Role of surface properties during biosorption of copper by pretreated Aspergillus niger biomass

In this work the sorption of copper by pretreated Aspergillus niger was studied towards understanding adsorption behavior as a consequence of the distinctive surface structure of biomass. Investigation of surface properties of the biomass was achieved by characterization studies as a function of pH and equilibrium concentration, combined with scanning electron micrograph (SEM), Fourier transform infrared (FTIR) spectroscopy, potentiometric titration and zeta potential measurements. The important functional groups present on pretreated biomass as determined through FTIR spectroscopy and interpreted from potentiometric titration data were carboxylic acid and amide groups. These groups in turn participated in the surface reaction of the adsorbent. The zeta potential was −7.5 to −19.1 mV in the pH range of 4–6.5. The maximum retention capacity of the biomass was determined to be equal to 23.6 mg copper/g of biomass, at initial pH 6.0 (equilibrium pH 4.6). The Langmuir and Freundlich isotherm equations were used to describe the behavior of the system. The theoretical maximum uptake from Langmuir isotherm was 0.42 mmol/g and the efficiency of sorption was 0.91. Considering the probability of different forms of functional group dominance based on pH of the solution, a surface reaction model was used to describe the surface structure as well as metal adsorption equilibrium. The model parameters were determined by nonlinear regression. The active site density on the biomass surface was computed using this model as 3.64 mmol/g.