Modeling chromium (VI) biosorption by acid washed crab shells

The removal of toxic heavy metals from the spent plating baths and the process rinse water is a growing concern for metal finishing industries. Among the pollutants, hexavalent chromium, such as chromate (CrO4 2 ) is considered to be a serious toxic form, as it can easily cross the cell and nuclear membranes and cause genetic damage. Niu and Volesky proposed an acid washed crab (ucides cordatus) shell (AWCS) biosorption technique for effectively adsorbing anionic chromium (VI) species from aqueous solution. The acid washed crab shells in this work contain about 78% of chitin. Chitin is the world’s second most abundant naturally occurring polysaccharide. Much of it is disposed of as waste from seafood crustaceans, mainly crabs, shrimps, prawns, and lobsters, where it occurs as a significant component in the shells/exoskeletons of these crustaceans. Study on crab shells could provide important information for understanding similar waste biomaterials, such as shells of shrimps, prawns, lobsters and other chitin-containing biomaterials with regard to biosorption application. It was determined that chromium biosorption by AWCS was strongly affected by solution pH and ionic strength. Chromate occurred in the forms of chromate and/or dichromate depending on the solution pH. Chromate or dichromate were mainly bound on the weak-base amino groups of chitin or protein present in AWCS. These observations indicate that a mathematical model which can effectively predict the chromium biosorption by AWCS has to be sensitive to parameters, such as solution pH, ionic strength, and chromium speciation. Earlier publications commonly used biosorption equilibrium models based on the Langmuir equation. The equilibrium uptakes in this model were determined without reference to any of the important factors such as pH, ionic strength, and speciation of ions. As a result, the equilibrium constants are dependent on the solution conditions seriously limiting the prediction capability of these models over more extended sorption conditions. Surface complexation modeling represents a body of knowledge that has been available for modeling of metal ion adsorption on various impenetrable surfaces. These models have been developed to provide a way to describe the activity coefficient of both the surface species and the species in the solution. They can generally operate over a range of pH and ionic strength. The biosorbent AWCS used in this biosorption system was semipenetrable. While it is a popular biosorbent, unfortunately, the aforementioned model could not successfully describe the behavior of such a system. Developing a mathematical model that could effectively predict the biosorption behavior of semipenetrable biosorbents, such as crab shells and related biomaterials represents an outstanding challenge. In this work, a mathematical model was developed which was exemplified by chromium (VI) biosorption system. The model effectively represented the influence of solution pH, ionic strength and chromium speciation on the equilibrium chromium uptake. The prediction capability of the model was demonstrated using the experimental data of chromium (VI) biosorption obtained by Niu and Volesky. Correspondence concerning this article should be addressed to C. H. Niu at catherine.niu@usask.ca.