A Comparison of Microbial Bioaugmentation and Biostimulation for Hexavalent Chromium Removal from Wastewater

Hexavalent chromium (VI) in wastewater is a great risk to human health and to the quality of water sources. However, adapted microorganisms can rapidly reduce this chemical species to the trivalent form (III) and make it less active. Our objective was to evaluate the capacity of bacterial isolates for Cr (VI) reduction in nutrient medium and in effluent and to compare indigenous microorganisms with those isolated from wastewater contaminated with Cr (VI). Cr (VI) reduction was also tested with different sources of carbon, nitrogen, and phosphorus at two temperatures (10 and 30 °C). Initially, the resistant microorganisms were isolated from the solution with 100 mg L⁻¹ of Cr (VI). Subsequently, we evaluated the effectiveness of the isolates in reducing Cr (VI) I in culture medium under temperature-controlled conditions, with concentrations of 10 and 100 mg L⁻¹ of Cr (VI). In the subsequent step, we studied the isolates and autochthonous microorganism efficiency to reduce Cr (VI) present in contaminated effluent, with the addition of nutrients and at different temperatures (10 and 30 °C). In the culture medium containing 10 mg L⁻¹ of Cr (VI), isolates were reduced by 100 % in 48 h. When tested against 100 mg L⁻¹ of Cr (VI), the decrease was 70 and 40 % at 120 h of incubation of the isolates 6 and 11, respectively. In the effluent, there was no significant reduction without nutritional biostimulation. When carbon and phosphorus were applied, isolates 6, 11, and indigenous microorganisms reduced 100 % of the Cr (VI) in 72 h. Nitrogen was not limited in terms of effluent characteristics. At 10 °C incubation temperature, Cr (VI) was completely reduced but slower compared to incubation at 30 °C. The results demonstrate that nutritional biostimulation aided by bioremediation is an excellent tool for reducing hexavalent chromium in wastewater.