Biodegradation Of Nitrobenzene Research Articles

Electrical stimulation promotes synchronous nitrobenzene bio-degradation and vanadate bio-reduction

The combined pollution of nitrobenzene (NB) and pentavalent vanadium [V(V)] in groundwater brings great risks to the ecological environment and human health. In this study, we investigated the biological removal of NB and V(V) from the groundwater with electrical stimulation. Up to 94.3 ± 1.32% of NB and 87.2 ± 0.856% of V(V) were removed at 0.6 V. Although the removal of NB and V(V) was reduced at increased loading of NB or V(V), supplementing more ethanol substantially improved the removal performance. A few genera (e.g., Alkalibacter, Anaerovorax, Dechloromonas, Xanthobacter, Acetobacterium, and Clostridium_sensu_stricto_12) were distinctively enriched in the bioreactors spiked with both NB and V(V). The metagenome analysis revealed functional genes (e.g., nbzAa, catA, CYC, pilA) that were affiliated with NB and V(V) removal. It is speculated that the NB was first electrochemically reduced to aniline, and then mineralized to other intermediates (e.g., catechol). Cathode donating electron, glutathione and nitrite-reductase contributed to the bioreduction of V(V) to tetravalent vanadium. This study can advance our understanding of management of combined pollution of heavy metals and refractory organics in the groundwater.

Nitrobenzene reduction promoted by the integration of carbon nanotubes and Geobacter sulfurreducens

Electron shuttles (ES) can mediate long-distance electron transfer between extracellular respiratory bacteria (ERB) and the surroundings. However, the effects of graphite structure in ES on the extracellular electron transfer (EET) process remain ambiguous. This work investigated the function of graphite structure in the process of nitrobenzene (NB) degradation by Geobacter sulfurreducens PCA, in which highly aromatic carbon nanotubes (CNTs) was studied as a typical ES. The results showed that the addition of 1.5 g L−1 of CNTs improved the NB biodegradation up to 81.2%, plus 18.8% NB loss due to the adsorption property of CNTs, achieving complete removal of 200 μM NB within 9 h. The amendment of CNTs greatly increased the EET rate, indicating that graphite structure exhibited excellent electron shuttle performance. Furthermore, Raman spectrum proved that CNTs obtained better graphite structure after 90 h of cultivation with strain PCA, resulting in higher electrochemical performance. Also, CNTs was perceived as the “Contaminant Reservoir”, which alleviated the toxic effect of NB and shortened the distance of EET process. Overall, this work focused on the effects of material graphite structure on the EET process, which enriched the understanding of the interaction between CNTs and ERB, and these results might promote their application in the in-situ bioremediation of nitroaromatic-polluted environment.

Roles of Pseudomonas aeruginosa and Ensifer adhaerens in accelerating nitrobenzene biodegradation by removing an inhibitory intermediate

Pseudomonas aeruginosa and Ensifer adhaerens were isolated from aerobic biomass acclimated to nitrobenzene (NB) based on ability to biodegrade NB and its intermediate, o-aminophenol. Bioaugmenting either of them into the acclimated biomass accelerated NB biodegradation. The NB removal rates were as 19% and 144% faster, respectively, with P. aeruginosa and E. adhaerens, compared to the acclimated biomass, even though the dry-weight concentrations of P. aeruginosa and E. adhaerens were only ∼10% of the acclimated biomass. Biodegradation was further accelerated by another 36%–217% when P. aeruginosa and E. adhaerens, respectively, were bioaugmented into the acclimated biomass at ∼10% of the dry weight. The primary benefit of P. aeruginosa and E. adhaerens came from their ability to biodegrade rapidly an inhibitory intermediate of NB biodegradation, o-aminophenol. Phylogenetic analysis showed that Thauera, which is known to be active in NB biodegradation, was by far the dominant genus in the acclimated biomass. Although the abundances of E. adhaerens and P. aeruginosa were minimal in the acclimated biomass (∼0.01%), they played an important role in accelerating NB biodegradation by bioaugmention once they were isolated, enriched, and bioaugmented.

Substantially enhanced degradation of nitrobenzene by biosponge iron system: Process and mechanisms

Considering the high reactivity, cost-efficiency, environmental friendliness, the zero-valent iron-microbe (bio-ZVI) technology for refractory organic wastewater treatment has obtained worldwide attentions. This paper submerged sponge iron (SFe, a typical ZVI material) in activated sludge to form biological sponge iron (bio-SFe) systems, while the iron sludge and ordinary activated sludge systems were taken as blank control. To study the strengthening effect and degradation mechanism of bio-SFe systems, Nitrobenzene (NB) was chose as a target contaminant. The interaction between SFe and microorganisms improved the NB removal efficiency, prompted the formation of Fe(II)/ROS and facilitated the biodegradation process. Specifically, the Fenton-like effects for NB removal increased from 3.6 % in mono-SFe system to 11.79 % in SFe coupled with iron sludge system. This facilitated the production of OH could constantly attack and eventually mineralize NB. Additionally, biodegradation of NB was the predominant pathway (64.25 %), when iron sludge combined with SFe, the removal of NB could ascend to 92.11 %. SFe created a more favorable environment for certain core functional genera related to recalcitrant pollutants reduction and iron cycle. According to the accumulation of NH4+-N content and the identification of the degradation intermediates of NB in bio-SFe system, the partial reduction biodegradation of NB was finally confirmed.

The elucidation of the biodegradation of nitrobenzene and p-nitrophenol of nitroreductase from Antarctic psychrophile Psychrobacter sp. ANT206 under low temperature

Psychrobacter is one important typical strain in the Antarctic environment. In our previous study, Psychrobacter sp. ANT206 from Antarctica with novel cold-adapted nitroreductase (PsNTR) could biodegrade nitrobenzene and p-nitrophenol in low temperature environment. In this study, the in-frame deletion mutant of psntr (Δpsntr-ANT206) that displayed well genetic stability and kanamycin resistance stability was constructed using allelic replacement method. Additionally, Δpsntr-ANT206 was more sensitive to nitrobenzene and p-nitrophenol in the comparison of heat and hyperosmolarity, suggesting that psntr gene participated in the regulation of the tolerance against nitro-aromatic compounds (NACs). Further analysis was conducted by integrated gas chromatography-mass spectrometry (GC-MS), and several metabolites were identified. Among them, ethylbenzene, L-Alanine, citric acid, aniline, 4-aminophenol and other metabolites were different between the wild-type strain and Δpsntr-ANT206 under nitrobenzene and p-nitrophenol stress at different time periods under low temperature, respectively. These data could increase the knowledge of the construction of deletion mutant strains and biodegradation mechanism of NACs of typical strains Psychrobacter from Antarctica, which would also provide the basis of the molecular technique on the regulation of bioremediation of the contaminants under low temperature in the future.

Enhancing nitrobenzene biodegradation in aquatic systems: Feasibility of using plain soil as an inoculant and effects of adding ascorbic acid and peptone

Nitrobenzene (NB) is recalcitrant to microbial biodegradation due to the electron-deficient character of the nitro group (NO2−). Prior work has found that the reductant could enhance NB biodegradation by providing excess electron donors. However, the existing theory couldn't explain the increase-and-decrease pattern of the NB biodegradation rate with an increase in a reductant concentration. Our results suggest that the reductant affects NB biodegradation by two mechanisms: the available electron donors and the stimulation or inhibition of biomass growth, which are linked by a pseudo-first-order reaction kinetics. In addition, the results showed that directly inoculating the plain soil into the aquatic system and then allowing the synergistic effect of the organic reductant (ascorbic acid) and the substrate (peptone) enhance NB biodegradation. Employing the new method, 200 mg L−1 NB was transformed in 72 h. GC–MS analysis detected two novel intermediate metabolites, indicating that NB was degraded into aniline and further transformed into acetanilide and 9-octadecenamide before its mineralization. This study sheds light on how to exploit the synergistic effects of the availability of excess electron donors and biomass growth by controlling the reductant and a substrate in the right concentration range (e.g., ascorbic acid < 0.8 mgL−1 + peptone).

Substantially enhanced anaerobic reduction of nitrobenzene by biochar stabilized sulfide-modified nanoscale zero-valent iron: Process and mechanisms

Nanoscale zero-valent iron (nZVI), although being increasingly used in anaerobic systems for strengthening the removal of various refractory pollutants, is limited by various inherent drawbacks, such as easy precipitation, passivation, poor mass and electron transfer. To address the above issues, biochar stabilized sulfide-modified nZVI (S-nZVI@BC) was added into an up-flow anaerobic sludge blanket (UASB) to investigate the enhancement of anaerobic biodegradation of nitrobenzene (NB) and its impacts on microbial community structure. The results demonstrated that both NB reduction and aniline formation could be substantially facilitated in S-nZVI@BC coupled system compared to other anaerobic ones coupled with nZVI or S-nZVI. The dosage of S-nZVI@BC resulted in the formation of densely packed aggregates, evidently increased the extracellular polymeric substances content, promoted the volatile fatty acids transformation and stimulated the methane yield. Furthermore, species related to fermentation (Bacteroides and Longilinea), methanogenesis (Methanosarcina and Methanomethylovorans), electroactivity (Pelobacter, Thiobacillus and Phaselicystis) as well as reduction (Desulfovibrio) were considerably enriched in S-nZVI@BC coupled system. The activities of electron transport, total adenosine triphosphate, nitroreductase and NAD(P)H, which were closely related to microbial activity and NB transformation, were increased noticeably in S-nZVI@BC coupled anaerobic system. This study demonstrated the promising potential for long-term operation and full-scale application of S-nZVI@BC coupled system for the treatment of NB containing wastewater.

Simultaneous anaerobic and aerobic transformations of nitrobenzene

Aerobic biodegradation of nitrobenzene (NB) produces nitrophenol (NP), which has stronger toxicity than NB. Anaerobic biodegradation of NB produces aniline (AN), which has weaker toxicity, but is a dead-end product in anaerobic conditions. Accumulation of AN should be overcome by coupling anaerobic and aerobic transformations: NB is transformed to AN in an anaerobic zone of the bioreactor, and AN is then transformed in an aerobic zone. A vertical baffled bioreactor (VBBR) was employed for NB biodegradation with a controlled dissolved oxygen (DO) concentration. NB biodegradation was accelerated by simultaneous anaerobic and aerobic transformations, since AN was biotransformed by a mono-oxygenase reaction. Adding exogenous electron donor (acetate) enhanced NB removals when the DO concentration was ∼0.5 mg/L, because the donor accelerated mono-oxygenations of NB and AN. Coupling anaerobic and aerobic transformations can be a valuable strategy for biodegrading organic compounds that undergo aerobic and anaerobic biotransformations.

A novel approach for enhancing bacterial strains’ Nitrobenzene degradation rate

This study presents a novel approach that could efficiently enhance the rate of the strains' nitrobenzene biodegradation by utilizing the synergistic effect of organic reductants and a co-substrate. The authors found that the rates of strains' nitrobenzene biodegradation were increased along with the enhancement of the electron-donating power of organic additives. Combining the electron-donating power of organic reductants acting as catalyst with the ability of a co-substrate to increase the biomass of the strains improved the rate of nitrobenzene biodegradation. Employing the new approach, the rates of nitrobenzene biodegradation of the five targeted strains (Staphylococcus carnosus S12, Bacillus amyloliquefaciens YX0, Bacillus subtilis YX3, Bacillus cereus Y10, and Bacillus cereus YX2) were enhanced from 8.4%, 16.4%, 23.8%, 11.1% and 8.3% in a salt medium up to 85.1%, 88.6%, 95.8%, 60.5%, and 58.6%, respectively. The process described in this research may offer a protocol useful for enhancing the strains’ biodegradation rate of other nitroaromatic compounds.

Nitrobenzene degradation pathways and their interaction with sulfur and nitrogen transformations in horizontal subsurface flow constructed wetlands

Nitrobenzene (NB) degradation pathways and their interaction with sulfur and nitrogen transformations in constructed wetlands are not fully understood. This study investigates the effectiveness of horizontal subsurface flow constructed wetlands (HSSFCWs) in NB biodegradation including the biodegradation pathways and microbial interactions. The investigation was based on two laboratory-scale wetlands planted with Juncus effusus. One wetland was intermittently aerated while the other was not. Data about NB degradation were collected at varying influent loading rates [35 and 140mg/L] for 120-day period. From the results, both wetlands exhibited an overall performance of 99% NB removal. More so, intermittent aeration did not significantly improve overall performance but rather it enhanced the buffer capacity of NB degradation to shock influent loading. The result also demonstrates that NB mineralization account for about 96% and 4% for volatilization. Formation of ammonium from NB degradation increased the ammonium concentration in the pore water. In unaerated wetland, 84mg/L of sulfide accumulated from complex microbial reaction involving sulfate as electron acceptor and NB as electron donor.

The fate of a nitrobenzene-degrading bacterium in pharmaceutical wastewater treatment sludge

This paper describes the fate of a nitrobenzene-degrading bacterium, Klebsiella oxytoca NBA-1, which was isolated from a pharmaceutical wastewater treatment facility. The 90-day survivability of strain NBA-1 after exposure to sludge under anaerobic and aerobic conditions was investigated. The bacterium was inoculated into sludge amended with glucose and p-chloronitrobenzene (p-CNB) to compare the bacterial community variations between the modified sludge and nitrobenzene amendment. The results showed that glucose had no obvious effect on nitrobenzene biodegradation in the co-metabolism process, regardless of the presence/absence of oxygen. When p-CNB was added under anaerobic conditions, the biodegradation rate of nitrobenzene remained unchanged although p-CNB inhibited the production of aniline. The diversity of the microbial community increased and NBA-1 continued to be one of the dominant strains. Under aerobic conditions, the degradation rate of both nitrobenzene and p-CNB was only 20% of that under anaerobic conditions. p-CNB had a toxic effect on the microorganisms in the sludge so that most of the DGGE (denaturing gradient gel electrophoresis) bands, including that of NBA-1, began to disappear under aerobic conditions after 90days of exposure. These data show that the bacterial community was stable under anaerobic conditions and the microorganisms, including NBA-1, were more resistant to the adverse environment.

Intimately coupling of photolysis accelerates nitrobenzene biodegradation, but sequential coupling slows biodegradation

Photo(cata)lysis coupled with biodegradation is superior to photo(cata)lysis or biodegradation alone for removal of recalcitrant organic compounds. The two steps can be carried out sequentially or simultaneously via intimate coupling. We studied nitrobenzene (NB) removal and mineralization to evaluate why intimate coupling of photolysis with biodegradation was superior to sequential coupling. Employing an internal circulation baffled biofilm reactor, we compared direct biodegradation (B), biodegradation after photolysis (P+B), simultaneous photolysis and biodegradation (P&B), and biodegradation with nitrophenol (NP) and oxalic acid (OA) added individually and simultaneously (B+NP, B+OA, and B+NP+OA); NP and OA were NB’s main UV-photolysis products. Compared with B, the biodegradation rate P+B was lower by 13–29%, but intimately coupling (P&B) had a removal rate that was 10–13% higher; mineralization showed similar trends. B+OA gave results similar to P&B, B+NP gave results similar to P+B, and B+OA+NP gave results between P+B and P&B, depending on the amount of OA and NP added. The photolysis product OA accelerated NB biodegradation through a co-substrate effect, but NP was inhibitory. Although decreasing the UV photolysis time could minimize the inhibition impact of NP in P+B, P&B gave the fastest removal of NB by accentuating the co-substrate effect of OA.

Biodegradation of nitrobenzene in a lysogeny broth medium by a novel halophilic bacterium Bacillus licheniformis

The Bacillus licheniformis strain YX2, a novel nitrobenzene-degrading halophilic bacterium, was isolated from active sludge obtained from a pesticide factory. Strain YX2 can withstand highly acidic and alkaline conditions and high temperatures. Degradation of nitrobenzene (200mgL−1) by YX2 exceeded 70% after 72h in lysogeny broth medium (pH 4–9). Under optimal degradation conditions (33°C, pH 7 in LB medium) YX2 degraded 50, 100, 200, and 600mgL−1 nitrobenzene within 36, 36, 72, and 156h, respectively. Even in the presence of benzene, phenol or aniline, strain YX2 efficiently degraded nitrobenzene. Furthermore, strain YX2 completely degraded 600mgL−1 nitrobenzene in 7% NaCl (w/w). Thus, our data show that strain YX2 may have promise for removing nitrobenzene from complex wastewaters with high salinity and variable pH.

Study of archaea community structure during the biodegradation process of nitrobenzene wastewater in an anaerobic baffled reactor

Nitrobenzene (NB) is an important industrial raw material in organic synthesis. However, successful biological treatment is challenging since NB wastewater is biologically toxic. During the experiment, the performance was examined during the acclimation process of NB in an anaerobic baffled reactor (ABR). The removal efficiencies of NB and chemical oxygen demand were 98% and 90%, respectively. Furthermore, by applying polymerase chain reaction-denaturing gradient gel electrophoresis (DGGE) technology of 16SrDNA, this paper analyzes the structural change of the archaea community in the ABR before and after NB acclimation and identifies the dominant community. The sequence structure analysis of archaea 16S rDNA in DGGE profiles shows that after NB biodegradation, the archaea-dominant community primarily consists of Methanothrix soehngenii from Methanosarcina, Methanosaeta concilii from Methanosaeta, Methanobacterium beijingense 8-2, uncultured Archaeon TA04, and uncultured Methanobacterium sp. isolated from environmental samples, which may be the important functional archaea in an ABR for NB biodegradation. The study of the population structure distribution and the dominant archaea community is helpful for elucidating the mechanisms of the anaerobic biodegradation mechanism of NB.

Characterization of the nitrobenzene-degrading strain Pseudomonas sp. a3 and use of its immobilized cells in the treatment of mixed aromatics wastewater

A bacterial strain Pseudomonas sp. a3 capable of degrading nitrobenzene, phenol, aniline, and other aromatics was isolated and characterized. When nitrobenzene was degraded, the release of NH(4) (+) was detected, but not of NO(2) (-). This result implied that nitrobenzene might have a partial reductive metabolic pathway in strain a3. However, aniline appeared as one of the metabolites during the aerobic degradation of nitrobenzene. Moreover, the appearance of 2-aminophenol during aniline degradation by strain a3 indicated that novel initial reactions existed during the degradation of nitrobenzene and aniline by strain a3. Strain a3 was immobilized in the mixed carrier of polyvinyl alcohol and sodium alginate to improve its degrading efficiency. The optimal concentrations of polyvinyl alcohol and sodium alginate in the mixed carrier were 9 and 3 %, respectively. The immobilized cells had stable degradation activity and good mechanical properties in the recycling tests. The immobilized cells also exhibited higher tolerances in acidic (pH 4-5) and highly saline (10 % NaCl) environments than those of free cells. The biodegradation of nitrobenzene mixed with aniline and phenol using immobilized cells of Pseudomonas sp. a3 was also greatly improved compared with those of free cells. The immobilized cells could completely degrade 300 mg L(-1) nitrobenzene within 10 h with 150 mg L(-1) aniline and 150 mg L(-1) phenol. This result revealed that the immobilized cells of Pseudomonas sp. a3 could be a potential candidate for treating nitrobenzene wastewater mixed with other aromatics.

Isolation and characterization of a nitrobenzene-degrading bacterium Klebsiella ornithinolytica NB1 from aerobic granular sludge

Strain NB1 was isolated from aerobic granular sludge capable of degrading nitrobenzene. Effects of several factors including pH, temperature, salinity, and second carbon/nitrogen source as co-substrate on the biodegradation of nitrobenzene by strain NB1 were investigated. Results showed that the optimal conditions for the biodegradation of nitrobenzene by strain NB1 were at pH 7.0 and 28–35°C. Under these conditions, the biodegradation rate of nitrobenzene could reach 9.29mgl−1h−1 when the initial nitrobenzene concentration was 600mgl−1. The addition of 1840mgl−1 glucose or 500mgl−1 ammonium chloride could slightly enhance the biodegradation efficiency, but even more addition of glucose or ammonium chloride delayed the biodegradation. Strain NB1 could degrade nitrobenzene in the presence of less than 3% NaCl (w/v %). Based on morphological and physiological characteristics as well as the phylogenetic analysis of 16S rDNA sequences, strain NB1 was identified as Klebsiella ornithinolytica.

Tolerance, uptake and removal of nitrobenzene by a newly-found remediation species Mirabilis jalapa L.

The growth, photosynthesis rate, and ultrastructure of Mirabilis jalapa L. as a newly-found remediation species under stress of nitrobenzene (NB) and its uptake and removal of NB by the plants were investigated. The results showed that M. jalapa plants could endure contaminated soils by lower than 10.0mgNBkg−1 because there was no decrease in the total length of the plant roots, the maximum length of the hypocotyle, the length of the first seminal root, the height of the shoots and the dry biomass of the seedlings as well as the photosynthesis rate of the plants compared with those in the control. In particular, the growth of the plants could be significantly (P<0.01) enhanced by 0.1mgNBkg−1 under unautoclaved and autoclaved soils. Ultrastructural observations on leaf cells of the plants found that these cells had smooth, clean and continuous cell membranes and cell walls, indicating that there was no obvious damage by NB in comparison with those in the control. Although the absorption of NB in shoots and roots of M. jalapa was weak, plant-promoted biodegradation of NB was considerable and the dominant contribution in the removal of NB from contaminated soils, suggesting the feasibility of M. jalapa applied to phytoremediation of NB contaminated soils.

The role of nitrobenzene on the yield of trihalomethane formation potential in aqueous solutions with Microcystis aeruginosa

Algae are one of the most important disinfection by-product (DBP) precursors in aquatic environments. The contents of DBP precursors in algae are influenced by not only environmental factors but also some xenobiotics. Trihalomethane formation potential (THMFP) in both the separate and interactive pollution of Microcystis aeruginosa and Nitrobenzene (NB) was investigated in batch experiment to discover the effects of xenobiotics on the yield of DBP precursors in the algal solution. The results show that in the separate NB solution, NB did not react with Cl 2 to form trihalomethane (THM), whereas in the algae solution, THMFP had a significant positive linear correlation with M. aeruginosa density in both solution and extracellular organic matter (EOM). The correlation coefficients were 0.9845 ( p = 3.567 × 10 −4) and 0.9854 ( p = 1.406 × 10 −4), respectively. According to regression results, about 77.9% of the total THMFP came from the algal cells, while the rest came from EOM. When the interactive pollution of M. aeruginosa and NB occurred, the growth of algae was inhibited by NB. The density of M. aeruginosa in a high concentration NB solution (280 μg/L) was only 71.1% of that in the solution without NB after 5 days of incubation. However, THMFP in the mixture (algae and NB) and the EOM did not change significantly, and the productivity of THMFP by the algae (THMFP/10 8cells) increased with the increase in NB concentration. There was a significant linear correlation between THMFP/10 8cell and NB concentration ( r = 0.9117, p < 0.01), which shows the contribution of the algae to THM formation was enhanced by NB. This result might be caused by the increased protein productivity and the biodegradation of NB by M. aeruginosa.

Biodegradation of nitrobenzene by aerobic granular sludge in a sequencing batch reactor (SBR)

The biodegradation of nitrobenzene by aerobic granular was investigated in this study. After cultivation in feeding-increasing form, nitrobenzene was degraded as sole carbon and nitrogen resource by aerobic granular sludge cultivated in SBR reactor and was not inhibited by nitrobenzene concentration up to 600mg/L. Aeration time has a significant effect on nitrobenzene removal, especially at lower than 15h. The highest nitrobenzene loading was 0.33kg/(m3d) attained complete removal. Feeding time and shock loading had little effect on nitrobenzene removal. Micrococcus luteus, Bacillus sp., Pseudomonas sp., Comamonas sp., Acinetobacter sp. and Rhodococcus sp. are the main nitrobenzene degrading bacteria in the granular sludge. SBR with granular sludge was an efficient, reliable and stable process for nitrobenzene treatment. These results may provide a guideline for nitrobenzene wastewater treatment.

Transport and fate modeling of nitrobenzene in groundwater after the Songhua River pollution accident

In 2005 a pollution accident occurred in the Songhua River, which is geographically located next to groundwater supply plants. This caused public concern about the transport and fate of nitrobenzene (NB) in the groundwater. This paper discusses the mechanisms and effects of the transport and fate of NB in groundwater based on pilot scale experiments conducted in the laboratory, including a simulation experiment, bench-scale batch tests and a one-dimensional numerical model. Parallel batch tests showed that the adsorption of NB to the clay and sand followed the Langmuir-type isotherm, and clay had a greater NB adsorption capacity than sand. NB biodegradation in different conditions was well fitted by the Monod equation and the q max values varied from 0.018 to 0.046 h −1. Results indicated that NB’s biodegradation was not affected by the initial NB concentration. Numerical modeling results indicated a good match between computed and observed data, and in the prediction model NB entered the groundwater after the pollution accident. However, the highest concentration of NB was much lower than the allowable limit set by the national standard (0.017 mg/L).