| Soil is the material basis of sustainable economic and social development,and it is related to everyone’s health.Recently,with the rapid development of industry and agriculture,heavy metal contaminated soil has become more and more serious,which has seriously threatened human health and the balance of regional ecosystems.The problem of soil contaminated by heavy metals has attracted increasing attention.In order to protect human health and ecological balanc e,the State Council has implemented the "Ten Articles of Earth" of the national action plan.It is easy for heavy metals to migrate into the soil environment,but heavy metals are not easy to be degraded.Understanding the migration and transformation mec hanism of heavy metals in the soil,the migration of heavy metals in the soil has become the focus of current scholars.However,the effects of dissolved organic matter in soil on the adsorption of heavy metals and the characteristics of soil adsorption when multiple pollutants coexist are lack of timely reported.According to the previous investigation of contaminated sites,this study focused on the effects of dissolved organic matter on the characteristics of lead adsorption in soils and the characterist ics of lead adsorption in soils when multiple pollutants coexist.Using biochar and hydroxyapatite as passivation materials,the effect of passivation materials on Pb2+ contamination in soil was studied.Adsorption and remediation mechanism of biochar and hydroxyapatite on Pb2+ were analyzed,which made the passivation stabilizer play a greater role in remediation of heavy metal contaminated soil.The main results of this study are as follows:1.A majority of adsorption occurred in the first 180 min.Subse quently,the adsorption became equilibrium within about 240 min.Electrostatic attraction led to the initial fast adsorption,which resulted in Pb(II)’s fast transportation from the solutions on the soil’s surface.Ligand and ion exchanges were Pb(II)adsorption’s dominant mechanism,as shown in the below slow adsorption.When lead enter the soil,they diffuse into the pore with the concentration gradient of ions in the aqueous solution,and then they are fixed in the soil.According to the findings,the molecule’s adsorption is an adsorption of monolayer on the surface of adsorbent.In the actual polluted soil,not only the single heavy metal or organic pollutants exist,but also two or more pollutants exist at the same time.In this paper,the adsorption characteristics of lead ion and methylene blue on soils and their coexistence were studied.The initial concentration of lead ion and methylene blue at 50 mg/L,100 mg/L,250 mg/L were selected respectively.The adsorption equilibrium time reached equilibrium within 180 minutes.Langmuir equation harbored a good fitness with both the mono-and multi-experimental outcomes.Based upon the Langmuir isotherm,the maximal adsorption abilities of Pb(II)and MB acquired from binary systems sorption were attenuated compared to those from the single system(Pb(II): 77.70 >65.96 mg g-1;MB: 242.31 >222.36 mg g-1).According to the above experimental results,when the pollutants exist simultaneously,the amount of adsorption of lead ion and methylene blue on soil affe cts each other,which leads to the decrease of adsorption.The reason may be that the competitive adsorption binding sites of lead ion and methylene blue reduce each other.2.Dissolved organic matter in soil can increase the adsorption capacity of soil.After the addition of DOM,the removal efficiencies of Soil + 10 m L DOM and Soil + 5 m L DOM were 94.43% and 89.25%,respectively.Lead ion is adsorbed on soil surface,but some dissolved organic matter complexes with lead ion in solution,which increases the adsorption capacity of lead ion.The results show that dissolved organic matter plays an important role in the adsorption of lead ion in black soil.Three-dimensional fluorescence spectroscopy,synchronous fluorescence spectroscopy and two-dimensional fluorescence spectroscopy were used to study the interaction mechanism between DOM and lead ions.Three-dimensional fluorescence spectroscopy was used to analyze the four main substances in DOM,such as aromatic proteins and tryptophan proteins,humic acids and fulvic acids.The results of synchronous fluorescence spectroscopy showed that humic acid and protein-like substances were the main fluorescent substances of DOM in soil.When the concentration of lead ion is 0 to 100 mg/L,the fluorescence intensity of dissolved organic matter decreases with the increase of the concentration of lead ion.Stern-Volmer equation can be used to model the data of fluorescence quenching to gain better understanding of the quenching mechanism.The quenching rate constants of Kq for peak A and B were 1.38×1011 L/mol/s and 3.93×1011 L/mol/s,severally in the Stern-Volmer regression curve’s linear range.The static fluorescence quenching was the major process of quenching process.To measure the process of static fluorescence quenching process,the modified Stern-Volmer equation.KA was considered as 0.58×103 and 0.51×103 L/mol,which showed the great interactions between Pb2+ and DOM.The maximal binding constants log Kb for peak A and B were 6.89 L/mol and 3.00 L/mol,severally.According to the results,there is one class of binding site for the humic-like substance and there are two classes of binding sites for the protein-like substance.The humic-like substance fluorescence fraction was more susceptible to the decreased intensity of fluorescence on the Pb2+ presence.The positive area centered on the pair of wavelength 346 nm/282 nm.The changes’ order is as below 282 nm < 346 nm.3.Biochar derive from corn straw.The static adsorption experiment of 150 mg biochar was carried out.Within a certain range,the removal rate of lead ion increased with the increase of p H,and p H=7 was the best adsorption environment.The leaching concentration of lead ion was low when the dosage of biochar was 100 mg.In the process of biochar adsorbing lead ion,after 40 minutes,biochar adsorbing lead ion reached equilibrium.In the subsequent experiments,40 minutes was taken as the best adsorption time.The results showed that the adsorption rate of biochar was the main control step.The adsorption isotherms belong to Langmuir model,and the adsorption of heavy metals by biochar is monolayer adsorption.In the process of passivating soil,with the increase of remediation days,the leaching concentration of heavy metal lead ion in soil gradually decreases,leaching lead ion from 16.12 mg/L to 0.598 mg/L.Because of the porous and abundant functional groups on the surface of biochar,biochar passivation remediation of heavy metal lead soil have a better passivation effect.The application of biochar passivation in remediation of heavy metal contaminated soil not only adsorbs heavy metals in soil to itself,but also adsorbs dissolved organic matter in soil to biochar.The relationship between biochar,soil and soluble organic matter was discussed.Soil adsorption of heavy metals would be affected by dissolved organic matter.The more DOM in soil,the greater the adsorption capacity of soil for heavy metals.The existence of DOM can improve the binding ability of DOM and lead,and it can be adsorbed by biochar to increase the adsorption capacity of biochar materials.4.Hydroxyapatite was selected as soil passivator.First,the experimental conditions were optimized,such as the dosage of passivator,p H,time.The results showed that hydroxyapatit e adsorbed Pb2+ to form covalent compounds or chemical adsorption caused by ion exchange.By fitting the isotherms,the adsorption of Pb2+ by hydroxyapatite conforms to Langmuir model.The results show that lead ion adsorbs on the surface of hydroxyapatite in a non-uniform manner through the monolayer.The activity of heavy metal lead ion decreases with the increase of the specific gravity of hydroxyapatite.Weak acid extraction state(CB),Reducible form(OX),oxidizable(OR),Water-soluble(EX)decrease obviously and conver to Residue(RES).Therefore,the increase of the specific gravity of hydroxyapatite effectively reduces the activity o f lead and achieves the passivation purpose.Remediation of heavy metal contaminated soil with hydroxyapatite has obvious passivation effect of heavy metal lead. |
