The Binding Of Hg To Humic Acid And Humic Acid-ferrihydrite Associations In Soil

Humic substances（HS）consist of abundant functional groups,such as carboxylic,carbonylic,methoxy,phenolic and sulfur groups.Significantly,sulfur groups are regarded as the dominant groups and proved to have the preference for Hg complexation compared to oxygen-containing groups.The concentration ratio of Hg/HS is very important as it affects the binding constant of Hg to organic matter.Under the condition of Hg concentrations in excess of sulfur groups concentrations in HS,it is expected that a small fraction of the total Hg saturates all HS sulfur groups,whether the majority of Hg will bind to oxygen functional groups that are present in HS at much higher concentrations than sulfur groups has not yet been systematically explored.Besides,Iron（hydr）oxides and natural organic matter（NOM）,both the important components in soil,are widely present in the form of colloidal particles.They can interact and combine with each other and,consequently,the formed iron（hydr）oxides-NOM composites can provide a solid foundation for controlling the mobility,transformation and speciation of Hg in soil.Therefore,humic acid（HA）was selected as target HS,the nature of Hg binding at high concentration ratio of Hg/HA was investigated through batch sorption experiments.The behavior of both Hg and HA in soil under high Hg to HA concentration ratios was simulated in a climate chamber through pots tests.Besides,many cases suggested that humic substances application in soil could stimulate plants growth,but we don’t confirm its effects on plant when exposed to Hg contamination at high Hg to HA concentration ratios.Thus,the contribution of HA on Hg accumulation and bioavailability in rice was performed based on culture experiments in greenhouse aimed to examine whether HA application could reduce Hg toxicity in plants under the given high Hg to HA concentration ratios.We hypothesize that HA has great potential to Hg binding under high Hg to HA ratio,it can also reduce mobility of Hg in soil and therefore inhibit the uptake of Hg by rice plants.Ferrihydrite was chosen as a typical iron（hydr）oxide.The effect of complexing way and morphology contribute to Hg adsorption on ferrihydrite-humic acid composite were analyzed using batch adsorption and kinetic sorption experiments combined with X-ray spectroscopy.The results of this study present new perspectives for understanding the mobility and fate of heavy metal in soil,and are of use to the design of remediation strategies for heavy metal contaminated.The main results are as follows:（1）Under high Hg to HA concentration ratios,the relatively high sorption capacity of26.07 mg/g for Hg due to ion diffusion,chemisorption and dissociation of acidic groups was determined through batch sorption tests.It also proved that the ion strength and solution p H were main factors influencing on Hg sorption resulting from the competition of sorption sites and neutralization of the surface charge.（2）The application of HA can effectively reduce Hg mobility in spiked soils when concentration ratios for Hg to HA ranged between 10-50μg of Hg/mg HA.All TCLP Hg contents were declined to 0.09-0.12 mg?L^-1 and 0.18-0.21 mg?L^-1 for initial Hg contents at50 and 500 mg?kg^-1 and could obey the TCLP maximum concentration criteria value.（3）The positive influence of HA on plant biomass and Hg accumulation in plant organs under these high Hg/HA concentration ratios in water and soil culture were visibly.The promoting effect was mainly performed in tillering stage with increase of 33.19%and 26.17%for fresh weight of straw and root,and 20.67%and 14.18%for dry weight.For different rice organs,the Hg concentrations are in the order of roots>straw>grains.The key role of HA was complexation with Hg via adsorption and/or co-precipitation in a direct way,and thus indirectly decrease the Hg stressing effect to rice plants,as was confirmed through SEM analysis of root tissues.In a word,the utilization of HA at high concentrations ratios of Hg to HA is significantly effective at reducing Hg mobility and bioavailability in soil-plants systems.（4）The binding differences of Hg on adsorption and coprecipitation ferrihydrite–humic acid composites were evaluated.XRD data show a weaker crystal structure of ferrihydrite,and a new peak at 0.49 nm is observed.The carbon content is more in coprecipitate complexes than adsorption ones,but not the specific surface and pore volume.The higher C loading and diffusion rate lead to an increase of adsorption rate in coprecipitate,but not yet the Hg adsorption capacity.The adsorption capacities of two complexes were negatively correlated with C/Fe molar ratio.When the C/Fe molar ratio was 0.1,the maximum adsorption capacities of adsorbed and co-precipitated composites were 119 mg?g^-1 and 110mg?g^-1.In the ternary complexes of ferrihydrite–humic acid-metal,O-C=O、C-O and C=O of HA and Fe（OH）O and Fe-O-Fe of Fh all participated in the adsorption of Hg.The contribution of ferrihydrite and humic acid alone on Hg adsorption of two complexes are different,and of which coprecipitate complex play the primary role.（5）The effects of morphologies on Hg binding to ferrihydrite-humic composites were obtained.Transmission electron microscopy images revealed the gel-like ferrihydrite is loosely aggregated with open structure,while the dense ferrihydrite compactly aggregated with more consolidated and thicker structure.The structure of gel-like increased the Hg adsorption rate and adsorption capacity,but not the rate of desorption.With the increase of C/Fe molar ratio,the adsorption capacities of two complexes decreased.When the C/Fe molar ratio was 0.1,the maximum adsorption capacities of gel-like and dense composites were 135mg?g^-1 and 132 mg?g^-1.Compared to dense composites,the function groups such as carboxyl and aromatic groups involved in Fe coordination were more in gel-like composites.In addition,for gel-like composites,more hydroxyl and carboxyl groups of HA are found to involved in the binding with Fe-containing groups on ferrihydrite surface.The binding mechanism of Hg on gel-like complex and dense ones are different.The Fe O-Hg-HA complexes are speculated to form in gel-like and dense composites.Besides,the morphologies of Fh can affect the distribution of carbon bonds and amount of Fe-containing groups on Fh surface involved in HA complexation.