An elastic semi IPN polymer hybrid for enhanced adsorption of heavy metals

An elastic semi IPN from guar gum (GG), xanthan gum (XG) and poly(acrylic acid) (PAA) was developed and used as a bioadsorbent for soluble Pb(II) and Hg(II) from water. The IPNs were developed using a photo-initiated crosslinking cum polymerization approach (benzophenone was used as an initiator) with a variable composition of the biopolymers (GG:XG fixed at 20:80 by mass) to PAA by mass (Biopolymers: PAA as 90:10, 80:20, 70:30 and 50:50, respectively). The hybrids were extensively characterized for microstructure, morphology, swelling, porosity and rheological behavior, both in dry and swelled conditions. Three grades of biosorbents namely 90:10, 80:20 and 70:30 showed an enhanced adsorption efficiency compared to the remaining ternary grades as well as the crosslinked binary grade of GG:XG (20:80 mass composition, used as a control), respectively. It was primarily due to high network strength (elasticity) cum flexibility (more liquid like behavior) of those three ternary grades under swollen condition followed by their higher swelling capacity and strongly negative zeta potential to attract the positively charged Pb(II) and Hg(II), respectively. The 80:20 grade (80 parts was the mass composed of XG:GG in 80:20 mass ratio and 20 parts was the mass of PAA) exhibited the maximum molecular effect and thus recorded the highest adsorption efficiency (93 % for Pb(II) with an adsorption capacity of 111.6 and 72 % of Hg(II) with an adsorption capacity of 86.4). Nevertheless, all the hybrids showed a strong pH dependant adsorption as the -COOH unit present in the network displayed a pH sensitive ionization. The adsorption was lower below the pH level of 4.0 (pKa of PAA at 4.5) while drastically improved beyond that. On additional note, the adsorbate dose was also found to affect the adsorption efficiency whereby a maximum dose of 300 ppm of both Pb(II) and Hg(II) was found to be most effective for adsorption.