The role of pectin in Cd binding by orange peel biosorbents: a comparison of peels, depectinated peels and pectic acid.

Biosorption by cheaply and abundantly available materials such as citrus peels can be a cost efficient method for removing heavy metals from wastewater. To investigate the role pectin plays in metal binding by citrus peels, native orange peels, protonated peels, depectinated peels, and extracted pectic acid were compared. Kinetic experiments showed that equilibrium was achieved in 1h. The 1st-order model was more effective in describing the kinetics than the 2nd-order model. Titrations showed two acidic sites with pK(a) values around 4 (carboxyl) and 10.5 (hydroxyl), respectively. The pH dependent surface charge was described well by a two-site model. Sorption isotherms were best modeled assuming a 1:2 binding stoichiometry, followed by the Langmuir and the Freundlich model. The binding capacity was highest for pectic acid (2.9 mequiv./g) followed by protonated peels and native peels, being lowest for depectinated peels (1.7 mequiv./g). This showed the importance of pectin in metal binding by citrus peels. However, even depectinated peels were still good sorbents which still provided carboxyl groups that were involved in metal binding. FTIR spectra confirmed the presence of carboxyl and hydroxyl groups in all materials and their involvement in metal binding.