Effect Of Humic Acid In Water On NO₃^-/Pb²⁺Removal By Resin-based Nano-Zero-valent Iron Composites

Lead ions and nitrate,as common pollutants in the environment,cause various extent damage to the growth and reproduction of plants and animals.Since the 1990s,nano zero-valent iron(nZVI)has been widely used in water/soil remediation due to its large specific surface area and strong reduction capacity.The use of ion-exchange resin as the carrier of nZVI not only overcomes the disadvantage that nZVI is easy to aggregate and difficult to achieve solid-liquid separation,but also provides sufficient sites for the nZVI particles due to the resin-rich internal network pore structure and its inherently charged functional groups.The nZVI particles can be uniformly dispersed to keep the smaller nZVI particle size,thereby exerting a higher activity,and also have a strong adsorption effect on the target pollutants,thereby greatly improving its removal efficiency of the pollutants.However,the stability of nanocomposites in practical applications is also related to the substances contained in groundwater.In particular,humic acids,which is the most widely distributed natural organic matter(NOM)on the earth,is present in soil,surface water,and groundwater and has a rich functional group that can interact with metal ions and can also be adsorbed on the anion resin,but there is no research about the effect of HA on the performance of ion-exchange resin-supported nZVI composites in removing nitrate or lead ions.Firstly,the way of fabrication of resin-based(D201/D001)nano-zero-valent iron(nZVI)was optimized and was successfully applied to the removal of NO3-and Pb2+.Then,humic acid(HA)was used as an interfering substance to investigate its effects on the removal of NO3-and Pb2+from D201-nZVI and D001-nZVI.The mechanism of the influence of HA on the nZVI particle structure,distribution,and activity of D201-nZVI and D001-nZVI was preliminarily elucidated,and the relationship between the concentration of HA and the distribution of nZVI on the support was analyzed.The mechanism of the effect of HA on the removal of NO3-and Pb2+ by D201-nZVI and D001-nZVI was initially revealed,thus providing a theoretical basis and technical reference for the effect of organics in water on the removal of contaminants by similar organic carrier-inorganic metal materials.The hybrid D201-nZVI and D001-nZVI were fabricated by two steps,including ion exchanging and chemical reduction.Through the adjustment of the ratio of precursor solution,NaBH4 concentration and reduction time,BET(Brunauer Emmett Teller),XRD(X-ray diffraction),TEM(Transmission electron microscope),and XPS(X-ray photoelectron spectroscopy)were used to characterize the morphological structure of the composites,thereby the way of fabrication of D201-nZVI and D001-nZVI were optimized.Firstly,for the fabrication of D201-nZVI,the ratio of precursor solution should be composed of 2 mol/L FeCl3,1 mol/L HCl,and 10%C2H5OH,while ensuring that NaCl is saturated.And the reducing agent NaBH4 concentration should be 3%and the reduction time is 15 min.The iron content of the composites obtained by this method can reach to 14.43%,and the nZVI particle size was about 10 nm.In addition,for the fabrication of D001-nZVI,the precursor solution is 2 mol/L FeCl3,and the NaBH4 concentration should be 4%,and the reduction time is controlled at 30 min.ZVI immobilized on D001 also reached the nanoscale,with a minimum particle size of up to about 5 nm,and the iron content of the composites was about 11.2%.To explore the interaction between HA and D201-nZVI or D001-nZVI,and the effect of target pollutants NO3-and Pb2+ on these interactions,the fresh composites was added into different concentrations of HA,respectively.The result shows that HA can be adsorbed by D201-nZVI,and the mechanism includes electrostatic attraction between HA and D201 resin and electrostatic adsorption or complexation of HA on the surface of nZVI particles.The HA coating formed by the electrostatic adsorption of HA on the surface of the nZVI oxidized shell can make the nZVI particles more dispersed,which is beneficial to increase the removal rate of pollutants.There is no effect between HA and D001,but the electrostatic attraction between HA and nZVI oxide layers should not be ignored.The competitive adsorption of HA and target contaminants on the surface of the resin or nZVI oxidized shell may reduce the removal rate of the corresponding contaminants;There is no reaction between HA and NO3-,and the complexation of HA and Pb2+ may increase the removal rate of Pb2+.It was further found that the effect of HA on the reduction of nitrate by D201-nZVI was concentration-dependent.At low HA concentration(<5 mg/L),HA coating formed by the HA adsorption on the surface of the nZVI particles enhanced the dispersion of the particles,which leaded to a more evenly distribution of nZVI particles in the solution,and thus a higher nitrate reduction activity,and the reduction rate is accelerated.When HA concentration was increased to 5 mg/L or more,the competitive adsorption of HA and NO3-on the surfaces of the D201-nZVI dominated,and ammonia nitrogen production and nitrate reduction rate were all decreased.When the HA concentration reached to a further high level(>20 mg/L),HA acted as an electron shuttle to accelerate the reduction of Fe(?)to Fe(?)in the D201-nZVI,and thus the nitrate reduction rate was accordingly enhanced,and the reduction rate is increased.The ammonia production increased by 24.8%at HA concentration of 40 mg/L as compared with that of the control(without addition of HA).At the same time,the effect of HA on the removal of lead ions by D001-nZVI is also related to its concentration.When the concentration of HA was low(0?30 mg/L),there was adsorption competition between HA and Pb2+ on the surface of nZVI particles,so the removal efficiency of Pb2+ with D001-nZVI increased with HA.When HA concentration is more than 30 mg/L,the negatively charged HA can be adsorbed on the surface of nZVI particles and form a layer of organic macromolecular coat,which makes the particles more dispersed and the activity is improved.Therefore,the removal efficiency changes little.In addition,the adsorption complexation between HA and Pb2+also played an active role in the removal of Pb2+ with higher concentrations of HA.