Effect Of Humic Acids On The Adsorption Of Heavy Metals To Natural Surface Coatings

Natural surface coatings exist widely on rocks, surfacial sediments andmineral particles in rivers, lakes, wetlands, estuaries and marine. The surfacecoatings in this research are considered to be the attachments on rocks and othersolid phases in aquatic environment that are composed of metal oxides (such asiron, manganese and aluminum oxides), organic matters and other minerals.Due to the high surface reactivity and large surface areas, the most importantrole of surface coatings in aquatic environments is the adsorption or enrichmentof heavy metals, organic compounds and other pollutants. The adsorptioncapacities of surface coatings are always higher than sediments, suspendedparticles and other solid phases. Natural organic matters (NOM) broadly existin aquatic environment, comprising all detrial organic matter in naturalecosystem, specifically excluding living organisms and compounds ofman-made origin. Humic substances are the most important and reactivelycomponents of NOM. NOM molecules have hydrophilicities, adsorptioncapacities and complexities. They can complex with organic and inorganicmatters, and play important role in the transference and transform of heavymetals. Humic substances are thought to be composed of humic acids (HA) andfulvic acids (FA). HA and FA are distinguished from each other operationallyby the solubility.The researches about the effects of HA and FA on the adsorption of heavymetals to solid phases in aquatic systems are focused on man-made mineralsand clay particles, however, there are few reports about surface coatings. Forthe purpose of comparing with the previous research of our team, and receivingmore information about the behavior of heavy metals in water, HA and FA areisolated from the sediments in Nanhu Lake as the represents of NOM. Batchesof experiments are carried out to study the effects of HA and FA on theadsorption of heavy metals to Nanhu Lake surface coatings.To study the effects of HA and FA on the adsorption of toxic metals onsurface coatings, HA and FA of batch concentrations are added simultaneouslywith Pb, Cd and Cu. The effects on the thermodynamic characteristic of Cdadsorption and the adsorption of Cu, Cd, Pb and Ni with HA and FA addedsimultaneously at different pH values are also investigated. The results showthat the presence of HA and FA decreased the adsorption of these four metals tosurface coatings. There are different effects observed from the four metals, Cuis affected most significantly, and the effects of humus on the adsorption of Cdare the least. The effects of HA and FA are also different. FA decreases more Pb,Ni and Cd than HA. On the contrary, the effect of HA on the adsorption of Cu ismore significantly than FA. The presence of HA and FA also reduce themaximum adsorption amount of Cd. The adsorption amounts of these fourmetals to surface coatings increase with increasing pH value either with orwithout HA or FA in the systems. The addition of HA and FA also decrease theadsorption of these four metals on surface coatings at different pH values.Tests covering Cd or Cu and two types of humic substances are completedunder different pH to observe the effects of loading order on Cd and Cuadsorption. Different results are obtained depending on the adsorption sequencefrom both Cd and Cu. For Cd, the effects of HA and FA are the most significantif they are added before Cd, and minor effects are observed if Cd and humicsubstances are added simultaneously. As for Cu, the effect of HA added with Cusimultaneously is the most significantly. But when FA added simultaneously,the effect is the least among the three adding sequences. It can be deduced thatthe rates of Cd, Cu, HA and FA adsorption to surface coatings are different. Thegeneral trend with respect to adsorption speed being: Cd>HA>Cu>FA. Thereare other two factors contribute to the metal adsorption on surface coatings inthe presence of HA and FA. One is the strength of bond forces of metal-surfacecoatings and humus-surface coatings, and the other one is the rate ofmetal-humus complexion.In a word, the interactions among heavy metals, humus, and surfacecoatings are very complicated. In this process, there are three mechanismsoccurred at one time: heavy metals binding to surface coatings, humus attachingto surface coatings and humus complexion with metals. In the ternary system,the one that is operative and play a dominant role in the three mechanisms leadsto a sequential adsorption amount of heavy metals. The conclusion in this paperlimits to restricted conditions and the humus from given sources. When itcomes to application, the characteristics of solid phases used as adsorbent andthe nature of humus must be considered.The effects of HA and FA on the adsorption of Cu to each component ofsurface coatings are also investigated. The adsorption characters of Cu tounextracted and extracted surface coatings are compared. It can be deducedfrom the results that organic phases contribute most to Cu adsorption, and thecontributions of iron oxides and manganese oxides are relatively small. There isalso a comparison for co-adsorptions of Cu with HA and FA to the surfacecoatings extracted by the same extractant. The analysis found that allcomponents of surface coatings are affected by both HA and FA. For surfacecoatings extracted by Na2S2O4, the effect of HA on Cu adsorption is higher thanFA. But for surface coatings extracted by NH2OH·HCl and H2O2/HNO3, theeffects of HA and FA are almost the same. So it can be deduced that the effectof HA on Cu adsorption to iron oxides maybe greater than FA.