Valorization of algal biomass to synthesize heterogeneous gC3N4-Biochar photocatalyst for the treatment of industrial wastewater using photocatalysis-persulfate oxidation process.

Textile wastewater, heavily contaminated with organic dyes, is generating severe problems to environment and human health. The implementation of gC3N4 with biochar (gC3N4-BC) for the treatment of textile wastewater is less effective due to the limited adsorption capacity and slower degradation kinetics. To tackle these problems, peroxydisulfate (PDS) is integrated with gC3N4-BC photocatalyst to enhance the process efficiency and kinetics. The synthesized gC3N4-BC-5 composite shows higher separation of charge carriers, light absorbance, and lower energy bandgap (2.62 eV). The results of photocatalytic degradation and rate constant are enhanced up to 99.9 % and 0.041 min-1 using gC3N4-BC-5 with PDS as compared to without PDS (96.8 % and 0.028 min-1, respectively). The radicals (SO4-•,O2-•, and OH•) are responsible to improve the degradation process efficiency and kinetics. The reusability of optimized sample indicates that gC3N4-BC-5 is stable and effective up to five cycles. The gC3N4-BC-5 composite attains highest adsorption (70.9 %) when compared to BC (62.3 %) and pure gC3N4 (27.1 %). The well-fitted models of adsorption (Pseudo-Second-Order and Freundlich) confirm the favorable, chemical, and multilayered adsorption process. The coupling of gC3N4-BC-5 with PDS is effective, efficient, and stable process to enhance the kinetics and degradation of textile wastewater.

Competitive inhibition of catalytic nitrate reduction over Cu-Pd-hematite by groundwater oxyanions.

The presence of various oxyanions in the groundwater could be the main challenge for the successive application of Cu-Pd-hematite bimetallic catalyst to aqueous NO3- reduction due to the inhibition of its catalytic reactivity and alteration of product selectivity. The batch experiments showed that the reduction kinetics of NO3- was strongly suppressed by ClO4-, PO43-, BrO3- and SO32- at low concentrations (>5 mg/L) and HCO3-, CO32-, SO42- and Cl- at high concentrations (20-500 mg/L). The presence of anions significantly changing the end-product selectivities influenced high N2 selectivity. The selectivity toward N2 increased from 55% to 60%, 60%, and 70% as the concentrations of PO43-, SO32-, and SO42- increased, respectively. It decreased from 55% to 35% in the presence of HCO3- and CO32- in their concentration range of 0-500 mg/L. The production of NO2- was generally not detected, while the formation of NH4+ was observed as the second by-product. It was found that the presence of oxyanions in the NO3- reduction influenced the reactivity and selectivity of bimetallic catalysts by i) competing for active sites (PO43-, SO32-, and BrO3- cases) due to their similar structure, ii) blockage of the promoter and/or noble metal (HCO3-, CO32-, SO42-, Cl- and ClO4- cases), and iii) interaction with the support surface (PO43- case). The results can provide a new insight for the successful application of catalytic NO3- reduction technology with high N2 selectivity to the contaminated groundwater system.

Stability of Sn-Pd-Kaolinite catalyst during heat treatment and nitrate reduction in continuous flow reaction.

In this study, a novel and highly reactive Sn-Pd catalyst supported by environmentally benign kaolinite (Sn-Pd-kaolinite) was developed and evaluated for stability for effective nitrate (NO3-) reduction in batch and continuous mode. Complete NO3- removal with fast reduction kinetics (k = 18.16 × 10-2 min-1) and 71% selectivity toward N2 were achieved by the Sn-Pd-kaolinite catalyst during batch reactions. During continuous tests, 100% NO3- removal and 80% N2 was achieved for 60 h. However, NO3- removal efficiency gradually decreased to 80% in170 h. The catalyst was then successfully regenerated in the system by increasing H2 flow which achieved a complete NO3- removal again. The metal leaching from catalyst surface was negligible (Sn 0.01% and Pd 0.006%) and the structure was stable during the continuous test, confirming that the Sn-Pd-Kaolinite catalyst had a superior reaction kinetics and operational durability.

Strategies to enhance the stability of nanoscale zero-valent iron (NZVI) in continuous BrO3- reduction.

The reduction of bromate to bromide was successfully achieved by bimetallic catalysts with NZVI support in continuous-flow reactors. The stability of NZVI-supported bimetallic catalysts was enhanced by decelerating the iron corrosion and sequential rapid passivation of the iron-Cu-Pd ensembles under optimized reaction conditions. Thus >99% bromate removal can be continuously achieved for 11 h. The lifetime of the bimetallic catalyst was further enhanced and tested under different hydraulic retention time, catalyst loading, and initial bromate concentrations. At the optimized operation conditions, the catalyst showed a complete bromate reduction by 24 h and then the reactivity slowly decreased to 20% over the next 100 h. X-ray diffraction and X-ray photoelectron spectroscopy showed that the reactive NZVI support was oxidized to Fe(II) and Fe(III) along with Cu(0) oxidation to CuO, while the oxidation state of Pd did not change. Therefore, bromate reduction occurred on the surface of reactive NZVI support and Cu(0) particle, while Pd played a role as a hydrogenation catalyst that prolonged the lifetime of the bimetallic catalyst.

Effect of promoter and noble metals and suspension pH on catalytic nitrate reduction by bimetallic nanoscale Fe(0) catalysts.

Experiments were conducted to investigate the effect of experimental factors (types of promotor and noble metals, H2 injection, and suspension pH) on catalytic nitrate reduction by bimetallic catalysts supported by nanoscale zero-valent iron (NZVI). NZVI without H2 injection showed 71% of nitrate reduction in 1 h. Cu/NZVI showed the almost complete nitrate reduction (96%) in 1 h, while 67% of nitrate was reduced by Ni/NZVI. The presence of noble metals (Pd and Pt) on Cu/NZVI without H2 injection resulted in the decrease of removal efficiency to 89% and 84%, respectively, due probably to the electron loss of NZVI for formation of metallic Pd and Pt. H2 injection into Cu-Pd/NZVI suspension significantly improved both catalytic nitrate reduction (>97% in 30 min) and N2 selectivity (18%), indicating that adsorbed H on active Pd sites played an important role for the enhanced nitrate reduction and N2 selectivity. The rapid passivation of NZVI surface resulted in a dramatic decrease in nitrate reduction (79-28%) with an increase in N2 selectivity (8-66%) as the suspension pH increased from 8 to 10.