Effects Of Coal Humic Acid And Ferrihydrite And Their Coprecipitate On Lead's Speciation And Bioavailability In Soil

Plant growth and human health threatened by the soil heavy metal pollution.The recovering of the contaminated soil has become a hotspot in related research field.Humic acid and iron oxide are the important influencing factors of soil properties and soil heavy metal migration and transformation,both of them can affect soil lead morphology and activity directly and indirectly.The coprecipitate of the humic acid and iron oxide have good adsorption effect on heavy metals' availability in solution.However,there are few studies on the changes of soil heavy metal morphology and bioavailability in soil.Lacking of the optimal passivation ratio of soil heavy metals' research under the combined influence of humic acid and iron oxide.The passivation mechanism is not complete yet.Therefore,it is of great significance to explore the effect of the complex of humic acid and iron oxide on the transformation and bioavailability of heavy metals in contaminated soil on assurance of crops quality and weakening toxicity of heavy metals.In the experiment,soil culture experiment was used for investigating the effects of addictives on soil heavy metal morphology.The surface characterization and chemical properties of additives' were analysized by Search Engine Marketing(SEM),X-ray diffraction(XRD),infrared spectroscopy(FTIR)and Specific surface area analyzer(BET).The effect of humic acid,ferrihydrite and humic acid-ferrihydrite coprecipitation on lead speciation in soil was detected by developed BCR method.The best concentration of lead in the passivated soil and the optimum ratio of carbon to iron was detected.The mechanism of the additive on the soil lead was discussed.The effect of additives' bioactivities passivation ability had been analysized by single step immersion to extract leachable lead concentration in the soil.The main conclusions are as follows:(1)The addition of coal humic acid reduced the specific surface area and capacity of the coprecipitate.The coprecipitate's pore size is bigger.And the surface of the coprecipitate is rougher than the surface of the ferrihydrite.(2)The effect of humic acid and ferrihydrite on the p H of soil was not obvious.The carbon-iron complex decreased the p H of soil weakly.Soil electrical conductivity increased with increasing of lead's concentration.The effect of humic acid on the soil conductivity was not changed irregularly.The conductivity of low-concentration lead's soil was inhibited byferrihydrite.The conductivity of lead-rich soil was enhanced.Soil electrical conductivity increased due to the increasing of the ratio of carbon and iron in the coprecipitate.(3)The exchangeable speciation of soil lead was decreased after adding humic acid.But the content of the reduced form,the oxidizable form and the residual lead's changes were unstable in the different additives' amount and different concentration of lead in soil.(4)The low amount of ferrihydrite promoted the exchangeable state and the recoverable state change into the oxidizable state and the residual state.After adding the amount of ferrihydrite the first three speciations were transformed into the residual speciation.(5)Under the low ratio of carbon iron(0.0214 ~ 0.3215),the other three speciations were gradually converted to the residual speciation.The degree of conversion from large to small were the oxidation speciation,exchangeable speciation,and reduced speciation.Under the lower Pb concentration,the deactivation effect was best when the ratio of carbon and iron is the smallest.The passivation speciation was the highest when the carbon and iron ratio is0.1072.(6)The smaller of the coprecipitate's addition,the more residue when the ratio of carbon and iron was between 0.2500 to 2.0000.The best passivation of the ratio of carbon and iron apeared at 0.2500.The coprecipitate's passivation is inhibited by the complex factors of soil,and the passivation effect of ferrihydrite is better than coprecipitate.(7)Coprecipitate was more effective than humic acid on reducing the soluble lead in soil.And the lower ratio of the carbon and iron,the better passivation effect.Under the same ferrihydrite addition,the coprecipitate's passivation effect is better than the ferrihydrite.