Phosphate interactions with iron-titanium oxide composites: Implications for phosphorus removal/recovery from wastewater.

Understanding the interactions between phosphate (P) and mineral adsorbents is critical for removing and recovering P from wastewater, especially in the presence of both cationic and organic components. To this end, we investigated the surface interactions of P with an iron-titanium coprecipitated oxide composite in the presence of Ca (0.5-3.0 mM) and acetate (1-5 mM), and quantified the molecular complexes and tested the possible removal and recovery of P from real wastewater. A quantitative analysis of P K-edge X-ray absorption near edge structure (XANES) confirmed the inner-sphere surface complexation of P with both Fe and Ti, whose contribution to P adsorption relies on their surface charge determined by pH conditions. The effects of Ca and acetate on P removal were highly pH-dependent. At pH 7, Ca (0.5-3.0 mM) in solution significantly increased P removal by 13-30% by precipitating the surface-adsorbed P, forming hydroxyapatite (14-26%). The presence of acetate had no obvious influence on P removal capacity and molecular mechanisms at pH 7. At pH 4, the removal amount of P was not obviously affected by the presence of Ca and acetate. However, acetate and high Ca concentration jointly facilitated the formation of amorphous FePO4 precipitate, complicating the interactions of P with Fe-Ti composite. In comparison with ferrihydrite, the Fe-Ti composite significantly decreased the formation of amorphous FePO4 probably by decreasing Fe dissolution due to the coprecipitated Ti component, facilitating further P recovery. An understanding of these microscopic mechanisms can lead to the successful use and simple regeneration of the adsorbent to recover P from real wastewater.

Health risk-oriented source apportionment of PM2.5-associated trace metals.

In health-oriented air pollution control, it is vital to rank the contributions of different emission sources to the health risks posed by hazardous components in airborne fine particulate matters (PM2.5), such as trace metals. Towards this end, we investigated the PM2.5-associated metals in two densely populated regions of China, the Yangtze River Delta (YRD) and Pearl River Delta (PRD) regions, across land-use gradients. Using the positive matrix factorization (PMF) model, we performed an integrated source apportionment to quantify the contributions of the major source categories underlying metal-induced health risks with information on the bioaccessibility (using simulated lung fluid) and speciation (using synchrotron-based techniques) of metals. The results showed that the particulate trace metal profiles reflected the land-use gradient within each region, with the highest concentrations of anthropogenically enriched metals at the industrial sites in the study regions. The resulting carcinogenic risk that these elements posed was higher in the YRD than in the PRD. Chromium was the dominant contributor to the total excessive cancer risks posed by metals while manganese accounted for a large proportion of non-carcinogenic risks. An elevated contribution from industrial emissions was found in the YRD, while traffic emissions and non-traffic combustion (the burning of coal/waste/biomass) were the common dominant sources of cancer and non-cancer risks posed by metals in both regions. Moreover, the risk-oriented source apportionment of metals did not mirror the mass concentration-based one, suggesting the insufficiency of the latter to inform emission mitigation in favor of public health. While providing region-specific insights into the quantitative contribution of major source categories to the health risks of PM2.5-associated trace metals, our study highlighted the need to consider the health protection goal-based source apportionment and emission mitigation in supplement to the current mass concentration-based framework.

Phosphorus Removal and Recovery from Wastewater using Fe-Dosing Bioreactor and Cofermentation: Investigation by X-ray Absorption Near-Edge Structure Spectroscopy.

A new phosphorus (P) removal and recovery process that integrates an FeCl3-dosing, membrane bioreactor (MBR), and side-stream cofermentation was developed for wastewater treatment. The Fe and P species and their transformation mechanisms via aerobic and anaerobic conditions were investigated with X-ray absorption near edge structure (XANES) spectroscopy. In the new treatment system, 98.4% of the total P in domestic wastewater was removed and retained in activated sludge in the MBR. During the subsequent acidogenic cofermentation with food waste, P in the MBR sludge was released and eventually recovered as vivianite, achieving an overall P recovery efficiency of 61.9% from wastewater. The main pathways for P removal and recovery with iron dosing and acidogenic fermentation were determined by XANES analysis. The results showed that Fe-enhanced P removal with the MBR was mainly achieved by precipitation as ferric phosphate (24.2%) and adsorption onto hydrous iron oxides (60.3%). During anaerobic fermentation, transition from Fe(III)-P to Fe(II)-P complex occurred in the sludge, leading to Fe(II) dissolution and P release. The pH decrease and microbial Fe reduction were crucial conditions for effective P extraction from the MBR sludge. The efficiency of P recovery increased with an increase in the fermentation time and organic load and a decrease of pH in the solution.