Enhanced reduction of sulfate by iron-carbon microelectrolysis: interaction mechanism between microelectrolysis and microorganisms.

Sulfate wastewater has a wide range of sources and greatly harms water, soil, and plants. Iron-carbon microelectrolysis (IC-ME) is a potentially sustainable strategy to improve the treatment of sulfate (SO42-) wastewater by sulfate-reducing bacteria (SRB). In this study, an iron-carbon mixed micro-electrolysis bioreactor (R1), iron-carbon layered bioreactor (R2), activated carbon bioreactor (R3), and scrap iron filing bioreactor (R4) were constructed by up-flow column experimental device. The performance and mechanism of removing high-concentration sulfate wastewater under different sulfate concentrations, hydraulic retention times (HRT), and chemical oxygen demand (COD)/SO42- were discussed. The results show that the iron-carbon microelectrolysis-enhanced SRB technology can remove high-concentration sulfate wastewater, and the system can still operate normally at low pH. In the high hydraulic loading stage (HRT = 12 h, COD/SO42- = 1.4), the SO42- removal rate of the R1 reactor reached 98.08%, and the ORP value was stable between - 350 and - 450 mV, providing a good ORP environment for SRB. When HRT = 12 h and influent COD/SO42- = 1.4, the R1 reactor sulfate removal rate reached 96.7%. When the influent COD/SO42- = 0.7, the sulfate removal rate was 52.9%, higher than the control group. Biological community analysis showed that the abundance of SRB in the R1 reactor was higher than that in the other three groups, indicating that the IC-ME bioreactor could promote the enrichment of SRB and improve its population competitive advantage. It can be seen that the synergistic effect between IC-ME and biology plays a vital role in the treatment of high-concentration sulfate wastewater and improves the biodegradability of sulfate. It is a promising process for treating high-concentration sulfate wastewater.

Reparation of nano-FeS by ultrasonic precipitation for treatment of acidic chromium-containing wastewater.

Nano-FeS is prone to agglomeration in the treatment of chromium-containing wastewater, and ultrasonic precipitation was used to synthesize nano-FeS to increase its dispersion. The optimization of the preparation method was carried out by single factor method (reaction temperature, Fe/S molar ratio and FeSO4 dropping flow rate) and response surface methodology. Dynamic experiments were constructed to investigate the long-term remediation effect and water column changes of nano-FeS and its solid particles. The changes of the remediation materials before and after the reaction were observed by SEM, and the mechanism of the remediation of chromium-containing wastewater by nano-FeS prepared by ultrasonication was revealed by XRD. The results showed that the reaction temperature of 12 °C, Fe/S molar ratio of 3.5 and FeSO4 dropping flow rate of 0.5 mL/s were the best parameters for the preparation of nano-FeS. The nano-FeS has efficient dispersion and well-defined mesoporous structure in the form of needles and whiskers of 40-80 nm. The dynamic experiments showed that the average removal of Cr(VI) and total chromium by nano-FeS and its immobilized particles were 94.97% and 63.51%, 94.93% and 45.76%, respectively. Fe2+ and S2- ionized by the FeS nanoparticles rapidly reduced Cr(VI) to Cr(III). Part of S2- may reduce Fe3+ to Fe2+, forming a small iron cycle that gradually decreases with the ion concentration. Cr(III) and Fe2+ form Cr(OH)3 and FeOOH, respectively, with the change of aqueous environment. Another part of S2- reacts with Cr(III) to form Cr2S3 precipitate or is oxidized to singlet sulfur. The FeS nanoparticles change from short rod-shaped to spherical shape. Compared with the conventional chemical precipitation method, the method used in this study is simple, low cost, small particle size and high removal rate per unit.

Preparation of lignite-loaded nano-FeS and its performance for treating acid Cr(VI)-containing wastewater.

In this study, lignite-loaded nano-FeS (nFeS@Lignite) was successfully prepared by ultrasonic precipitation, and its potential for treating acid Cr(VI)-containing wastewater was explored. The results showed that the 40--80-nm rod-shaped nFeS was successfully loaded onto lignite particles, and the maximum adsorption capacity of Cr(VI) by nFeS@Lignite reached 33.08 mg∙g-1 (reaction time = 120 min, pH = 4, temperature = 298.15 K). The adsorption process of Cr(VI) by nFeS@Lignite fitted the pseudo-second-order model and the Langmuir isotherm model, and thermodynamic results showed that the adsorption process was an endothermic process with an adsorption enthalpy of 28.0958 kJ·mol-1. The inhibition intensity of coexisting anions on Cr(VI) removal was in the order of PO43- > NO3- > SO42- > Cl-, and the increase of ionic strength resulted in more pronounced inhibition. Electrostatic adsorption, reduction, and precipitation were synergistically engaged in the adsorption of Cr(VI) by nFeS@Lignite, among which reduction played a major role. The characterization results showed that Fe2+, S2-, and Cr(VI) were converted to FeOOH, S8, SO42-, Fe2O3, Cr2O3, and Fe(III)-Cr(III) complexes. This research demonstrates that nFeS@Lignite is a good adsorbent with promising potential for application in the remediation of heavy metal-contaminated wastewater.