Insights into reductive dechlorination of triclocarban in river sediments: Field measurements and in vitro mechanism investigations

A Review on the Fate of Legacy and Alternative Antimicrobials and Their Metabolites during Wastewater and Sludge Treatment

Antimicrobial compounds are used in a broad range of personal care, consumer and healthcare products and are frequently encountered in modern life. The use of these compounds is being reexamined as their safety, effectiveness and necessity are increasingly being questioned by regulators and consumers alike. Wastewater often contains significant amounts of these chemicals, much of which ends up being released into the environment as existing wastewater and sludge treatment processes are simply not designed to treat many of these contaminants. Furthermore, many biotic and abiotic processes during wastewater treatment can generate significant quantities of potentially toxic and persistent antimicrobial metabolites and byproducts, many of which may be even more concerning than their parent antimicrobials. This review article explores the occurrence and fate of two of the most common legacy antimicrobials, triclosan and triclocarban, their metabolites/byproducts during wastewater and sludge treatment and their potential impacts on the environment. This article also explores the fate and transformation of emerging alternative antimicrobials and addresses some of the growing concerns regarding these compounds. This is becoming increasingly important as consumers and regulators alike shift away from legacy antimicrobials to alternative chemicals which may have similar environmental and human health concerns.

Comparison of anaerobic, cycling aerobic/anoxic, and sequential anaerobic/aerobic/anoxic digestion to remove triclosan and triclosan metabolites from municipal biosolids

The antimicrobial triclosan (TCS) is a pervasive and persistent environmental micropollutant which can contaminate land, biota, and water through the land application of biosolids. Many existing sludge management techniques have limited effectiveness against TCS and TCS metabolites including triclosan-sulfate (TCS-SO₄). The objective of this study was to evaluate the impacts of different digestion types (anaerobic, aerobic/anoxic, and sequential anaerobic + aerobic/anoxic), temperatures, and digester sludge retention times (SRTs) on the destruction of organic matter, and on TCS/TCS metabolites. Conventional mesophilic anaerobic digesters (AD), room temperature cycling aerobic/anoxic digesters (AERO/ANOX), and sequential AD + AERO/ANOX digesters were all effective in removing organic matter. The optimum single-stage AD, and AERO/ANOX scenarios were both 20-day SRTs which had 52.3 ± 1.4 and 47.1 ± 3.7% chemical oxygen demand (COD) removals, respectively. Sequential AD + AERO/ANOX digesters improved organic matter destruction, removing up to 68.2 ± 2.1% of COD at an 8-day AD + 12-day AERO/ANOX second-stage (mesophilic) SRTs. While AD showed modest levels of TCS removals (all <40%), TCS was substantially more degradable aerobically with AERO/ANOX removing up to 80.3 ± 2.5% of TCS and nearly all TCS-SO₄ entering the digester at a 20-day SRT. Sequential AD + AERO/ANOX removed virtually all TCS-SO₄ entering the system and improved TCS removals from first stage ADs. However, they were less effective than a single-stage AERO/ANOX digester operating at the same overall SRT. These results demonstrate that AERO/ANOX and sequential AD + AERO/ANOX processes could be used to reduce the amount of TCS, TCS-SO₄ and TCS-related compounds in digested sludge, minimizing the environmental burden of the land application of biosolids.

Fate, risk and removal of triclocarban: A critical review

The halogenated antimicrobial triclocarban (TCC) has large production and consumption over last decades. Its extensive utilization in personal care products and insufficient treatment in conventional wastewater treatment plants (WWTPs) has led to its listing as one of emerging organic contaminants (EOCs). Due to the hydrophobicity and chemical stability of TCC, it has been omnipresent detected in terrestrial and aquatic environments, and its prolonged exposure has thrown potential pernicious threat to ecosystem and human health. Considering its recalcitrance, especially under anoxic conditions, both biological and non-biological methods have been exploited for its removal. The efficiency of advanced oxidation processes was optimistic, but complete removal can rarely be realized through a single method. The biodegradation of TCC either with microbial community or pure culture is feasible but efficient bacterial degraders and the molecular mechanism of degradation need to be further explored. This review provides comprehensive information of the occurrence, potential ecological and health effects, and biological and non-biological removal of TCC, and outlines future prospects for the risk evaluation and enhanced bioremediation of TCC in various environments.

Occurrence and fate of antimicrobial triclocarban and its transformation products in municipal sludge during advanced anaerobic digestion using microwave pretreatment

This study, for the first time, investigated the impact of microwave pretreatment on the fate of the pervasive antimicrobial triclocarban (TCC) that was already present in municipal sludge, before and during advanced anaerobic digestion (AD) under thermophilic and mesophilic conditions. A range of microwave temperature (80 and 160 °C) and exposure duration (1 and 30 min) configurations were studied by employing ten bench-scale anaerobic digesters fed with mixed sludge at three different solids retention times (SRTs) including 20, 12, and 6 days. Seasonal changes influenced the levels of TCC in municipal sludge sampled from a plant employing the biological nutrient removal. Initial batch pretreatment studies showed that microwave irradiation itself can achieve TCC removal efficiencies up to 30 ± 4 and 64 ± 5% at 80 and 160 °C, respectively. The control digesters utilizing un-pretreated mixed sludge showed limited TCC removals, between 18 and 32% and 11-26% respectively, under thermophilic and mesophilic temperatures. On the other hand, the highest TCC elimination (78 ± 2%) was obtained from the thermophilic digester utilizing microwaved sludge at 160 °C for 30 min at SRT of 12 days. The non-chlorinated carbanilide (a transformation product of TCC) was detected and quantified for the first time during conventional and microwave-pretreated anaerobic sludge digestion. The formation of carbanilide in biosolids through reductive dechlorination could be an indicator of efficient and complete TCC transformation. This research demonstrated that AD coupled with microwave pretreatment can be used to reduce environmental concentrations of TCC in municipal sludge and biosolids.

Bioaugmentation of triclocarban and its dechlorinated congeners contaminated soil with functional degraders and the bacterial community response

Partial removal of haloaromatic antimicrobial triclocarban (TCC) during wastewater treatment caused the final introduction of residual TCC into soils. Bioaugmentation has been proposed for the biodegradation of TCC and its dechlorinated congeners 4,4'-dichlorocarbanilide (DCC) and carbanilide (NCC) in soil. The isolated TCC-degrading strain Ochrobactrum sp. TCC-2 and chloroanilines-degrading strain Diaphorobacter sp. LD72 were used to study the removal efficiency of TCC, DCC and NCC mixture and their chloroanilines intermediates, respectively. The potential degradation competition between TCC and its dechlorinated congeners, and the response of bacterial community during the bioremediation were also investigated. The biodegradation of DCC and TCC was significantly enhanced for soil with inoculums compared with sterilized and natural soils. Chloroanilines products could also be effectively removed. For the degradation of combined substrates in the aqueous medium, NCC had negative effect on the degradation of TCC and DCC, while TCC and DCC negatively influenced each other. The bioaugmentation with two degraders obviously changed the phylogenetic composition and function of indigenous soil microbiome. Importantly, the inoculated degraders could be maintained, suggesting their adaptability and potential application in bioaugmentation for such recalcitrant contaminants. This study offers new insights into the enhanced bioremediation of TCC and its dechlorinated congeners contaminated soils by the bioaugmentation of functional degraders and the structure and function response of the indigenous soil microbiome to the bioremediation process.

Occurrence, fate and risk assessment of biocides in wastewater treatment plants and aquatic environments in Thailand

This study investigated the occurrence and fate of 19 biocides in 8 wastewater treatment plants and receiving aquatic environments (both freshwater and estuarine systems) in Thailand. The predominant compound in wastewater and surface water was methylparaben with the maximum concentration of 15.2 μg/L detected in the receiving river, while in sludge and sediment was triclocarban with the maximum concentration of 8.47 μg/g in sludge. Triclosan was the main contaminants in the fish samples with the maximum concentration of 1.20 μg/g. Similar results of biocides were found in the estuarine system in Pattaya city, with the maximum concentration of 185 ng/L in sea water for methylparaben, and 242 ng/g in estuarine sediment for triclocarban. The aqueous removal rates for the biocides ranged from 15% to 95% in average. The back estimated-usage and total estimated emission of Ʃ19 biocides in Thailand was 279 and 202 tons/year, respectively. Preliminary ecological risk assessment showed that clotrimazole and triclosan could pose high risks to aquatic organisms in the receiving aquatic environments.

Fate of Triclocarban (TCC) in aquatic and terrestrial systems and human exposure

Triclocarban (TCC) is considered as contaminant of emerging concern (CEC), and ranked in the top 10 CEC occurrence. TCC is a high production volume synthetic chemical used extensively in various personal care products. This chemical will be released into the environment via incomplete wastewater treatment and untreated wastewater discharge. TCC and its transformation products (4,4'-dichlorocarbilide (DCC),1-(3-chlorophenyl)-3-phenylurea (MCC) and carbanilide (NCC),2'OH-TCC, 3'OH-TCC) were detected in the environmental matrices. Sediment organic carbon will influence TCC concentrations in suspended and bed sediments. TCC is an antimicrobial agent and also emerging endocrine disruptor that can cause immune dysfunction and affect human reproductive outcomes. Furthermore, TCC alters the expression of proteins related to binding and metabolism, skeletal muscle development and function, nervous system development and immune response. TCC has potential health risks in wildlife and humans. Several animal studies illustrate that it can cause various adverse effects, which can be monitored by antioxidant biomarkers (CAT, GST and LPO). Accumulation of TCC in organisms depends on the lipophilicity and bioavailability of TCC in sediment and water. TCC was continuously detected in aquatic system. TCC is a lipophilic compound, which can efficiently bind with lipid content. Women are more vulnerable to TCC due to substantially higher frequency and extended exposure to TCC. This review provides basic information of occurrence of TCC and the exposure levels in aquatic organisms. Several literature have shown the higher usage and human exposure levels of TCC, which provides useful information for the chemical management approaches.

Occurrence of triclocarban and benzotriazole ultraviolet stabilizers in water, sediment, and fish from Indian rivers

Triclocarban and benzotriazole ultraviolet stabilizers (BUVSs) are listed as high production volume synthetic chemicals, used extensively in personal care products. Many of these chemicals persist in the aquatic environment as micropollutants. Knowledge on their fate in freshwater ecosystems is still lacking, especially in the Indian Rivers. Our intention is to study the seasonal distribution, hazard quotient, risk assessment, and bioaccumulation of triclocarban and BUVSs (UV-9, UV-P, UV-326, UV-327, UV-328, and UV-329) during wet and dry seasons in water, sediment and fish from the Kaveri, Vellar, and Thamiraparani rivers in Tamil Nadu State, India. Triclocarban and BUVSs were identified in all matrices analysed. Triclocarban was found in water, sediment, and fish up to 1119ng/L, 26.3ng/g (dry wt.), and 692ng/g (wet wt.), respectively. Among BUVSs, UV-329 was found up to 31.3ng/L (water samples), UV-327 up to 7.3ng/g (sediment samples), and UV-9 up to 79.4ng/g (fish samples). The hazard quotient (HQ_env.) for triclocarban in surface water was found to be at risk level (HQ_env. >1) in the Kaveri, and Thamiraparani rivers during dry season. Bioaccumulation factors indicate that target compounds (triclocarban and BUVSs) could bio-accumulate in organisms.

Effects of 1,1,1-Trichloroethane and Triclocarban on Reductive Dechlorination of Trichloroethene in a TCE-Reducing Culture

Chlorinated compounds were generally present in the environment due to widespread use in the industry. A short-term study was performed to evaluate the effects of 1,1,1- trichloroethane (TCA) and triclocarban (TCC) on trichloroethene (TCE) removal in a reactor fed with lactate as the sole electron donor. Both TCA and TCC inhibited TCE reduction, but the TCC had a more pronounced effect compared to TCA. The TCE-reducing culture, which had never been exposed to TCA before, reductively dechlorinated TCA to 1,1-dichloroethane (DCA). Below 15 μM, TCA had little effect on the transformation of TCE to cis-dichloroethene (DCE); however, the reduction of cis-DCE and vinyl chloride (VC) were more sensitive to TCA, and ethene production was completely inhibited when the concentration of TCA was above 15 μM. In cultures amended with TCC, the reduction of TCE was severely affected, even at concentrations as low as 0.3 μM; all the cultures stalled at VC, and no ethene was detected. The cultures that fully transformed TCE to ethene contained 5.2–8.1% Dehalococcoides. Geobacter and Desulfovibrio, the bacteria capable of partially reducing TCE to DCE, were detected in all cultures, but both represented a larger proportion of the community in TCC-amended cultures. All cultures were dominated by Clostridium_sensu_stricto_7, a genus that belongs to Firmicutes with proportions ranging from 40.9% (in a high TCC (15 μM) culture) to 88.2%. Methanobacteria was detected at levels of 1.1–12.7%, except in cultures added with 15 and 30 μM TCA, in which they only accounted for ∼0.4%. This study implies further environmental factors needed to be considered in the successful bioremediation of TCE in contaminated sites.

Role of plant growth-promoting Ochrobactrum sp. MC22 on triclocarban degradation and toxicity mitigation to legume plants

Triclocarban (TCC) is an emerging and persistent pollutant once released into environment. In this study, TCC-degrading Ochrobactrum sp. MC22, was isolated and characterized. This is the first report on plant-growth promoting bacterium with versatile capability of TCC degradation under aerobic and anaerobic conditions. The aerobic degradation of TCC occurred completely of which the kinetic analysis revealed a non-self-inhibitive substrate effect, and broad-concentration-range degradation efficiency (ranging from 0.16-30mgL-1). Anaerobic TCC degradation was feasible, but was significantly enhanced up to 40-50% when ferric, or acetate was provided as electron donor, or acceptor, respectively. TCC biodegradation under both conditions was proposed to initially occur through hydrolysis leading to transient accumulation of chloroanilines, which could be completely metabolized and detoxified. With concern on TCC adverse effect to plants, role of MC22 on toxicity mitigation was investigated using two legume plants: Vigna radiata and Glycine max (L.) Merr. Upon TCC exposure, damage of both plant structures, especially root system was observed, but was substantially mitigated by MC22 bioaugmentation. This study not only provides thorough TCC degradation characteristic and kinetics of MC22, but also suggests a potential role of this bacterial strain for a rhizoremediation in crop area with TCC contamination.

Functional Characterization of a Novel Amidase Involved in Biotransformation of Triclocarban and its Dehalogenated Congeners in Ochrobactrum sp. TCC-2

Haloaromatic antimicrobial triclocarban (3,4,4'-trichlorocarbanilide, TCC) is a refractory contaminant which is frequently detected in various aquatic and sediment environments globally. However, few TCC-degrading communities or pure cultures have been documented, and functional enzymes involved in TCC biodegradation hitherto have not yet been characterized. In this study, a bacterial strain, Ochrobactrum sp. TCC-2, capable of degrading TCC under both aerobic and anaerobic conditions was isolated from a sediment sample. A novel amidase gene (tccA), responsible for the hydrolysis of the two amide bonds of TCC and its dehalogenated congeners 4,4'-dichlorocarbanilide (DCC) and carbanilide (NCC) to more biodegradable chloroaniline or aniline products, was cloned and characterized. TccA shares low amino acid sequence identity (27 to 38%) with other biochemically characterized amidases and contains the conserved catalytic triad (Ser-Ser-Lys) of the amidase signature enzyme family. TccA was stable over a pH range of 5.0 to 10.0 and at temperatures lower than 60 °C, and it was constitutively expressed in strain TCC-2. In contrast to the halogenated TCC and DCC, the nonchlorinated NCC was the preferred substrate for TccA. TccA also had hydrolysis activity to a broad spectrum of amide bonds in herbicides, insecticides, and chemical intermediates. The constitutive expression and broad substrate spectrum of TccA suggested strain TCC-2 may be potentially useful for bioremediation applications.