Coagulation/co-catalytic membrane integrated system for fouling-resistant and efficient water purification.

Low-pressure catalytic membranes allow efficient rejection of particulates and simultaneously removing organics pollutant in water, but the accumulation of dissolved organic matters (DOM) on membrane surface, which cover the catalytic sites and cause membrane fouling, challenges their stable operation in practical wastewater treatment. Here we propose a ferric salt-based coagulation/co-catalytic membrane integrated system that can effectively mitigate the detrimental effects of DOM. Ferric salt (Fe3+) serving both as a DOM coagulant to lower the membrane fouling and as a co-catalyst with the membrane-embedded MoS2 nanosheets to drive perxymonosulfate (PMS) activation and pollutant degradation. The membrane functionalized with 2H-phased MoS2 nanosheets showed improved hydrophilicity and fouling resistance relative to the blank polysulfone membrane. Attributed to the DOM coagulation and co-catalytic generation of surface-bound radicals for decontamination at membrane surface, the catalytic membrane/PMS/ Fe3+ system showed much less membrane fouling and 2.6 times higher pollutant degradation rate in wastewater treatment than the catalytic membrane alone. Our work imply a great potential of coagulation/co-catalytic membrane integrated system for water purification application.

Heavy-metal resistant bio-hybrid with biogenic ferrous sulfide nanoparticles: pH-regulated self-assembly and wastewater treatment application.

Self-assembled bio-hybrids with biogenic ferrous sulfide nanoparticles (bio-FeS) on the cell surface are attractive for reduction of toxic heavy metals due to higher activity than bare bacteria, but they still suffer from slow synthesis and regeneration of bio-FeS and bacterial activity decay for removal of high-concentration heavy metals. A further optimization of the bio-FeS synthesis process and properties is of vital importance to address this challenge. Herein, we present a simple pH-regulation strategy to enhance bio-FeS synthesis and elucidated the underlying regulatory mechanisms. Slightly raising the pH from 7.4 to 8.3 led to 1.5-fold higher sulfide generation rate due to upregulated expression of thiosulfate reduction-related genes, and triggered the formation of fine-sized bio-FeS (29.4 ± 6.1 nm). The resulting bio-hybrid exhibited significantly improved extracellular reduction activity and was successfully used for treatment of high-concentration chromium -containing wastewater (Cr(VI), 80 mg/L) at satisfactory efficiency and stability. Its feasibility for bio-augmented treatment of real Cr(VI)-rich electroplating wastewater was also demonstrated, showing no obvious activity decline during 7-day operation. Overall, our work provides new insights into the environmental-responses of bio-hybrid self-assembly process, and may have important implications for optimized application of bio-hybrid for wastewater treatment and environmental remediation.

Electron delocalization triggers nonradical Fenton-like catalysis over spinel oxides.

Nonradical Fenton-like catalysis offers opportunities to overcome the low efficiency and secondary pollution limitations of existing advanced oxidation decontamination technologies, but realizing this on transition metal spinel oxide catalysts remains challenging due to insufficient understanding of their catalytic mechanisms. Here, we explore the origins of catalytic selectivity of Fe-Mn spinel oxide and identify electron delocalization of the surface metal active site as the key driver of its nonradical catalysis. Through fine-tuning the crystal geometry to trigger Fe-Mn superexchange interaction at the spinel octahedra, ZnFeMnO4 with high-degree electron delocalization of the Mn-O unit was created to enable near 100% nonradical activation of peroxymonosulfate (PMS) at unprecedented utilization efficiency. The resulting surface-bound PMS* complex can efficiently oxidize electron-rich pollutants with extraordinary degradation activity, selectivity, and good environmental robustness to favor water decontamination applications. Our work provides a molecule-level understanding of the catalytic selectivity and bimetallic interactions of Fe-Mn spinel oxides, which may guide the design of low-cost spinel oxides for more selective and efficient decontamination applications.

Anaerobic self-assembly of a regenerable bacteria-quantum dot hybrid for solar hydrogen production.

Inorganic-biological hybrid systems (bio-hybrids), comprising fermentative bacteria and inorganic semiconductor photosensitizers for synergistic utilization of solar energy and organic wastes, offer opportunities for sustainable fuel biosynthesis, but the low quantum efficiency, photosensitizer biotoxicity and inability for self-regeneration are remaining hurdles to practical application. Here, we unveil a previously neglected role of oxygen in suppressing the biosynthesis of cadmium selenide quantum dots (CdSe QDs) and the metabolic activities of Escherichia coli, and accordingly propose a simple oxygen-regulation strategy to enable the self-assembly of bacterial-QD hybrids for efficient solar hydrogen production. Shifting from aerobic to anaerobic biosynthesis significantly lowered the intracellular reactive oxygen species level and increased NADPH and thiol-protein production, enabling a two-order-of-magnitude higher bio-QD synthesis rate and resulting in CdSe-rich products. Bacteria with abundant biocompatible intracellular bio-QDs naturally formed a highly active and self-regenerable bio-hybrid and achieved a quantum efficiency of 28.7% for hydrogen production under visible light, outperforming all the existing bio-hybrids. It also exhibited high stability during cyclic operation and robust performance for treating real wastewater under simulated sunlight. Our work provides valuable new insights into the metallic nanomaterial biosynthesis process to guide the design of self-assembled bio-hybrids towards sustainable energy and environmental applications.

Self-regenerable bio-hybrid with biogenic ferrous sulfide nanoparticles for treating high-concentration chromium-containing wastewater.

Biogenic ferrous sulfide nanoparticles (bio-FeS) as low-cost and green-synthesized nanomaterial are promising for heavy metals removal, but the need for complicated extraction, storage processes and the production of iron sludge still restrict their practical application. Here, a self-regenerable bio-hybrid consisting of bacterial cells and self-assembled bio-FeS was developed to efficiently remove chromium (Cr(VI)). A dense layer of bio-FeS was distributed on the cell surface and in the periplasmic space of Shewanella oneidensis MR-1, endowing the bacterium with good Cr(VI) tolerance and unusual activity for bio-FeS-mediated Cr(VI) reduction. An artificial transmembrane electron channel was constituted by the bio-FeS to facilitate extracellular electron pumping, enabling efficient regeneration of extracellular bio-FeS for continuous Cr(VI) reduction. The bio-hybrid maintained high activity within three consecutive treatment-regeneration cycles for treating both simulated Cr(VI)-containing wastewater (50 mg/L) and real electroplating wastewater. Importantly, its activity can be facilely and fully restored through bio-FeS re-synthesis or regeneration with replenished fresh bacteria. Overall, the bio-hybrid merges the self-regeneration ability of bacteria with high activity of bio-FeS , opening a promising new avenue for sustainable treatment of heavy metal- containing wastewater.

Risk assessment concerning the heavy metals in sediment around Taihu Lake, China.

The present study sought to determine the systematic pollution status of the heavy metals (HMs) found in the sediment around Taihu Lake, China. The concentrations and speciations of the HMs in the sediment were measured. The Ni and Cr concentrations exceeded the probable effect level (PEL), while the concentrations of the other HMs (except Hg) were between the threshold effect level and the PEL. The enrichment factor values for all HMs indicated slight enrichment. The geoaccumulation index revealed that the HMs represented light pollution, while the average contaminant factor value of the HMs indicated moderate pollution. The ecological risk factor value indicated a medium ecological risk, with Cd and Hg being associated with a high and considerable potential ecological risk, respectively. The ecological risk index indicated that the ecological risk posed by the HMs was high. The contamination degree revealed a moderate level of metal pollution, while the pollution load index indicated a heavy pollution level. The extractable proportion of the HMs (except for Cr and As) exceeded 50%. In the case of Cd, the exchangeable proportion was 51.11%, which indicated it to be extremely unstable. PRACTITIONER POINTS: Heavy metals (except for Cr and As) have a high potential ecological risk. All selected heavy metals implying varying degrees of instability. Cd was the most serious pollutant based on the Igeo, Cf and EF analyses.

Long-term, selective production of caproate in an anaerobic membrane bioreactor.

Fermentative caproate production from wastewater is attractive but is currently limited by the low product purity and concentration. In this work, continuous, selective production of caproate from acetate and ethanol, the common products of wastewater anaerobic fermentation, was achieved in an anaerobic membrane bioreactor (AnMBR). The reactor was continuously operated for over 522 days without need for chemical cleaning. With an ethanol-to-acetate ratio of 3.0, the effluent caproate concentration was 2.62 g/L on average and the caproate ratio in liquid products reached 74%. Further raising the influent ethanol content slightly increased the effluent caproate level but lowered the product selectivity and resulted in microbial inhibition. The Clostridia (the major caproate-producing bacteria) and Methanobacterium species (which consume hydrogen to alleviate microbial inhibition) was significantly enriched in the acclimated sludge. Our results imply a great potential of utilizing AnMBR to recover caproate from the effluent of wastewater acidogenic fermentation process.

A single microbial electrochemical system for CO2 reduction and simultaneous biogas purification, upgrading and sulfur recovery.

In this work, a single microbial electrochemical system was developed for multiple goals simultaneously - CO2 reduction, biogas purification, upgrading and sulfur recovery. This system consists of a methanogen-inoculated biocathode for CO2 reduction and a ferrous ion (Fe2+)-mediated abiotic anode for hydrogen sulfide (H2S) oxidation. In the cathodic chamber, methane production rate of 20.6 ± 1.0 µmol·h-1 and high upgrading level (up to 98.3% methane content) were achieved. In the anodic chamber, H2S was completely removed and selectively converted into elemental sulfur particles. The system showed stable performance during continuous operation for treating both pure CO2 and mixed gases, with a cathodic coulombic efficiency of up to 85.2%. This simple system holds a great potential for practical application for biogas upgrading and sulfur recovery from waste water/gases.

Optimizing operation of municipal wastewater treatment plants in China: The remaining barriers and future implications.

China's national development strategy now prioritizes environmental protection over economic growth, which has driven a rapid development of China's wastewater sector. In particular, the treatment capacity of municipal wastewater treatment plants (WWTPs) has been substantially strengthened and stricter effluent quality control enforced. However, the operating performance of most WWTPs is still poor and does not meet the sustainable development demands. In this study, the current status of WWTPs operation in China was comprehensively analyzed, the key barriers to improving the plants operating efficiency were identified by taking into account the different plant scales, geographic distribution, discrepancy between cities and counties, and the influence of environmental policies and supplementary facilities. The underdeveloped sewer network was mainly responsible for the low operating ratios (i.e., utilization degree of the designed treatment capacity) of the plants (76% in counties and 85% in cities) especially for those in north China, although the situation is plant specific because a considerable fraction of plants (19%) are still running under overload condition. Other challenges include the high energy consumption of the plants (0.313 kWh/m3), and severely lagged implementation of sludge disposal (up to 40% sludge was still improperly disposed), arising mainly from the poor management on the sewer and sludge. Lastly, several possible directions of improvement to overcome these barriers were discussed. This work may provide valuable implications for optimizing municipal wastewater management in China towards higher efficiency and sustainability.

Interactions between chlorophenols and peroxymonosulfate: pH dependency and reaction pathways.

A non-radical reaction between peroxysulfates and phenolic compounds, as important structural moieties of natural organic matters, has been reported recently, implying new opportunities for environmental remediation without need for catalyst or energy input. However, this approach seems to be ineffective for halogenated aromatic compounds, an important disinfection by-products (DBPs). Here, we shed light on the interactions between peroxymonosulfate (PMS) and chlorophenols and the influential factors. The results show that the chlorophenols transformation kinetics were highly dependent on the solution pH and chlorophenol species: raising the pH significantly accelerated the chlorophenols degradation, and at alkaline pH the removal rates of different chlorophenols were in the order of trichlorophenol > dichlorophenol > chlorophenol > tetrachlorophenol. The faster degradation of pollutants with more chlorine groups was mainly due to their relatively higher dissociation degree, which favors a direct pollutant-PMS interaction to generate radicals for their degradation. The chlorophenol degradation intermediate (i.e. benzoquinone) further mediated the generation of singlet oxygen at alkaline pH, thereby contributing to accelerated pollutant removal. The slower degradation of tetrachlorophenol than other chlorophenols was likely due to its strong electrostatic epulsion to PMS which restricted the reaction. Our work unveils the chlorophenols degradation mechanisms in PMS reaction system, which may facilitate a better understanding and optimization of advanced oxidation processes for pollution control to reduce potential DBPs accumulation.

Recovery of high-concentration volatile fatty acids from wastewater using an acidogenesis-electrodialysis integrated system.

Recovery of volatile fatty acids (VFAs) from wastewater is an important route for wastewater valorization. Selective acidogenic fermentation enables an efficient production of VFAs from wastewater, whereas electrodialysis (ED) provides an effective approach to concentrate VFAs. However, these two processes have not been coupled in one single system previously. In this study, an acidogenesis-ED integrated system that coupled a continuous acidogenesis with a batch process of VFA concentration was developed for recovery of high-concentration VFAs from wastewater. Under 20.0 V voltage, the acetate was concentrated by 4-fold and the propionate and butyrate were concentrated by over 3-fold in the integrated system after 528-h operation. The declined VFAs recovery ratios at the later stage due to significant reverse diffusion indicate a need to prevent product over-accumulation. This work demonstrated the feasibility of the acidogenesis-ED integrated reactor for wastewater valorization and discussed the remaining challenges and opportunities.

Risk Assessment of Metals in Urban Soils from a Typical Industrial City, Suzhou, Eastern China.

Risk of metals in urban soils is less studied, compared to that in other types of soils, hindering accurate assessment of human exposure to metals. In this study, the concentrations of five metals (As, Cd, Cr, Pb, and Hg) were analyzed in 167 surface soil samples collected from Suzhou city and their potential ecological and human health risks were assessed. The mean concentrations of As, Cd, Pb, and Hg except Cr, were higher than the background values in Jiangsu Province. Metal concentrations varied among districts, where sites of high contamination showed a punctate distribution. Principal components and correlation analyses revealed that As, Pb, and Cd could originate from the same sources. The geo-accumulation (Igeo) and potential ecological risk indices (RI) were calculated and the relatively low values of Igeo (<0) and RI (<150) suggested generally low ecological risk. The noncarcinogenic risks of the metals were relatively low for Suzhou residents (i.e., average hazard index or HI: 0.1199 for adults and 0.5935 for children, <1), while the total carcinogenic risks (TCR) of Cr and As were acceptable (TCR in the range of 1.0 × 10-6 to 1.0 × 10-4). Children faced a higher threat than adults. Results of Monte-Carlo simulations were lower than those obtained from models using deterministic parameters. Of all the uncertain parameters, the ingestion rate and body weight were the most sensitive for adults and children, respectively, while As was an important factor for both. The results as well as the factors controlling risks of metals could help better understand the risks of metals in urban soils of industrial cities in China.

Spatial distribution and removal performance of pharmaceuticals in municipal wastewater treatment plants in China.

Municipal wastewater treatment plants (WWTPs) are an important source of pharmaceuticals released into the environment. Understanding how various pharmaceuticals are distributed and handled in WWTPs is a prerequisite to optimize their abatement. Here we investigated the spatial distribution and removal efficiencies pharmaceuticals in China's WWTPs. A total of 35 pharmaceuticals in wastewater samples from 12 WWTPs at different cities of China were analyzed. Among these detected pharmaceuticals, caffeine showed the highest concentration (up to 1775.98ngL-1) in the WWTP influent. In addition, there were significant regional differences in pharmaceutical distribution with higher influent concentrations of total pharmaceuticals detected in WWTPs in the northern cities than the southern ones. The state-of-the-art treatment processes were generally inefficient in removing pharmaceuticals. Only 14.3% of pharmaceuticals were removed effectively (mean removal efficiency>70%), while 51.4% had a removal rate of below 30%. The anaerobic/anoxic/oxic (AAO)-membrane bioreactor (MBR) integrated process and sequencing batch reactor (SBR) showed better performance than the AAO and oxidation ditch (OD) processes. Ofloxacin, erythromycin-H2O, clarithromycin, roxithromycin and sulfamethoxazole in WWTP effluents exhibited a high or medium ecological risk and deserved special attention.

Competitive sorption of heavy metals by water hyacinth roots.

Heavy metal pollution is a global issue severely constraining aquaculture practices, not only deteriorating the aquatic environment but also threatening the aquaculture production. One promising solution is adopting aquaponics systems where a synergy can be established between aquaculture and aquatic plants for metal sorption, but the interactions of multiple metals in such aquatic plants are poorly understood. In this study, we investigated the absorption behaviors of Cu(II) and Cd(II) in water by water hyacinth roots in both single- and binary-metal systems. Cu(II) and Cd(II) were individually removed by water hyacinth roots at high efficiency, accompanied with release of protons and cations such as Ca2+ and Mg2+. However, in a binary-metal arrangement, the Cd(II) sorption was significantly inhibited by Cu(II), and the higher sorption affinity of Cu(II) accounted for its competitive sorption advantage. Ionic exchange was identified as a predominant mechanism of the metal sorption by water hyacinth roots, and the amine and oxygen-containing groups are the main binding sites accounting for metal sorption via chelation or coordination. This study highlights the interactive impacts of different metals during their sorption by water hyacinth roots and elucidates the underlying mechanism of metal competitive sorption, which may provide useful implications for optimization of phytoremediation system and development of more sustainable aquaculture industry.

Human embryonic mesenchymal stem cells participate in differentiation of renal tubular cells in newborn mice.

Stem cells are used with increasing success in the treatment of renal tubular injury. However, whether mesenchymal stem cells (MSC) differentiate into renal tubular epithelial cells remains controversial. The aims of the present study were to observe the localization of human embryonic MSCs (hMSCs) in the kidneys of newborn mice, and to investigate hMSC differentiation into tubular epithelium. Primary culture hMSCs were derived from 4-7-week-old embryos and labeled with the cell membrane fluorescent dye PKH-26. The degree of apoptosis, cell growth, differentiation and localization of hMSCs with and without this label were then determined using immunohistochemical methods and flow cytometry. hMSCs and PKH26-labeled hMSCs were revealed to differentiate into chondrocytes and adipocytes, and were demonstrated to have similar proliferative capability. In the two cell types, the antigens CD34 and CD45, indicative of hematopoietic lineages, were not expressed; however, the expression of the mesenchymal markers CD29 and CD90 in MSCs, was significantly increased. During a 4-week culture period, laser confocal microscopy revealed that PKH26-labeled hMSCs in the kidneys of newborn mice gradually dispersed. Two weeks after the injection of the PKH26-labeled cells, the percentage of PKH26-labeled hMSCs localized to the renal tubules was 10±2.1%. In conclusion, PKH26 labeling has no effect on hMSC differentiation, proliferation and mesenchymal cell surface features, and hMSCs injected into the kidneys of newborn mice may transform to renal tubule epithelium.

Synergetic decolorization of reactive blue 13 by zero-valent iron and anaerobic sludge.

Efficient decolorization of reactive blue 13 was achieved by a combined use of anaerobic sludge and zero-valent iron (ZVI), and the underlying mechanism of this process was elucidated, Addition of 1.0 g/L ZVI into sludge accelerated the decolorization, with the decoloration ratio after 1-h treatment increased by 29.4% compared with the sum of the individual systems, indicating a synergy between the sludge and ZVI. Meanwhile, substantial changes in sludge morphology and microbial community were observed. The increased dye removal by ZVI was mainly attributed to: (1) a directly chemical reduction and aggregation; and (2) creation of a more favorable pH and anaerobic environment for microorganisms. The acid production by acidogens also partially offset the pH rise in ZVI corrosion and would alleviate ZVI deactivation. This work might offer valuable implications for the optimization and practical application of ZVI-anaerobic sludge processes for treatment of azo dyes or other recalcitrant pollutants.

N-acetylcysteine-pretreated human embryonic mesenchymal stem cell administration protects against bleomycin-induced lung injury.

INTRODUCTION: The transplantation of mesenchymal stem cells (MSCs) has been reported to be a promising approach in the treatment of acute lung injury. However, the poor efficacy of transplanted MSCs is one of the serious handicaps in the progress of MSC-based therapy. Therefore, the purpose of this study was to investigate whether the pretreatment of human embryonic MSCs (hMSCs) with an antioxidant, namely N-acetylcysteine (NAC), can improve the efficacy of hMSC transplantation in lung injury. METHODS: In vitro, the antioxidant capacity of NAC-pretreated hMSCs was assessed using intracellular reactive oxygen species (ROS) and glutathione assays and cell adhesion and spreading assays. In vivo, the therapeutic potential of NAC-pretreated hMSCs was assessed in a bleomycin-induced model of lung injury in nude mice. RESULTS: The pretreatment of hMSCs with NAC improved antioxidant capacity to defend against redox imbalances through the elimination of cellular ROS, increasing cellular glutathione levels, and the enhancement of cell adhesion and spreading when exposed to oxidative stresses in vitro. In addition, the administration of NAC-pretreated hMSCs to nude mice with bleomycin-induced lung injury decreased the pathological grade of lung inflammation and fibrosis, hydroxyproline content and numbers of neutrophils and inflammatory cytokines in bronchoalveolar lavage fluid and apoptotic cells, while enhancing the retention and proliferation of hMSCs in injured lung tissue and improving the survival rate of mice compared with results from untreated hMSCs. CONCLUSIONS: The pretreatment of hMSCs with NAC could be a promising therapeutic approach to improving cell transplantation and, therefore, the treatment of lung injury.

Increased cleavage rate of human nuclear transfer embryos after 5-aza-2'-deoxycytidine treatment.

As an abundant source that involves fewer ethical considerations, human abnormally fertilized zygotes are superior to oocytes as therapeutic cloning recipients of nuclear transfer. However, more effective manipulation conditions should be developed for somatic cell nuclear transfer (SCNT) studies using human abnormally fertilized zygotes as recipients. The present study found that the use of cytochalasin B was not necessary for, and even harmful to, the enucleation of human zygotes. This study also decreased the DNA methylation levels in reconstructed embryos using a DNA methyltransferase inhibitor, 5-aza-2'-deoxycytidine (5-aza-dC), in an attempt to correct the abnormalities in DNA methylation that might play an important role in the failure of embryo development. After 5-aza-dC treatment and nuclear transfer (NT-Aza group), 32.7% of reconstructed embryos developed to the 8-cell stage, which is a much higher percentage than that of the nuclear transfer only (NT) group (11.1%). The DNA methylation level in the NT-Aza group was significantly lower than that of the NT group, as determined by 5-methylcytosine immunodetection. Based on the present results, this study recommends performing the enucleation procedure without cytochalasin B treatment and using 5-aza-dC in the culture of reconstructed embryos in human SCNT studies.

VEGF-modified human embryonic mesenchymal stem cell implantation enhances protection against cisplatin-induced acute kidney injury.

The implantation of mesenchymal stem cells (MSC) has been reported as a new technique to restore renal tubular structure and improve renal function in acute kidney injury (AKI). Vascular endothelial growth factor (VEGF) plays an important role in the renoprotective function of MSC. Whether upregulation of VEGF by a combination of MSC and VEGF gene transfer could enhance the protective effect of MSC in AKI is not clear. We investigated the effects of VEGF-modified human embryonic MSC (VEGF-hMSC) in healing cisplatin-injured renal tubular epithelial cells (TCMK-1) with a coculture system. We found that TCMK-1 viability declined 3 days after cisplatin pretreatment and that coculture with VEGF-hMSC enhanced cell protection via mitogenic and antiapoptotic actions. In addition, administration of VEGF-hMSC in a nude mouse model of cisplatin-induced kidney injury offered better protective effects on renal function, tubular structure, and survival as represented by increased cell proliferation, decreased cellular apoptosis, and improved peritubular capillary density. These data suggest that VEGF-modified hMSC implantation could provide advanced benefits in the protection against AKI by increasing antiapoptosis effects and improving microcirculation and cell proliferation.

Rat marrow-derived multipotent adult progenitor cells differentiate into skin epidermal cells in vivo.

Wound repair and functional reconstruction are two key aspects for treatment of skin injury. Research on cell source for skin repair has become a focus of study. The immune rejection induced by allograft cells and the limited source of autologous epidermal stem cells have led to more attention on the multipotent adult progenitor cells (MAPC). In this study, we examined the influence of the local environment of skin injury on the migration and differentiation of MAPC in nude mice. The homing of MAPC to the wounds and the epidermal differentiation of MAPC were investigated by detecting the expression of specific antigens of rat major histocompatibility complex I (MHC-I) antigen and the tracing markers. Three weeks after transplantation, hair follicle-like structure appeared and rat MHC-I antigen was positive in the follicles of the healed skin. PKH26-labeled cells expressing cytokeratin were found in the regenerated follicle-like structures, sebaceous glands and sweat glands. Our findings indicate that MAPC can migrate to the skin injury site and the hair follicles, and participate in skin wound healing by differentiating into epidermal cells, which contributes to the theoretical research of MAPC plasticity and provides theoretical evidence for clinical application of transplantation therapy with MAPC.