Mercury(II) removal from aqueous solution by sorption onto alginate,pectate and polygalacturonate calcium gel beads. A kinetic and speciation based equilibrium study

Gel beads of calcium alginate, pectate and polygalacturonate salts have been tested as sorbent materials for mercury(II) removal from aqueous solutions. Physico-chemical properties of gel beads, defined by SEM–EDX, TGA and texture and density analysis, were correlated with gel beads sorption capacity towards Hg²⁺ ion. A speciation study in aqueous solution was carried out to define the strength of interaction of mercury ion with the polymers investigated and to assess the more suitable experimental conditions to achieve the best effectiveness of Hg²⁺ sorption by gel beads. On the basis of the speciation study, pH values in the 3–5.5 pH range were considered appropriated for mercury(II) sorption by gel beads. Kinetics of mercury(II) sorption and calcium(II) release from the sorbent materials were studied at pH 3, 3.6 and 5.2. The highest sorption rate (K) and amount of mercury(II) adsorbed were obtained at pH 3 and 3.6; therefore, pH 3.3 was chosen for the equilibrium study of Hg²⁺ sorption at 25 °C. The results obtained by using Langmuir and Freundlich isotherm equations show the following sorption capacity trend: Ca–Pect > Ca–PGA > Ca–AA.