Reusable isotype heterojunction g-C3N4/alginate hydrogel spheres for photocatalytic wastewater treatment.

Various visible-light-driven photocatalysts have been studied for practical applications in photocatalytic wastewater treatment via solar irradiation. Among them, g-C3N4 has attractive features, including its metal-free and environmentally friendly nature; however, it is prone to charge recombination and has low photocatalytic activity. To solve these problems, isotype heterojunction g-C3N4 was recently developed; however, the methods employed for synthesis suffered from limited reproducibility and efficiency. In this study, isotype heterojunction g-C3N4 was synthesized from various combinations of precursor materials using a planetary ball mill. The isotype heterojunction g-C3N4 synthesized from urea and thiourea showed the highest photocatalytic activity and completely decolorized Rhodamine B (RhB; 10 ppm) in 15 min under visible-light irradiation. Furthermore, to improve recyclability, isotype heterojunction g-C3N4 was immobilized in alginate hydrogel spheres. The isotype heterojunction g-C3N4/alginate hydrogel beads were used in 10 repeated RhB degradation experiments and were able to maintain their initial photocatalytic activity and mechanical strength. These achievements represent an advance towards practical, sustainable photocatalytic wastewater treatment.

Nitrogen removal from wastewater by an immobilized consortium of microalgae-bacteria in hybrid hydrogels.

The high content of nitrogen in wastewater brings some operational, technical, and economical issues in conventional technologies. The aim of this study was to evaluate the nitrogen removal by hybrid hydrogels containing consortium microalgae-nitrifying bacteria in the presence of activated carbon (AC) used as an adsorbent of inhibitory substances. Hybrid hydrogels were synthesized from polyvinyl alcohol (PVA), sodium alginate (SA), biomass (microalgae-nitrifying bacteria), and AC. The hybrid hydrogels were evaluated based on the change in ammonium (NH4), nitrate (NO3), and chemical demand of oxygen (COD) concentrations, nitrification rate, and other parameters during 72 h. Results indicated that NH4 removal was more effective for hydrogels without AC than with AC, without significant differences regarding consortium biomass concentration (5 or 16%), presenting final concentrations of 3.13 and 3.75 mg NH4/L for hydrogels with 5 and 16% of the biomass, respectively. Regarding NO3 production, hydrogels without AC reached concentrations of 25.9 and 39.77 mg NO3/L for 5 and 16% of the biomass, respectively, while treatments with AC ended with 2.17 and 1.37 mg NO3/L. This confirms that hydrogels can carry out the nitrification process and do not need AC to remove potential inhibitors. The best performance was observed for the hydrogel with 5% of biomass without AC with a nitrification rate of 0.43 mg N/g TSS·h.

Advanced light-tolerant microalgae-nitrifying bacteria consortia for stable ammonia removal under strong light irradiation using light-shielding hydrogel.

The consortium of microalgae and nitrifying bacteria has attracted attention owing to its advantages, such as energy- and cost-efficiency in terms of using only light irradiation without aeration. However, high light intensity can easily cause photoinhibition of nitrifying bacteria, resulting in process breakdown of the consortium. This challenge limits its practical application in outdoor environment. In a previous study, we developed a "light-shielding hydrogel" which entrapped nitrifying bacteria in carbon black-added alginate hydrogel beads and confirmed its effectiveness of protecting the nitrifying bacteria from intense light up to 1600 µmol photons m-2 s-1. However, the applicability of the light-shielding hydrogel to microalgae-nitrifying bacteria consortia under strong light irradiation has not yet been clarified. In this study, we aimed to establish consortia of Chlorella sorokiniana and nitrifying bacteria immobilised in light-shielding hydrogel and evaluate their nitrification performance under strong light. Three nitrifying bacteria conditions were used: light-shielding hydrogel, hydrogel containing only nitrifying bacteria without carbon black ('hydrogel'), and dispersed nitrifier without immobilisation ('dispersion') as a control. At 1600 µmol photons m-2 s-1, the dispersion afforded a significant decrease in nitrification activity and subsequent process breakdown. In contrast, light-shielding hydrogel achieved complete nitrification without nitrite accumulation and had nitrification rates of approximately nine and two times higher than those for the dispersion and hydrogel conditions, respectively. Based on the overall evaluation, a possible sequence of process breakdown under strong light was also proposed. This study demonstrated for the first time that the light-shielding hydrogel/consortia combination had potential for applications, which require mitigation of photoinhibition under strong light irradiation. Further, it is expected that the proposed method will contribute to realise the practical application of microalgae-nitrifying bacteria consortia in various countries that experience high sunlight intensity due to their location in the sunbelt areas.

Cumulative effects of titanium dioxide nanoparticles in UASB process during wastewater treatment.

Titanium dioxide nanoparticles (TiO2 NPs) are widely used in consumer products and one of their major fate is the wastewater treatment plants. However, NPs eventually arrive to aquatic and terrestrial ecosystems via treated water and biosolids, respectively. Since low concentration of NPs is accumulating in the upflow anaerobic sludge blanket (UASB) reactors that treat wastewater and reclaim water quality, the accumulation of TiO2 NPs in these reactors may impact in their performance. In this work, the long-term effects of TiO2 NPs on the main benefits of treating wastewater by UASB reactors such as, biogas production, methane fraction in biogas and organic matter removal were evaluated. Evaluation consisted of monitoring such parameters in two identical UASB reactors, one UASB-Control (without NPs) and the experimental one (UASB-TiO2 NPs) that received wastewater with TiO2 NPs. The fate of NPs in the UASB reactor was also determined. Results indicated that biogas production increased by 8.8% due to the chronic exposure of UASB reactor to TiO2 NPs during the first 44 days of experiment. However, the methane content in the biogas had no significant differences between both UASB, ranging between 78% and 90% of methane during the experiment. The removal of organic matter in both UASB was similar and ranged 92-98% along the experimental time. This means that accumulation of TiO2 NPs did not altered the biogas production and organic matter removal. However, the content of total volatile solids (TVS) in UASB-TiO2 NPs dropped off from 137.8 g to 64.2 g in 84 days, while for control reactor that decreased from 141.6 g to 92.4 g in the same period. Hence, the increased biogas production in the UASB exposed to TiO2 was attributed to hydrolysis of the TVS in this reactor. The main fate of TiO2 NPs was the granular sludge, which accumulated up to 8.56 mg Ti/g, which represent around 99% of the Ti spiked to the reactor and the possible cause of the biomass hydrolyzation in the UASB. Disposal of UASB sludge containing NPs from may raise ecotoxicological concerns due to the use of biosolids in agricultural activities.

Effects and fate of TiO2 nanoparticles in the anaerobic treatment of wastewater and waste sludge.

The increasing use of TiO2 nanoparticles (NPs) in customer products has also increased the concerns about their effects in the environment. Anaerobic digestion is a process probably exposed to high concentrations of TiO2 NPs due to its application for wastewater and waste sludge treatment. In this work, it was studied the anaerobic digestion performance and the extracellular polymeric substances (EPS) production in presence of TiO2 NPs, as well as the fate of TiO2 NPs in anaerobic reactors. Results showed that methane production enhanced an average of 14.9% in presence TiO2 NPs, which is considered a positive effect. A strong affinity between TiO2 NPs and EPS was found, especially for proteins (PRO) and polysaccharides (PS) in the loosely and tightly bound EPS layers of microorganisms (LB-EPS and TB-EPS). Ti quantification indicated that 92% of the TiO2 NPs are removed by anaerobic sludge, while 8% remain in the treated effluent.

Influence of wastewater type on the impact generated by TiO2 nanoparticles on the oxygen uptake rate in activated sludge process.

Physicochemical characteristics of wastewater have a relationship with the stability of TiO2 nanoparticles (NPs). This in turn has an effect on the toxicity of TiO2 NPs in microorganisms. In this work, the effect of TiO2 NPs on activated sludge process was evaluated using three different types of wastewater: synthetic, raw, and filtered. The results showed that aggregate size of TiO2 NPs and their specific adsorption of substrates were influenced by the type of substrates and the presence of suspended solids in the wastewater. It was also shown that TiO2 NPs in raw wastewater severely inhibited oxygen uptake by microorganisms as compared to uptake in synthetic or filtered wastewater. The attachment of TiO2 NP aggregates on cell membranes was observed for all types of wastewater. However, the internalization of TiO2 NPs by microorganisms was observed only for raw and filtered wastewater. These results indicate that the effects caused by TiO2 NPs on activated sludge were different depending on the wastewater used for the experiment.

Tailoring pore properties of MCM-48 silica for selective adsorption of CO2.

Four different types of amine-attached MCM-48 silicas were prepared and investigated for CO(2) separation from N(2). Monomeric and polymeric hindered and unhindered amines were attached to the pore surface of the MCM-48 silica and characterized with respect to their CO(2) sorption properties. The pore structures and amino group content in these modified silicas were investigated by XRD, FT-IR, TGA, N(2) adsorption/desorption at 77 K and CHN/Si analysis, which confirmed that in all cases the amino groups were attached to the pore surface of MCM-48 at 1.5-5.2 mmol/g. The N(2) adsorption/desorption analysis showed a considerable decrease of the pore volume and surface area for the MCM-48 silica containing a polymeric amine (e.g., polyethyleneimine). The CO(2) adsorption rates and capacities of the amine-attached MCM-48 samples were studied employing a sorption microbalance. The results obtained indicated that in addition to the concentration of surface-attached amino groups, specific interactions between CO(2) and the surface amino groups, and the resultant pore structure after amine group attachment have a significant impact on CO(2) adsorption properties of these promising adsorbent materials.

Mechanism of high-temperature CO2 sorption on lithium zirconate.

Lithium zirconate (Li2ZrO3) is one of the most promising materials for CO2 separation from flue gas at high temperature. This material is known to be able to absorb a large amount of CO2 at around 400-700 degrees C. However, the mechanism of the CO2 sorption/desorption process on Li2ZrO3 is not known yet. In this study, we examined the CO2 sorption/desorption mechanism on Li2ZrO3 by analyzing the phase and microstructure change of Li2ZrO3 during the CO2 sorption/desorption process with the help of thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) analyses. Li2ZrO3 powders were prepared from lithium carbonate (Li2CO3) and zirconium oxide (ZrO2) by the solid-state method, and the CO2 sorption/desorption property was examined by TGA. It was shown that pure Li2ZrO3 absorbs a large amount of CO2 at high temperature with a slow sorption rate. Addition of potassium carbonate (K2CO3) and Li2CO3 in the Li2ZrO3 remarkably improves the CO2 sorption rate of the Li2ZrO3 materials. DSC analysis for the CO2 sorption process indicates that doped lithium/potassium carbonate is in the liquid state during the CO2 sorption process and plays an important role in improving the CO2 uptake rate. XRD analysis for phase and structure change during the sorption/desorption process shows that the reaction between Li2ZrO3 and CO2 is reversible. Considering all data obtained in this study, we proposed a double-shell model to describe the mechanism of the CO2 sorption/desorption on both pure and modified Li2ZrO3.