Influence Of Proteins On The Adsorption Of Heavy Metal Ions Onto Manganese Oxide-water Interface

In general, the adsorption of heavy metal ions onto environmental particulates controls their concentration, transportation, transformation, bioavailability and toxicity in the environments. Natural organic material (NOM) is one of the constituents in environmental system such as soil, water and sediment. Many properties of NOM such as category, concentration and shape, affect the migration, transformation, bioavailability and biological toxicity of heavy metal ions. Protein is a vital macromolecule involved in NOM. Protein is composed of many amino acids and there are a number of function groups, e.g. amino groups, carboxyl and hydroxyl groups, which are relation to the fate of organic pollutants, migration and transformation of heavy metal ions.Three kinds of manganese oxides with different crystal structures were synthesized. Using the bovine serum albumin (BSA) and lysozyme (LSZ) as model protein, we studied the influence of protein on the adsorption equilibrium of Cu²⁺, Cd²⁺, Zn²⁺ and Ni²⁺ onto manganese oxides. The results showed that the presence of BSA decreased the adsorption of the heavy metal ions onto manganese oxides and there were different effect on different metals. The effect of BSA and LSZ was also different. In addition, different adding order had different effect on the adsorption of heavy metal ions. It was found that the complicated reactions of adsorption and complexation occured in the ternary system of manganese oxide - heavy metal ion– protein, and the reaction of all substance had direct effect on change law of adsorption capacity. The results had important significance on further recognition of migration and transformation.The influence of pH on BSA adsorption ontoδ-MnO2,α-MnO2 andγ-MnOOH and LSZ adsorption ontoδ-MnO2 had been investigated. It was found that pH had great influence on the adsorption of the two proteins onto manganese oxides.The adsorption of Cu²⁺ ontoδ-MnO2,α-MnO2 andγ-MnOOH was investigated in the absence and presence of BSA. The adsorption-pH curves of Cu²⁺ onto manganese oxides were"S-shape"and the leaping range of adsorption would move to high value of pH in the presence of BSA. Cu²⁺ adsorption percentage obviously decreased in the presence of BSA compared with that in the absence of BSA. It may be the reason of the competition for adsorption sites between BSA and Cu²⁺. The adsorption isotherms of Cu²⁺ onto the three manganese oxides were fit for Langmuir adsorption model. The maximum capacity in the presence of BSA was lower than that without BSA.The influencing factors on Cu²⁺, Cd²⁺ and Zn²⁺ adsorption ontoδ-MnO2 were studied in the presence of BSA or LSZ. It was showed that the adsorption-pH curves of Cu²⁺, Cd²⁺, Zn²⁺ ontoδ-MnO2 were all"S-shape"in the presence and absence of proteins, and the leaping range of adsorption would move to high value of pH in the presence of BSA. It was believed that the adsorption of heavy metal ions ontoδ-MnO2 was described by surface hydroxyl complexation, and pH was the main factor of influencing adsorption. The Langmuir adsorption model fit the experimental date reasonably. And the addition of protein made the adsorption capacity decrease. The adsorption percentage of Cu²⁺, Cd²⁺ and Zn²⁺ decreased with the increase of ionic strength. In addition, the effect of concentration of BSA on the adsorption of heavy metal ions was also investigated.Comparison of different adding order, the influence of various factor on Ni²⁺, Cu²⁺ adsorption was investigated. The adsorption-pH curves of Ni²⁺, Cu²⁺ontoδ-MnO2 were"S-shape", and the leaping range of adsorption would move to high pH value in the presence of BSA. The extent of effect had relation to the adding order and the effect of the mode that BSA was added first was more obvious. The Langmuir adsorption model fit the experimental date reasonably and the maximum adsorption capacity related to the BSA adding order:â… (without BSA)>â…¡(heavy metal ion added first)≈Ⅲ(heavy metal ion and BSA added simultaneously)>â…£(BSA added first). In addition, the adsorption of Ni²⁺ or Cu²⁺ ontoδ-MnO2 increased with the rise of temperature. It was found that the adsorption reversibility of Ni²⁺ or Cu²⁺ adsorption ontoδ-MnO2 depended on pH. BSA had no effect on the adsorption reversibility.