by means of liquid-phase reduction method,thereafter,their capacity to remove nitrates and phosphates ions from water were evaluated.The results of different characterization techniques showed that the reactional properties of nZVI/BC towards contaminants were improved compared to nZVI or BC alone.The experimental results on effect due to mass ratio showed that all the three nZVI/BCs samples exhibited different removal efficiencies of nitrate and phosphate,among which the sample prepared with ratio of 1:2 developed the nitrate and phosphate removal efficiency of89.82%and 97.99%respectively,which was the optimum removal capacity observed.The nZVI/BC（1:2）was then preferred to be used for the next step of experimental tests provided its removal capacity over contaminants.The results showed that the increasing initial concentration resulted in significant decrease on the removal of efficiency of both nitrate and phosphate.The positive correlation was noted between contact time and removal efficiency of both nitrate and phosphate particularly during the first 2 hours according to the result,however,the phosphate removal efficiency was more enhanced than that of nitrate.The nZVI/BC maintained preferable nitrate and phosphate removal of 89.3%-95.0%and 95.2%-99.2%respectively over a wide p H range of 2-10.According the results,the positive correlation was observed between removal efficiency and adsorbent dose with 0.1 g be the optimum nZVI/BC dose note during this study,however,the rate of phosphate removal by nZVI/BC was noted to be higher than that of nitrate.Isotherm experimental data of nitrate and phosphate were well fitted to Freundlich isotherm model,indicating that the simultaneous removal of nitrate and phosphate using nZVI/BC was mainly controlled by multiple processes and that chemical adsorption occurred between adsorbate and the heterogeneous surface.The main products of nitrate reduction considered in this study were:nitrite,ammonium and nitrogen gas.The analysis of nitrate removal by-products selectivity revealed that the 60.5%of removed nitrate selectively became nitrogen gas which represent the majority among other products.The coexistence of nitrate had the promotional effect on the removal of phosphate removal.The results showed that the estimated maximum adsorption capacity for both nitrate（NO₃^-）and phosphate(PO₄^3-)reached 77.26 mg/g and 42.15 mg/g respectively,all the results point out the potential ability of nZVI/BC and its effectiveness for remediation of eutrophic water.