Enhancement of phosphate rock solubility through biological processes

Biological acidulation of a reactive phosphate rock (PR), Sechura, was investigated by exposing ground PR to various combinations of elemental sulphur, soil suspension enriched with Thiobacilli, nutrient solution and lactic casein whey. The incubation was carried out at 30°C for 4 weeks. Weekly measurements were made of the pH of the mixture, phosphorus extractable with water (water-P) and 0.5 M NaHCO, (bicarb-P) solution, biomass-P, residual elemental sulphur and the Thiobacillus population. Treatments without elemental S or Thiobacilli culture had little effect on the dissolution of PR. The combination of elemental S and Thiobacilli increased extractable P to nine times that of the untreated PR and with the addition of nutrient solution this was enhanced to 15 times. This was equivalent to 9–10% of stoichiometric acidulation of the PR with sulphuric acid. However, the amount of acid produced, calculated from the sulphur oxidised, was equivalent to that of 14% acidulation. Use of lactic whey with PR alone or in combination with elemental S showed either an immobilisation of soluble P or very little increase in water-P compared with control values. Amounts of P dissolved during incubation which might have been immobilised were determined by the fumigation-incubation technique for recovering P in the microbial biomass. This was found to be a very small fraction of the total P dissolved during the incubation.