Efficient nitrogen removal pathways and corresponding microbial evidence in tidal flow constructed wetlands for saline water treatment.

The artificial tidal wetlands ecosystem was believed to be a useful device in treating saline water, and it played a significant part in global nitrogen cycles. However, limited information is available on nitrogen-cycling pathways and related contributions to nitrogen loss in tidal flow constructed wetlands (TF-CWs) for saline water treatment. This study operated seven experimental tidal flow constructed wetlands to remove nitrogen from saline water at salinities of 0-30‰. Stable and high NH4+-N removal efficiency (∼90.3%) was achieved, compared to 4.8-93.4% and 23.5-88.4% for nitrate and total nitrogen (TN), respectively. Microbial analyses revealed the simultaneous occurrence of anaerobic ammonium oxidation (anammox), dissimilatory nitrate reduction to ammonium (DNRA), nitrification and denitrification, contributing to nitrogen (N) loss from the mesocosms. The absolute abundances were 5.54 × 103-8.35 × 107 (nitrogen functional genes) and 5.21 × 107-7.99 × 109 copies/g (16S rRNA), while the related genera abundances ranged from 1.81% to 10.47% (nitrate reduction) and from 0.29% to 0.97% (nitrification), respectively. Quantitative response relationships showed ammonium transformation were controlled by nxrA, hzsB and amoA, and nitrate removal by nxrA, nosZ and narG. Collectively, TN transformation were determined by narG, nosZ, qnorB, nirS and hzsB through denitrification and anammox pathways. The proportion of nitrogen assimilation by plants was 6.9-23.4%. In summary, these findings would advance our understanding of quantitative molecular mechanisms in TF-CW mesocosms for treating nitrogen pollution that caused algal blooms in estuarine/coastal ecosystems worldwide.

Molecular analysis of microbial nitrogen transformation and removal potential in the plant rhizosphere of artificial tidal wetlands across salinity gradients.

This study explored the microbial nitrogen transformation and removal potential in the plant rhizosphere of seven artificial tidal wetlands under different salinity gradients (0-30‰). Molecular biological and stable isotopic analyses revealed the existence of simultaneous anammox (anaerobic ammonium oxidation), nitrification, DNRA (dissimilatory nitrate reduction to ammonium) and denitrification processes, contributing to nitrogen loss in rhizosphere soil. The microbial abundances were 2.87 × 103-9.12 × 108 (nitrogen functional genes) and 1.24 × 108-8.43 × 109 copies/g (16S rRNA gene), and the relative abundances of dissimilatory nitrate reduction and nitrification genera ranged from 6.75% to 24.41% and from 0.77% to 1.81%, respectively. The bacterial 16S rRNA high-throughput sequencing indicated that Bacillus, Zobellella and Paracoccus had obvious effects on nitrogen removal by heterotrophic nitrifying/aerobic denitrifying process (HN-AD), and autotrophic nitrification (Nitrosomonas, Nitrospira and Nitrospina), conventional denitrification (Bradyrhizobium, Burkholderia and Flavobacterium), anammox (Candidatus Brocadia and Candidatus Scalindua) and DNRA (Clostridium, Desulfovibrio and Photobacterium) organisms co-existed with HN-AD bacteria. The potential activities of DNRA, nitrification, anammox and denitrification were 1.23-9.23, 400.03-755.91, 3.12-35.24 and 30.51-300.04 nmolN2·g-1·d-1, respectively. The denitrification process contributed to 73.59-88.65% of NOx- reduction, compared to 0.71-13.20% and 8.20-15.42% via DNRA and anammox, as 83.83-90.74% of N2 production was conducted by denitrification, with the rest through anammox. Meanwhile, the nitrification pathway accounted for 95.28-99.23% of NH4+ oxidation, with the rest completed by anammox bacteria. Collectively, these findings improved our understanding on global nitrogen cycles, and provided a new idea for the removal of contaminants in saline water treatment.

Comparison of microbial communities in different sulfur-based autotrophic denitrification reactors.

Sulfur-based autotrophic denitrification is a useful approach for the eutrophication control in lakes and rivers, yet the microorganisms in this process are still not clearly known. In order to reveal the bacterial composition in these denitrification reactors, high-throughput sequencing was performed over the sludge samples. And the results indicated that when using thiosulfate, elemental sulfur, and sulfide as electron donors, the microbial communities were clearly different. Besides the well-known Thiobacillus, many other genera of denitrifiers were identified. Chlorobaculum, Dechloromonas, and Acinetobacter were the most predominant genera in thiosulfate, elemental sulfur, and sulfide systems, respectively, while Janthinobacterium accounted for the most in the heterotrophic reactor with ethanol as electron donor. Thiobacillus existed abundantly in every system, even in the heterotrophic one. PCA comparison revealed that the microbial communities in the denitrification systems may vary greatly according to the electron donor, the running condition, sampling position, and other factors.

Development of a constructed wetland water treatment system for selenium removal: incorporation of an algal treatment component.

On the basis of the fact that algae have the ability to volatilize substantial quantities of selenium (Se), we investigated the concept of including an algal pretreatment unit into a constructed wetland system for the removal of Se from river water entering the Salton Sea. Of six different algal strains tested, the most effective in terms of Se volatilization and Se removal from the water column was a Chlorella vulgaris strain (designated Cv). Cv removed 96% of Se (supplied as selenate) from the microcosm water column within 72 h, with up to 61% being removed by volatilization to the atmosphere. X-ray absorption spectroscopy revealed that the major forms of Se likely to be accumulated in an algal-wetland system are selenomethionine, a precursor of volatile Se formation, and elemental Se. Our results suggest that the inclusion of an algal pretreatment unit within a constructed wetland water treatment system should not only enhance the efficiency of Se removal but also significantly reduce the risk of the buildup of ecotoxic forms of Se by promoting the biological volatilization of Se.

Development of a constructed wetland water treatment system for selenium removal: use of mesocosms to evaluate design parameters.

The Salton Sea in California is an important habitat for fish and waterfowl. Its ecosystem is threatened due to diminishing water supplies and increasing salinity. An alternative source of water to support species conservation habitat may be obtained from local rivers (e.g., Alamo or New Rivers), provided that a wetland treatment system can be developed to remove selenium (Se), fertilizer nutrients, and other contaminants. Here, we used mesocosms to evaluate a number of potential design options (e.g., plant species selection, sediment composition and arrangement, forced aeration, organic amendments, etc.) to improve the efficiency of Se removal using treatment wetlands. Our results show that, of five different substrate arrangements tested for Se removal, the most efficient was obtained for cattails growing in a substrate of cattail litter overlying sand and peat moss sediment (water column Se was reduced from 15 µg Se/L to <0.1 µg Se/L in 72 h). The addition of organic amendments in the form of alfalfa hay or alfalfa meal was also helpful in lowering Se levels. These results suggest that it may be possible to design constructed wetland water treatment systems capable of reducing Se concentrations in river water to values below 1 µg Se/L.

Bioremediation of selenium-contaminated soil using earthworm Eisenia fetida: Effects of gut bacteria in feces on the soil microbiome.

Selenium (Se) contamination in the soil poses a food safety risk to humans. The present study was to investigate the role of earthworm Eisenia fetida in soil Se remediation. When exposed to selenite at 4 mg Se/kg, E. fetida efficiently concentrated Se in tissues (24.53 mg Se/kg dry weight), however, only accounting for a minor portion of the added Se. Microbial analysis shows 12 out of 15 functional genera became more abundant in the worm-inhabited soil when exposed to Se, suggesting E. fetida contributed to Se remediation mainly by introducing Se-reducing bacteria to the soil via feces, which were dominated by the genera Pseudomonas (∼62.65%) and Aeromonas (∼29.99%), whose abundance was also significantly boosted in the worm-inhabited soil. However, when isolated from worm feces at 200 mg Se/L, Pseudomonas strains only displayed a high tolerance to Se rather than removal capacity. In contrast, among 4 isolated Aeromonas strains, A. caviae rapidly removing 85.74% of the added selenite, mainly through accumulation (67.38%), while A. hydrophila and A. veronii were more effective at volatilizing Se (27.77% and 24.54%, respectively), and A. media performed best by reducing Se by ∼49.00% under anaerobic conditions. Overall, our findings have highlighted the importance of E. fetida as a key contributor of functional bacteria to the soil microbiome, building a strong foundation for the development of an earthworm-soil system for Se bioremediation.

Selenium uptake, volatilization, and transformation by the cyanobacterium Microcystis aeruginosa and post-treatment of Se-laden biomass.

With a narrow margin between beneficial and toxic effects, selenium (Se) is of great concern due to its increasing level in aquatic environments. The accumulation and transformation of Se by the cyanobacterium Microcystis aeruginosa and effects of nutrients, particularly sulfate, were investigated. The nutrient-deprived cyanobacterium removed water-borne selenate (82.2 ± 0.93%) faster than selenite (58.9 ± 1.77%), with 86.0 ± 1.41% and 77.2 ± 1.00%, respectively, of the Se accumulated in the biomass and the rest volatilized. When supplied with excess nutrients, the Se accumulation and volatilization rates were significantly inhibited, with the removal efficiency dropping to 50.2 ± 2.59% and 7.37 ± 0.93% for selenite and selenate, respectively. When M. aeruginosa was tested with inadequate, appropriate, and adequate levels of sulfate, Se uptake decreased with increasing sulfate concentrations, particularly for selenate (from 34.1 to 4.81%). Using X-ray absorption near-edge structure to speciate biomass Se, selenite and selenate were transformed to organo-Se (87.3-100%), with or without nutrients present, suggesting M. aeruginosa could efficiently reduce Se oxyanions to more bioavailable forms. With increasing sulfate levels (5.0 and 10.0 mg S/L), percentages of SeMet converted from selenite decreased by 28.2-33.0%, with 19.1-33.2% as elemental Se, while organo-Se remained dominant (93.6-95.1%) in selenate-treated M. aeruginosa. Transmission electron microscopy shows structural damage in the cell wall at exposure to selenite (1600 µg Se/L), with the intracellular structure intact. To prevent Se biomagnification along aquatic food chains, the Se-laden biomass was combusted as a post-treatment, leading to a significant reduction in Se content (∼99.2%) and Se bioavailability, with inorganic Se (45.0-70.5%) predominant in the residue.

Remediation of selenium-contaminated soil through combined use of earthworm Eisenia fetida and organic materials.

Selenium (Se)-polluted soils pose serious threats to terrestrial ecosystems through food chains. This study evaluated the use of earthworm Eisenia fetida and organic materials for Se remediation. The greater mortality (6.7%) and weight loss (28.6%) were observed for earthworms exposed to selenate than selenite at 20 mg Se Kg-1 over 21 d, while selenate was taken up 2.5-fold faster than selenite. Compared with peat moss, Se accumulation increased by 119% in selenite-exposed earthworms supplied with cow manure. Earthworm activity caused greater reduction in selenite (17.4%), with little impact on selenate-treated soil. X-ray absorption spectroscopy speciation analysis shows 87-91% of tissue Se was transformed to organo-Se, i.e., SeMet and SeCys, in earthworms exposed to either selenite or selenate, posing great risks to their predators. The study also found selenium increased bacterial diversity in earthworm casts, while greater relative abundances (~37.57%) of functional genera were obtained for selenite. Over 24 h, two bacteria strains, Bacillus cereus and Aeromonas encheleia, isolated from casts, rapidly reduced selenite by ~94%, compared to ~25% for selenate. Elemental Se was present only in strains (~27%), casts (~11%) and worm-inhabited soil (~2.7%) of selenite treatments, suggesting earthworm gut microbiota could buffer earthworms and other soil fauna from selenite toxicity.

Nitrogen removal through collaborative microbial pathways in tidal flow constructed wetlands.

Constructed wetlands are efficient in removing nitrogen from water; however, little is known about nitrogen-cycling pathways for nitrogen loss from tidal flow constructed wetlands. This study conducted molecular and stable isotopic analyses to investigate potential dissimilatory nitrate reduction to ammonium (DNRA), denitrification, nitrification, anaerobic ammonium oxidation (anammox), and their contributions to nitrogen removal by two tidal wetland mesocosms, PA (planted with Phragmites australis) and NP (unplanted), designated to treat Yangtze River Estuary water. Our results show the mesocosms removed ~22.6% of TN from nitrate-dominated river water (1.19 mg·L-1), with better performance obtained in PA than that in NP, which was consistent with the molecular and stable isotopic data. The potential activities of DNRA, anammox, denitrification and nitrification varied between 0.6 and 1.6, 4.6-37.3, 36.4-305.7, and 463.7-945.9 nmol N2 g-1 dry soil d-1, respectively, with higher values obtained in PA than NP. Nitrification accounted for 94.3-99.4% of NH4+ oxidation, with the rest through anammox. Denitrification contributed to 77.9-90.3% of NOx- reduction, compared to 9.2-21.6% and 0.5-1.5% via anammox and DNRA, respectively; 78.4-90.9% of N2 was produced through denitrification, with the rest via anammox. Pearson correlation analyses suggest NH4+ was the major factor regulating nitrification, while NO3- played an important role in the competition between denitrification and DNRA, and NO2- was a key restrictive factor for anammox. Overall, this study reveals the importance of nitrification, denitrification, anammox and DNRA in nitrogen removal, providing new insight into the nitrogen-cycling mechanisms in natural/artificial tidal wetlands.

Salinity-driven nitrogen removal and its quantitative molecular mechanisms in artificial tidal wetlands.

The present study investigated the performance in nitrogen removal and associated nitrogen transformation processes in seven mesocosms fed with saline water (0‰ to 30‰) to simulate tidal flow constructed wetlands (TF CWs). The highly effective and steady removal of NH4+-N (84.74% averagely) was obtained at various salinities, while the rates varied from 6.34% to 89.19% and 22.54% to 87.48% for NO3--N and total nitrogen (TN), respectively. Overall, nitrogen removal efficiencies were greater at lower salinities. Molecular biological analyses verified the co-occurrence of dissimilatory nitrate reduction to ammonium (DNRA), denitrification, anaerobic ammonium oxidation (anammox) and nitrification in the mesocosms, reportedly contributing to nitrogen removal in TF CWs. The absolute copy numbers of nitrogen functional genes and total bacterial 16S rRNA were 2.54 × 103-7.35 × 107 and 3.21 × 107-7.82 × 109 copies g-1 dg (dry gravel), respectively, with the dominant phyla, i.e., Chloroflexi, Proteobacteria, Actinobacteriota, Cyanobacteria, and Firmicutes, accounting for over 80% of the sequences. The relative abundances of the genera related to nitrification and dissimilatory nitrate reduction processes, i.e., denitrification, anammox and DNRA, varied from 0.16% to 0.89% and from 3.66% to 11.59%, respectively, while quantitative relationships confirmed NH4+-N transformation rate was jointly controlled by amoA, hzsB, nxrA and nrfA, and NO3--N removal rate by nirS, nosZ, narG, qnorB and nxrA. These findings may shed light on quantitative molecular mechanisms for nitrogen removal in TF CWs for the saline water treatment, providing a sustainable solution to nitrogen pollution problem in the estuary ecosystem.

Se transformation and removal by a cattail litter treatment system inoculated with sulfur-based denitrification sludge: Role of the microbial community composition under various temperature and aeration conditions.

With a narrow margin between deficiency and toxicity, rising levels of selenium (Se) are threatening aquatic ecosystems. To investigate the role of microorganisms in Se bioremediation, a cattail litter system inoculated with the sulfur-based denitrification sludge was conducted. The results show the litter, as a carrier and nutrient source for bacteria, efficiently removed Se by ~ 97.0% during a 12-d treatment with water circulating. As the major removal pathways, immobilization rates of selenite were ~ 2.9-fold higher than selenate, and the volatilization, contributing to ~ 87.7% of the total Se removal, was significantly correlated with temperature (positively) and oxidation-reduction potential (ORP; negatively). Using X-ray absorption spectroscopy to speciate litter-borne Se, more Se0 formed without aeration due to abundant Se-reducing bacteria, among which Azospira and Azospirillum were highly related to the removal of both Se oxyanions, while Desulfovibrio, Azoarcus, Sulfurospirillum, Thauera, Geobacter, Clostridium, and Pediococcus were the major contributors to selenate removal. Overall, our study suggests microbial Se metabolism in the litter system was significantly affected by temperature and ORP, which could be manipulated to enhance Se removal efficiency and the transformation of selenate/selenite into low toxic Se0 and volatile Se, reducing risks posed by the residual Se in the system.

Enhancement of a constructed wetland water treatment system for selenium removal.

Selenium (Se) is essential to most animals, whereas the gap between necessity and toxicity is narrow. Our previous work showed constructed wetlands were a promising solution to Se contamination in aquatic habitats. This study further examined effects of organic amendments and hydrologic regimes on Se removal by constructed wetlands. Our results suggest the removal efficiency exceeded 94% within 8 days for the systems with moderate and low organic carbon contents in the substrate, as a 98% removal of Se was obtained in three weeks for the system subjected to the 2-day wet/dry cycle. To mimic field wetlands, a litter layer was added to the cattail treatment system, which reduced waterborne Se much more rapidly than control, achieving a 77% removal of Se within 4 days. XAS results show all sediment Se was transformed to Se0 in the presence of litter, as SeMet (47%) dominated the Se adsorbed by the litter. The findings indicate the Se removal capacity of a constructed wetland would improve over time, especially via Se volatilization into the atmosphere and Se stabilization in the sediment with litter accumulating at the surface. Another mesocosm experiment showed the cattail floating system effectively removed Se, particularly selenate, by 99% in 48 h. To confirm that high performance, seven constructed wetland types were set up for comparison. The results show the cattail floating system was most effective in Se removal (93-100% at around 35 °C in summer and 51-100% at about 5 °C in winter). More research is needed to test the floating system under more field conditions and investigate the biomagnification and biotransformation of the removed Se along food chains. Seven constructed wetland types were set up for comparison.

Interactive effects of earthworm Eisenia fetida and bean plant Phaseolus vulgaris L on the fate of soil selenium.

Selenium (Se) is an essential micronutrient for animals with a narrow margin between essentiality and toxicity. Se toxicity is largely related to inorganic forms of Se in soil, i.e., selenite and selenate that enter food chains through plant uptake, threatening higher trophic level organisms. This experiment investigated effects of earthworm activity on Se bioavailability in soil and the subsequent plant uptake, using earthworm Eisenia fetida and bean plant Phaseolus vulgaris L, both exposed to either selenite or selenate at 1 or 4 mg Se kg-1 for 16 weeks. Plants took up selenate (up to 221-fold) faster than selenite, with up to 84% of the Se rapidly transported to shoots. In the presence of earthworms, Se accumulation obviously increased for selenate-supplied plants, leading to an up to 4% increase in Se translocation factor for all treatments except for 1 mg kg-1 selenite treatment. Earthworms also concentrated Se faster in tissues (up to 274 mg kg-1 DW) at exposure to selenate. For Se toxicity, Se speciation analysis was conducted on the plants and earthworms using XAS. Compared to worm-free treatments, the percentage of organo-Se, i.e., SeMet and CysSeSeCys, increased in beans (up to 34%) in the presence of earthworms for selenate, while the elemental Se portion was significantly reduced or absent, opposite to the results for selenite. Surprisingly, elemental Se (up to 65%) dominated earthworms, regardless of the form of Se supplied. In conclusion, earthworms clearly enhanced Se uptake and translocation in plants, leading to elevated Se levels in shoots. To prevent resulting hazards to humans and other animals, caution should be taken while consuming the shoots, particularly beans, harvested from the Se contaminated soil where earthworm activity is high. Finally, the significant reduction in soil Se suggests phytoextraction of Se from the soil could be improved using earthworms as an aid to plants.

Trophic transfer and biotransformation of selenium in the mosquito (Aedes albopictus) and interactive effects with hexavalent chromium.

As an essential micronutrient for animals with a narrow range between essentiality and toxicity, selenium (Se) usually coexists with chromium (Cr) in contaminated aquatic environments. This study investigated effects of three diets (Microcystis aeruginosa, Chlorella vulgaris and biofilms) exposed to Se or/and Cr on Aedes albopictus as a vector for the aquatic-terrestrial transfer of Se and Cr. Se(IV)-exposed mosquitoes concentrated Se up to 66-fold faster than Se(VI)-exposed ones, corresponding to the greater Se enrichment in Se(IV)-treated diets. Analysis using synchrotron-based X-ray absorption spectroscopy (XAS) showed that Se(0) (61.9-74.6%) dominated Se(VI)-exposed mosquitoes except for the C. vulgaris-fed larvae (organo-Se, 94.0%), while organo-Se accounted for 93.3-100.0% in Se(IV)-exposed mosquitoes. Cr accumulation in larvae (56.40-87.24 µg Cr/g DW) or adults (19.41-50.77 µg Cr/g DW) was not significantly different among all Cr(VI) treatments, despite varying diet Cr levels. With Cr(0) being dominant (57.7-94.0%), Cr(VI)-exposed mosquitoes posed little threat to predators. Although mosquitoes exposed to Se or Cr had shorter wings, adults supplied with C. vulgaris or biofilms co-exposed to Se(VI) and Cr(VI) had wings significantly (1.1-1.2 fold) longer than Se(VI) only exposed ones. Overall, our study reveals the role of Ae. albopictus in transferring waterborne Se and Cr from the contaminated aquatic ecosystem to the terrestrial ecosystem with the resulting eco-risks to wildlife in both ecosystems.

Mutual effects of selenium and chromium on their removal by Chlorella vulgaris and associated toxicity.

The release of selenium (Se) and chromium (Cr) into the environment from anthropogenic activities has posed a hazard to aquatic ecosystems. In this study, we used Chlorella vulgaris for Se/Cr bioremediation and evaluated their mutual effects on the removal efficiency. Our results found C. vulgaris highly effective in removing selenite-Se(IV) (49.5 ± 1.9%), selenate-Se(VI) (93.0 ± 0.5%), chromic nitrate-Cr(III) (89.0 ± 3.2%) and dichromate-Cr(VI) (88.1 ± 1.3%) over a 72 h period. Cr(VI) significantly impeded Se removal, particularly for selenate, due to competition between both for algal uptake, whereas Cr(III) obviously enhanced Se removal, increasing Se volatilization by ~29%. Similarly, Se significantly increase Cr removal rates, with a maximum of 94.6 ± 0.2% for the algal co-exposed to Se(IV) and Cr(III). To reduce residual pollutants in the alga, we applied combustion as a post-treatment to burn off >99% of the biomass Se for all Se treatments, whereas most of the biomass Cr (54.7-81.6%) remained in the ash at significantly higher levels (~7430 µg Cr/g DW). For toxicity, our speciation analysis found organo-Se (SeCys and SeMet) dominant in the alga exposed to Se, particularly selenite. No Cr(VI) but Cr(III) forms were detected in all Cr-exposed alga. Elemental Se disappeared from all Se-exposed alga in the presence of Cr(VI), while Se resulted in the emergence of Cr-acetate in all Cr(III)-treated alga. After combustion, mineral Se, particularly elemental Se dominated most of the ash; likewise, elemental Cr, along with Cr2O3, was found in all the ash. Overall, our research would contribute to developing a low ecotoxic algal treatment system for Se/Cr contaminated water.

Depth-specific distribution and significance of nitrite-dependent anaerobic methane oxidation process in tidal flow constructed wetlands used for treating river water.

Tidal flow constructed wetlands (TF CWs) have been considered an effective approach to treat contaminated river water, as well as a significant role in global matter cycles, especially for carbon and nitrogen. Notably, it has been thought that methane oxidation was completely catalyzed by the aerobic process, ignoring the anaerobic methane oxidation, such as the nitrite-dependent anaerobic methane oxidation (n-damo) process. In our current work, therefore, we used molecular and stable isotopes to investigate the biodiversity, quantity and potential rate of n-damo bacteria in the TF CWs located in the Xisha Wetland Park in the Yangtze River estuary, China. The results revealed that n-damo process was active in the collected soil cores, with a decreasing trend along water depths and rates ranging from 8.48 to 23.45 nmol CO2 g-1 dry soil d-1. The n-damo bacterial contributions to CH4 oxidation and N2 production in TF CWs reached 9.49-26.26% and 20.73-47.11%, respectively, suggesting that n-damo bacteria was an important nitrogen and methane sink in the TF CWs, but had been previously overlooked. The copy numbers of total bacterial 16S rRNA and pmoA genes were 1.84-11.21 × 109 and 0.59-2.72 × 106 copies g-1 ds, respectively, as the higher abundance was found in the soil at lower water levels during tidal submergence. Diverse n-damo bacterial 16S rRNA gene sequences belonged to group B, C and D were measured, and it was found that group B and C were the most frequently measured n-damo clusters in the TF CWs. In addition, nitrite was the key factor regulating the n-damo bacterial distribution in the TF CWs. This study would broaden our horizons and help us better understand the nitrogen and methane cycles in tidal ecosystems.

Selenium removal by clam shells and gravels amended with cattail and reed litter.

Increasing selenium (Se) levels in aquatic environments raise concerns all over the world. This study investigated effects of organic amendments (cattail and reed litter) and porous media (gravels and clam shells) on Se removal efficiency of horizontal subsurface flow constructed wetlands. Our results show clam shells reduced Se (by mass) up to 2.4-fold faster than gravels within 19 days. Using clam shells as the sole substrate, 96.3% removal efficiency was obtained for cattail litter as an amendment, compared to 88.7% for reed litter over 10 days, although the latter released carbon and nitrogen at least 1.4-fold faster than the former. Meanwhile, speciation analysis suggests Se0 (~75%) and organo-Se (~94%) dominated the biofilms on shells and plant litter, respectively, as substrates. Overall, this study suggests clam shells and cattail litter as an effective medium and carbon source, respectively, can enhance microbial Se removal without posing risks to wildlife health.

Relationship between selenium removal efficiency and production of lipid and hydrogen by Chlorella vulgaris.

In our previous studies, Chlorella vulgaris had proven highly efficient in removing selenium (Se) from water, while the disposal of Se containing in algal biomass was still an issue of concern. Firstly, this research suggests algal Se could be released back to water, posing risks to aquatic wildlife. Thus, we further explored the possibility of using C. vulgaris to remove Se and produce lipid and hydrogen simultaneously. Our results show the higher percentage of saturated fatty acids, especially palmitic acid, was found in the sulfur (S) deprived algae exposed to either selenate or selenite, although the highest lipid content (21.9%) was found in the selenite treated algae in full BG11 medium. In addition, compared with the Se free algae, hydrogen production rate was 2.1- and 4.3-fold higher for the selenate and selenite treated algae, respectively. Se removal efficiency achieved by the selenite treated algae through accumulation and volatilization was 2.3-fold higher than the selenate treatment under hypoxic condition with S deprived, which is in contrast to the results obtained under aerobic conditions.

Selenium removal and biotransformation in a floating-leaved macrophyte system.

Selenium (Se) is an essential micronutrient for animals with a relatively narrow margin between essentiality and toxicity. To evaluate Se removal efficiency by a constructed wetland treatment system and its potential eco-risk, a floating-leaved macrophyte system was constructed, consisting of three main trophic levels. Over 21-d treatment, water Se concentration was gradually reduced by 40.40%, while 24.03% and 74.41% of the removed Se were found in the plant Nymphoides sp. and sediment, respectively. Among plant tissues, roots accumulated the highest Se level, although the greatest total Se was found in stems, followed by leaves, roots and rhizomes. X-ray absorption spectroscopy revealed that 82.65% of the absorbed selenite by the plants was biotransformed to other forms, as organo-Se species accounted for 45.38% of the Se retained in the sediment, which was primarily responsible for the entry of Se into the detritus food chain. The proportion of organo-Se compounds increased with trophic levels from sediments to fish, indicating, instead of direct uptake of selenite, the food chain transfer and biotransformation of Se may serve as a key exposure route for Se in aquatic organisms. When exposed to organo-Se compounds, i.e., SeCys and SeMet, the plants, shrimp and fish tended to accumulate more Se. However, the greater trophic transfer factor was obtained for selenate, leading to higher Se levels accumulated in fish. Overall, in addition to key mechanisms involved in Se removal, our research also provides a much better understanding of the potential eco-risk that may be posed by the floating-leaved plant system for bioremediation of Se via food chain transfer and biotransformation, paving the way for a low eco-toxic treatment system for Se remediation.

Development of an algal treatment system for selenium removal: Effects of environmental factors and post-treatment processing of Se-laden algae.

In developing an algal treatment system, selenium (Se) removal efficiency by Chlorella vulgaris was evaluated under various conditions such as Se concentration, algal density, temperature and pH. A maximum removal efficiency plateau of ∼90% was observed between 1000-3000 µg Se/L while the tolerance of Se toxicity was found at 6000 µg Se/L. C. vulgaris of 0.75 g DW/L showed the highest removal efficiency (84%), and volatilization was dominant below 1.37 g DW/L. Se volatilization was two times higher at 25 °C than at 20 °C in the first 24 h. Moreover, the highest removal efficiency (77%) was obtained at pH 8.0, compared to 66.5% at pH 6.5 and 40% at pH 10.0. To prevent ecotoxicity, Se laden algae were further burned to ashes or filtered out by Anodonta woodiana. After burning, biomass Se was reduced by 99%, with organo-Se entirely converted into inorganic Se, lowering Se bioavailability. A. woodiana removed 54% of Se in 24 h, leading to Se bioaccumulation in soft tissues, which may serve as dietary Se supplements for human health. Our results suggest the cleanup of Se-contaminated water from either agricultural runoff or industrial discharge could be achieved using an algal treatment system with minimum potential ecotoxicity.