Interactions of calcium and aluminium ions with alginate

The binding properties of alginate with calcium and aluminium ions have been studied, as a step towards understanding how algal extracellular organic matter (EOM) interferes with conventional aluminium-based coagulation/flocculation drinking water treatment. Alginate has been used as a model for algal EOM. Binding strengths and binding capacities have been determined, using an ion selective electrode for calcium and equilibrium dialysis for aluminium. Calcium and aluminium ions begin to bind above pH 3.5, co-incident with alginic acid deprotonation. Steady-state binding is achieved above pH 6. The variation in percent and absolute calcium ion uptake as a function of calcium to carboxyl ratio indicates that additional binding sites can be accessed at higher metal loadings. Alginate reaches a limiting capacity for calcium ions of approximately one calcium ion per eight carboxyl groups (or eight sugar units). Unlike calcium binding, a fixed number of aluminium binding sites are available for aluminium, irrespective of total concentration, corresponding to approximately one aluminium per seven carboxyl groups (or seven sugar units). On a carboxyl group concentration basis, alginate and fulvic acids (a fraction of natural organic matter) have similar capacities for aluminium ions, indicating that algal EOM can compete for aluminium intended for consumption by natural organic matter during coagulation. The addition of calcium ions prior to aluminium coagulation may minimise the influence of algal EOM on the coagulation process