A coupled photocatalytic system using niobium oxide and microalga: Cr (VI)-contaminated wastewater treatment

The increasing development of industries has caused unusual exploitation of natural resources, associated with the unregulated discharge of wastewater into the environment. Heavy metals, such as chromium, are among the pollutants known for their toxicity, non-biodegradability, durability in nature, and tendency to bioaccumulate. Cr contamination has intensified, and alternative processes, such as bioremediation and heterogeneous photocatalysis, are emerging as an option to traditional methods. The current work aimed to investigate the synergetic photocatalytic process in the reduction and removal of Cr (VI) using two independent phases: the initial addition of niobium oxide (Nb2O5) in a photoreactor under UV-C light, and the application of green alga Chlamydomonas reinhardtii, under another condition and time/space as a polish process. The synthetic wastewater was prepared with Cr (VI) concentrations of 10, 30, and 50 mg/L and introduced into the photoreactor under UV-C light, along with Nb2O5 at concentrations of 0.5 and 1.0 g/L; pH 3; recirculation of Cr (VI) solution at a continuous flow rate of 800 mL/min for 72 h. Aliquots were withdrawn at specific times and the remaining concentration of Cr (VI) was measured using a colorimetric method with 1,5-diphenylcarbohydrazide reagent, and the total Cr concentration was determined by flame atomic absorption spectrometry. Cr (VI) reduction results found for 1.0 g/L Nb2O5 were 51 %, 13 %, and 11 % in 10, 30, and 50 mg/L Cr (VI), respectively. The synthetic wastewater from the photoreactor was collected, centrifuged, and submitted to a batch reactor where C. reinhardtii was added at 1.0 g/L with pH 7 for 120 h, alternating 12/12 h in light/dark conditions. The efficiency of C. reinhardtii was verified by the analysis of cell density, Cr (VI) concentration, and total Cr concentration, as well as morphological analysis using SEM and EDS. Results of Cr (VI) reduction using microalgae were 50 % and 18 % for the concentration of 10 mg/L and 50 mg/L of Cr (VI) respectively; 81 % of reduction for 0.5 g/L and 39 % for 1.0 g/L of Nb2O5, both at a concentration of 30 mg/L of Cr (VI). In the end, the two processes provided, on average, results of 71 %, 31 %, and 18 % total removal of the heavy metal Cr at initial concentrations of 10, 30, and 50 mg/L of Cr (VI), respectively. Thus, the new proposed model, associating agents with distinct characteristics in two processes, an initial heterogeneous photocatalysis using Nb2O5 and subsequent biosorption by microalgae C. reinhardtii, proved to be a suitable new model for reducing and removing Cr.