Study On The Site Energy Distribution Of Competitive Adsorption Of Heavy Metal On Natural Sediment

Site energy distribution theory?SEDT?is a method for studying the mechanism of adsorption processes from the energy perspective.It can provide the adsorption energy and the corresponding distribution function.Therefore,the application of SEDT on the adsorbent whose surface contains heterogeneous adsorption energy has distinctly unique advantages.As an important place for migration and transformation of contaminants in water environment,the natural sediment is also an heterogeneous adsorbent.It is significantly important to research the adsorption behavior of contaminants on sediments.SEDT has been used to study the adsorption progresses of contaminants on sediments.However,the existing researches are limited in the species of pollutants and the application mode of the theory.Therefore,the purpose of this research is to further develop the application of SEDT in the study of contaminants adsorption on sediment in water environment,and to create a new analytical discussion method.The Langmuir model has been widely used to describe the adsorption of sediments.However,due to the contradiction between the heterogeneity of sediment and the hypothesis of surface homogeneity in Langmuir model,the model has obvious defects.This study recognized the error of the Langmuir model???and discussed the applicability of the model used in the adsorption of contaminants onto sediments quantitatively through SEDT.The results indicate that?mainly depends on the energy distribution heterogeneity in the sediment system.The Langmuir model can be applied in sediments when each one of the conditions below is satisfied:?1?m and n lie in the area which signifies that?is less than 10%;?2?the site energy heterogeneity index???of sediment is under 5.668.These findings are vital for the proper choice of models fitting the adsorption process of sediments.Secondly,SEDT was introduced into the competitive adsorption study of heavy metals in sediments.The site energy distributions of Cu²⁺,Cd²⁺and Zn²⁺in sediments with single,binary and ternary component systems were investigated.It illustrated that site energy distributions provide much more information than adsorption isotherms:?1?The metal with competitive advantage in each energy area.In the presence of Cu²⁺,Cd²⁺and Zn²⁺,Cu²⁺showed an absolute competitive advantage over the whole energy range,especially in the high energy area,and the competitive advantage of Zn²⁺compared with Cd²⁺mainly occurred in the low energy region.?2?Effect of competitive heavy metals on energy distribution of the adsorption system.The effect is mainly reflected in the low energy range and high energy range.The effect of Cd²⁺and Zn²⁺on Cu²⁺was small,however,both Cd²⁺and Zn²⁺showed significant changes in the energy distribution due to the presence of other heavy metal ions.?3?The ratio of each heavy metal on the adsorption sites in any energy area.In the high-energy area,which has great environmental significance,the ratios of heavy metals in the competitive adsorption sites obtained for various competitive systems were as follows:slightly more than 3:1?Cu-Cd?,slightly less than 3:1?Cu-Zn?,slightly more than 1:1?Cd-Zn?,and nearly 7:2:2?Cu-Cd-Zn?.Besides,taking the adsorption of Cu²⁺in ternary competitive adsorption system on different sediments as an example,the influence of sediment particle size and organic matter content on the site energy distribution were analyzed.It was found that the adsorption energy is positively correlated with the fractal dimension?D?and the organic matter content of sediment.There is a negative correlation between?and D,and there is no obvious negative correlation between?and organic matter content of sediment.The results from this study can provide more comprehensive information for understanding the transport and transformation of heavy metals(Cu²⁺,Cd²⁺,Zn²⁺)in sediments and helping to control heavy metal pollution in water environment.Furthermore,this study was effective in presenting a general pattern for future reference in competitive adsorption studies on sediments.