Halophilic bacterial community and their ability to remove zinc oxide and titanium dioxide nanoparticles from wastewater

Abstract

The microbial abundance of the brine samples in all the stages of plants was assessed using 16S ribosomal ribonucleic acid gene sequence analysis. A total of 35 genera (47% of the total population) except uncultured genera populated collected samples with Halomonas (8.48%) as the most domi-nant genus followed Nesterenkonia (6.29%), Pseudomonas (4.75%), Serratia (4.15%), Shewanella (3.64%), Burkholderia (2.47%), Thauera (2.14%), Delftia (2.18%), Arthrospira (2.18%) and others. Five moderately halophilic bacterial species namely Serratia sp. INBio 4041, Bacillus cereus strain CASA51-1, Morganella morganii strain AP28, Citrobacter freundii strain C09 and Lysinibacillus sp. NOSK was later isolated and assessed for the removal of nano-zinc oxide (nZnO) and nano-titanium dioxide (nTiO2) in highly polluted wastewater samples. Prior removal experiment isolates showed the ability to thrive under an extensive range of operating conditions with 30°C, pH 7, 100 rpm (agitation speed), 4% NaCl and 2% sucrose as optimum growth conditions. Moderately halophilic bacterial isolates were gradually able to uptake nZnO and nTiO2 at concentrations ranging from 1 to 200 mg/L over time. Bacillus sp. (100%–60%) appeared to have the highest nZnO-removal range, followed by Serratia sp. (100%–58%) and Morganella sp. (100%–50%), while Citrobacter freundii (97%–49%) and Lysinibacillus sp. (99%–43%) were observed to have the lowest removal efficiency. Individual isolates further demonstrated the following nTiO2 removal trend: Bacillus sp. (98%–54%), Serratia sp., (99%–52%) and the lowest removal efficiency appeared to be associated with Morganella sp. (99%–43%) and Citrobacter freundii (99%–40%). The consortium of halophilic bacteria showed higher removal efficiency of nZnO (100%–63%) and nTiO2 (100%–65%) with 100% over a wide range of metal oxide nanoparticles (NPs) concentration when compared to individual bacterial isolates. Bacterial isolates were also able to remove additional chemical pollutants with 100%–56% for Ag, Co, Cu, Ba, Ni, Li, Sr, and Ba, and 97%–1% for Na, S, Fe, P, Ca, Mg, As, and Sr upon exposure to various concentrations of both nZnO and nTiO2 over exposure time. An increase of pH value was noted in the presence of nTiO2 (pH 7 to pH 4) than in the presence of nZnO (pH 7 to pH 6). This study demonstrates the potential use of moderately halophilic bacteria for the bioremediation of highly polluted wastewater containing metal oxide nanoparticles and other metals.