Water-Soluble Synthetic Polymers: Their Environmental Emission Relevant Usage, Transport and Transformation, Persistence, and Toxicity

Water-soluble synthetic polymers (WSPs) are distinct from insoluble plastic particles, which are both critical components of synthetic polymers. In the history of human-made macromolecules, WSPs have consistently portrayed a crucial role and served as the ingredients of a variety of products (e.g., flocculants, thickeners, solubilizers, surfactants, etc.) commonly used in human society. However, the environmental exposures and risks of WSPs with different functions remain poorly understood. This paper provides a critical review of the usage, environmental fate, environmental persistence, and biological consequences of multiple types of WSPs in commercial and industrial production. Investigations have identified a wide market of applications and potential environmental threats of various types of WSPs, but we still lack the suitable assessment tools. The effects of physicochemical properties and environmental factors on the environmental distribution as well as the transport and transformation of WSPs are further summarized. Evidence regarding the degradation of WSPs, including mechanical, thermal, hydrolytic, photoinduced, and biological degradation is summarized, and their environmental persistence is discussed. The toxicity data show that some WSPs can cause adverse effects on aquatic species and microbial communities through intrinsic toxicity and physical hazards. This review may serve as a guide for environmental risk assessment to help develop a sustainable path for WSP management

Uncovering the Mechanisms of How Capsaicin Affects Short-Chain Fatty Acid Production during Food Waste Valorization

During food waste valorization to produce short-chain fatty acids (SCFAs), organic substrates are biotransformed step by step, but the effects of food additives that are present at high concentrations on these steps and targeted SCFA production have not been investigated. Therefore, this study aimed to address this knowledge gap by selecting capsaicin, a widely used food additive worldwide, as a representative additive. The results demonstrated a dose-dependent hormesis-like effect of capsaicin on the production of SCFAs. Specifically, the presence of 4 mg of capsaicin/g of volatile solids (VSs) increased SCFAs yield by 12.5%, while higher concentrations (8–64 mg of capsaicin/g of VS) caused a decrement of 22.5–62.9%. Notably, low levels of capsaicin accelerated the solubilization, hydrolysis, and acidification processes with the exception of solubilization and protein hydrolysis, which remained unaffected at higher levels. Additionally, the presence of capsaicin altered the bacterial cell membrane fluidity and led to an increase in reactive oxygen species (ROS). Slight accumulation of ROS stimulated metabolic activities, while excessive accumulation increased the level of microbial apoptosis. The low level of capsaicin enhanced SCFA production by upregulating gene expression associated with hydrolysis and SCFA bioconversion. Conversely, at high capsaicin levels, genes encoding components of the ribosomal subunit involved in polypeptide synthesis, cell migration, and survival regulation were downregulated, resulting in reduced microbial cell activity and SCFA production. This study provides valuable insights into the response mechanisms of complex microbial processes to capsaicin at the genetic level, specifically in SCFA production during anaerobic fermentation of food waste, and has implications in resource utilization of food waste (FW) containing exogenous additives

The Impact of Bisphenol A on the Anaerobic Sulfur Transformation: Promoting Sulfur Flow and Toxic H₂S Production

Bisphenol A (BPA), as a typical leachable additive from microplastics and one of the most productive bulk chemicals, is widely distributed in sediments, sewers, and wastewater treatment plants, where active sulfur cycling takes place. However, the effect of BPA on sulfur transformation, particularly toxic H2S production, has been previously overlooked. This work found that BPA at environmentally relevant levels (i.e., 50–200 mg/kg total suspended solids, TSS) promoted the release of soluble sulfur compounds and increased H2S gas production by 14.3–31.9%. The tryptophan-like proteins of microbe extracellular polymeric substances (EPSs) can spontaneously adsorb BPA, which is an enthalpy-driven reaction (ΔH = −513.5 kJ mol–1, ΔS = −1.60 kJ mol–1K –1, and ΔG = −19.52 kJ mol–1 at 35 °C). This binding changed the composition and structure of EPSs, which improved the direct electron transfer capacity of EPSs, thereby promoting the bioprocesses of organic sulfur hydrolysis and sulfate reduction. In addition, BPA presence enriched the functional microbes (e.g., Desulfovibrio and Desulfuromonas) responsible for organic sulfur mineralization and inorganic sulfate reduction and increased the abundance of related genes involved in ATP-binding cassette transporters and sulfur metabolism (e.g., Sat and AspB), which promoted anaerobic sulfur transformation. This work deepens our understanding of the interaction between BPA and sulfur transformation occurring in anaerobic environments

Insights into the Occurrence, Fate, Impacts, and Control of Food Additives in Food Waste Anaerobic Digestion: A Review

The recovery of biomass energy from food waste through anaerobic digestion as an alternative to fossil energy is of great significance for the development of environmental sustainability and the circular economy. However, a substantial number of food additives (e.g., salt, allicin, capsaicin, allyl isothiocyanate, monosodium glutamate, and nonnutritive sweeteners) are present in food waste, and their interactions with anaerobic digestion might affect energy recovery, which is typically overlooked. This work describes the current understanding of the occurrence and fate of food additives in anaerobic digestion of food waste. The biotransformation pathways of food additives during anaerobic digestion are well discussed. In addition, important discoveries in the effects and underlying mechanisms of food additives on anaerobic digestion are reviewed. The results showed that most of the food additives had negative effects on anaerobic digestion by deactivating functional enzymes, thus inhibiting methane production. By reviewing the response of microbial communities to food additives, we can further improve our understanding of the impact of food additives on anaerobic digestion. Intriguingly, the possibility that food additives may promote the spread of antibiotic resistance genes, and thus threaten ecology and public health, is highlighted. Furthermore, strategies for mitigating the effects of food additives on anaerobic digestion are outlined in terms of optimal operation conditions, effectiveness, and reaction mechanisms, among which chemical methods have been widely used and are effective in promoting the degradation of food additives and increasing methane production. This review aims to advance our understanding of the fate and impact of food additives in anaerobic digestion and to spark novel research ideas for optimizing anaerobic digestion of organic solid waste