Cadmium removal from dilute aqueous solution by gel beads of sugar beet pectin

Abstract

The metal-accumulative capabilities of polysaccharide gel entrapped microorganisms may be of interest for the treatment of waste water containing low amounts of metal ions. The cost of the gel matrix, however, seriously impedes the large-scale development of such immobilized-cell systems. We have therefore considered the use of pectin extracted from sugar beet pulp as a cost-effective alternative to commercial algal alginate and citrus pectin. A simple, inexpensive alkaline hydrolysis was sufficient to reduce by 80% the acetylation degree of sugar beet pectin (SBP) so that the rheological properties of the polymer in aqueous solution became close to those of citrus pectin. For instance, the intrinsic viscosity of saponified SBP was 157 cm 3 g −1, that of citrus pectin 197 cm 3 g −1. Hydrolyzed SBP was able to form gels (at concentrations of 2% w/v or higher) in the presence of divalent cations (Ca 2+). The cadmium-binding capacity ( Q a, mg Cd 2+ g −1 dry gel) of SBP gel beads compared favourably to that of algal alginate but remained somewhat lower than that of citrus pectin: Q a SBP=4.6; Q a alginate=4.1; Q a citrus pectin=7.6. The desorption of Cd 2+ ions from SBP gel beads in 0.3 M calcium chloride was easy, reaching 81% of the metal accumulated. The viability of microbial cells entrapped in SBP gel beads was maintained.