Biosorption of heavy metals by citrus fruit waste materials

Abstract

Conventionally used processes for removing heavy metals from wastewater are usually either expensive, such as ion exchange, or inefficient, such as precipitation. An innovative technique that is both efficient and economical is biosorption, in which living and dead biomass can act as biosorbents through physical-chemical processes like ion exchange and micro-precipitation. Pectin, a structural polysaccharide present in plant cell walls, is similar to alginate, a molecule that is often responsible for the high metal uptake by algae. Based on the structural similarity between alginate and pectin, it was expected that pectin rich bio-wastes may be as good a biosorbent material as brown algae. A comparison between different pectin-rich materials showed high stability and metal binding capacity of citrus peels. Sorption isotherms for citrus peels showed higher metal uptake capacity at pH 5 compared to pH 3. Kinetic studies revealed the time required to reach equilibrium for lemon fruit waste (0.177 mm) was 20 min while for larger particles the time increased to 30 min-60 min. For lemon fruit waste, the content and pKa values of acidic groups were determined by using a pKa spectrum technique. Isotherm modeling was carried out by using Langmuir isotherms and pH sensitive modeling.