Adsorption Behaviors Of Toxic Heavy Metals On Citric Acid-schwertmannite Complex

Acid mine drainage(AMD)originate from the weathering,oxidation and rain washing of sulfide minerals.AMD is a serious threat to ecosystem and human health due to its low acidity and high concentration of sulfate and heavy metals.Schwertmannite(Sch)is one of the secondary iron hydroxy sulfate mineral in AMD environment with great adsorption abilities for heavy metals.Citric acid(CA-Sch)is a typical natural organic matter and widely distributed in various soil and aquatic environments,microorganism and plants would secret CA to reduce the harm of heavy metal and ensure their normal growth and development in high heavy metal concentration environments.CA could combine with minerals and affect its adsorption characteristics of heavy metals.Therefore,clarifying the difference in the adsorption of heavy metals on the complex formed by the combination of natural organic matter and Sch will help to better understand the impact of impure Sch on the migration of heavy metals in the actual AMD environments,and provide guiding significance for the risk prediction and remediation of heavy metals in AMD contaminated areas.The main research results were as follows:(1)The characteristics of citric acid-schwertmannite complex: In this study,synthesized Sch was added to different concentration(CA)solutions to prepare citric acid-schwertmannite complexes.Results of X-ray diffraction(XRD)and Scanning electron microscopy(SEM)images show that complexes maintains the original mineralogical characteristics of Sch,and CA would not dissolve the Fe(?)of Sch,confirming the existence of the complex formed by the combination of CA and Sch in the actual environment.As the CA content of complex increased,its sulfate content and zeta potential would decrease,while the particle size,specific surface area and pore volume would increase.Results of Fourier transform infrared(FTIR)spectra showed that carboxyl groups could be found in the complexes,and the intensity of both the adsorbed and structural sulfate peaks were decreased,indicating the binding process of CA was accompanied by the release of different forms of sulfate.(2)The adsorption and reduction of Cr(?)on the complexes: As the CA content increased,the maximum Cr(?)adsorption capacity of the complexes decreased,the adsorption isotherm data fitted the Langmuir model,indicating the monolayer adsorption of chemical effect.The kinetics data showed the adsorption equilibrium time for CA-Sch was about 120 min and fit pseudo-second-order model which the chemical adsorption was dominant,and the rate constant was negatively correlated to the CA content.Cr(?)adsorption capability was also inversely correlated to the CA content of complexes at different p H value,and the optimal adsorption p H was decreased due to the lower zeta potential of CA-Sch.Results of X-ray photo electron spectroscopy(XPS)showed that CA was adsorbed to Sch by surface complex and result in the complexes tented to adsorb Cr(?)by sulfate exchange due to the occupation of surface sites on the mineral by CA,the adsorbed Cr(?)would be partially reduced to Cr(?)by carboxyl groups of CA,and the reduction ratio is positively related to the content of CA while the reduced Cr(?)would not released into solution.(3)The adsorption of Cd(II)?Pb(II)?Zn(II)?Cu(II)on the complexes: The adsorption capability of Cd(II)on complexes would increase with the increasing Cd(II)concentration or decreasing p H value.The Pb(II)adsorption amount of complexes was lower than that of Sch at different Pb(II)concentration and p H,and the higher CA content the stronger inhibition effect on adsorption,indicating CA competed the sorption sites with Pb.The adsorption amount of Zn(II)on complexes would increase compared to that of Sch at Zn(II)concentration lower than 20 mg/L but decrease at concentration higher than 50 mg/L.At different concentration and p H,adsorption of Cu(II)on complexes would decrease compared to Sch,the inhibitory effect was positively related to CA content due to sorption site competition,and the poor adsorption difference of the complexes at p H 6.0 justified the specific Cu(II)sorption on Sch.The coexisting sulfate will inhibit Cd(II)adsorption on complexes,promote the adsorption of Pb(II),Zn(II),and Cu(II),and the promoting effect would be lager as sulfate concentration increase.To sum up,in AMD environments containing various heavy metals,natural organic matter would affect the interaction between Sch and heavy metals after combining with the mineral.For Cr(?),although the adsorption capacity decreases,there is a reduction mechanism to reduce its environmental toxicity.For Cd(II),Pb(II),Zn(II)and Cu(II),CA content,metal concentration,p H value and coexisting sulfate would result in different sorption behavior of complexes compared to original Sch.This study is helpful to understand the influence of natural organic matter on the adsorption of heavy metals on Sch,and provides theory for pollution assessment and remediation of AMD environment.