Microbial extracellular polysaccharides and plagioclase dissolution

Bytownite feldspar was dissolved in batch reactors in solutions of starch ( glucose polymer), gum xanthan (glucose, mannose, glucuronic acid), pectin ( poly-galacturonic acid), and four alginates (mannuronic and guluronic acid) with a range of molecular weights (low, medium, high and uncharacterized) to evaluate the effect of extracellular microbial polymers on mineral disso lution rates. Solutions were analyzed for dissolved Si and Al as an indicat or of feldspar dissolution. At neutral pH, feldspar dissolution was inhibited by five of the acid polys accharides, gum xanthan, pectin, alginate low, alginate medium, alginate hi gh, compared to an organic-free control. An uncharacterized alginate substa ntially enhanced both Si and Al release from the feldspar. Starch, a neutra l polysaccharide, had no apparent effect. Under mildly acidic conditions, i nitial pH approximate to 4, all of the polymers enhanced feldspar dissoluti on compared to the inorganic controls. Si release from feldspar in starch s olution exceeded the control by a factor of three. Pectin and gum xanthan i ncreased feldspar dissolution by a factor of 10, and the alginates enhanced feldspar dissolution by a factor of 50 to 100. Si and Al concentrations in creased with rime, even though solutions were supersaturated with respect t o several possible secondary phases. Under acidic conditions, initial pH ap proximate to 3, below the pK(a) of the carboxylic acid groups, dissolution rates increased, but the relative increase due to the polysaccharides is lo wer, approximately a factor of two to ten. Microbial extracellular polymers play a complex role in mineral weathering. Polymers appear to inhibit dissolution under some conditions, possibly by irreversibly binding to the mineral surfaces. The extracellular polysacchar ides can also enhance dissolution by providing protons and complexing with ions in solution. Copyright (C) 1999 Elsevier Science Ltd.