Phosphogypsum as sulfate source for sulphate-reducing bacteria in a continuous fluidized-bed reactor

Phosphogypsum (PG) is one of the major byproducts of phosphoric acid production for fertilizers. It consists mainly of calcium sulfate dihydrate and may also contain toxic metals and residual acidity (pH 2-3). Generally, this waste is disposed in stacks near plants; however, this does not solve the environmental problems as there may be leaching of the waste piles by rainwater, contaminating the soil and water bodies. Therefore, PG disposal is a challenge for the fertilizer industry. This waste can be used as a sulfate source for sulfide production in anaerobic reactors using sulfate-reducing bacteria (SRB). The sulfide generated can be used later for the production of elemental sulfur or used in the precipitation of transition metals. This work evaluated the feasibility of using PG as a sulfate source for SRB in a continuous fluidized-bed reactor fed with glycerol p.a. as carbon source, and the hydraulic retention time (HRT) effect in the sulphate bioreduction process. The latter was determined at 15 +/- 2.6 and 9.7 +/- 1.2 h, respectively, for the theoretical values of 10 and 8 hours, from the use of lithium chloride as tracer. Sulfate removal at DQO/SO42- ratio of 1.9 was 73 +/- 11 and 75 +/- 12%, respectively, for HRT of 15 +/- 2.6 and 9.7 +/- 12 h. Specific sulfate removal rates were 0.106 +/- 0.04 and 0.179 +/- 0.07 gSO(4)(2)(-)/gSSV.d, respectively, for HRT 15 +/- 2.6 and 9.7 +/- 12 h.