Reduction of eggshell protein for adsorbing metals in water

Abstract

Eggshell (ES) membranes are rich in proteins consisting of many disulfide bonds and are reducible by various reductants to thiol ligands if adequately treated. This study adopted factorial experiments and response surface design to verify the most critical factors and determine the optimal conditions in eggshell reduction operations. Also, isothermal and kinetic adsorption models were used to demonstrate the metal adsorptive characteristics of the non-reduced and optimally reduced eggshells that adsorbed silver, copper, and chromium in water. Analysis-wise, metals in water were treated by aqua regia digestion and analyzed through inductively coupled plasma atomic emission spectroscopy (ICP-AES). Results revealed that thioglycol (i.e., 2-mercaptoethanol) and reaction time were the two most critical eggshell-modifying factors. Results of the response surface experiments indicated that the optimal eggshell reduction conditions were at the initial reductant concentration and reaction time equal to 9.75 M and 1.9 h, respectively. As for the isothermal metal adsorption using the modified eggshells, it fit the Langmuir model the best with the maximum adsorption capacities (q_m) of silver, copper, and chromium equal to 1.35, 2.09, and 2.39 mmole/g-ES, respectively. Data revealed that the reduced eggshells could completely adsorb silver, copper, and chromium within five hours and adsorbed around 6 and 2.6 times more copper and silver than the unreduced eggshells. These results demonstrate the much better uses of the reduced eggshells than plain eggshells for water metal purification.