Heavy metals (Ni, Pb, Cu) adsorption from aqueous solution using cross-linked carboxymethyl kappa-carrageenan

Kappa-carrageenan is extracted from red seaweeds (Eucheuma colonii). It is abundant in the Visayas region in the Philippines. Although it is known to be a binder used for the food and medicine industry, it is proven in this study that kappa-carrageenan can be used for waste water treatment as well. A multi-step carboxy methylation of kappa-carrageenan was performed under heterogeneous reaction conditions, in isopropyl alcohol/water slurry medium, with aqueous sodium hydroxide solution for activation, and monochloroacetic acid for etherification. Three (3) sets of carboxymethyl-κ-carrageenan (cmκC) paste were prepared and were irradiated using the cobalt-60 unit at the Philippine Nuclear Research Institute (PNRI) at 30, 50 and 70 kGy at a 10kGy/hr dose. Upon cross-linking, the carboxymethyl κ-carrageenan turned into hydrogels insoluble in distilled water, sodium hydroxide, and hydrochloric acid. An aqueous solution at pH 6 containing 50ppm of nickel (Ni), Lead (Pb) and Copper (Cu) were prepared for testing the hydrogels. The CMkC hydrogels irradiated at 30, 50 and 70 kGy were placed separately into an aqueous solutions containing Nickel (Ni), Lead (Pb) and Copper (Cu). Liquid samples from each irradiated doses were taken every 40 minutes for 120 minutes, and were analyzed using the Inductively Coupled Plasma (ICP)-Mass Spectrometer. Results showed that all three (3) doses of gamma-irradiated hydrogels adsorbed all three (3) heavy metals contained in the aqueous solutions. Among the three doses, the 70 kGy irradiated hydrogel gave the best gel characteristics, since it had the least swelling ratio. Results were fitted into three (3) kinetic isotherms namely the Elovich isotherm, the lagergren Pseudo-First Order isotherm and the Lagergren Pseudo-Second Order isotherm. Using linear regression, it is on the Lagergren Pseudo-First Order isotherm where adsorption of all three (3) metals were of highest correlation, thus proving that this isotherm is the best fit for the adsorption of these metals. With this, it is proven that the length of the contact time is directly proportional to adsorption capacity, q. Prolonged period of contact between the aqueous solution and the CMkC hydrogels would yield to more heavy metal adsorption. (author)