Calcium carbonate crystallization in the presence of biopolymers

The influence on calcium carbonate crystallization of a series of biopolymers that contain carboxylic acid or sulfate functional groups was studied using pH and turbidity measurements, optical microscopy, and scanning electron microscopy. Without biopolymer, single calcite (104) rhombohedra were formed. In the presence of nongelling biopolymers (xanthan and gellan) in the conditions used. (104) rhombohedra formed aggregates that were "stack-like", but in the presence of gelling biopolymers (pectin, kappa-carrageenan, and sodium alginate) the aggregates were "rosette-like". The "rosettes" were proposed to form by the nucleation of calcite on a gelled microparticle template to form a hollow shell. Low methoxy (LM) pectin was particularly effective at directing the growth of calcite rosettes and led to aggregates of radially aligned crystals. The influence of biopolymer concentration on calcite crystallization was studied for LM pectin and kappa-carrageenan. In the former case, an increasingly favorable influence of the pectin molecules on the surface energy of calcite nuclei was proposed to result in an enhanced propensity for nucleation, until the pectin concentration was so high that all of the calcium was sequestered. In the latter case, an increase in calcium binding with increasing K-carrageenan concentration decreased the solution supersaturation and hence decreased the propensity for calcite formation.