Aquatic Acute Toxicity Assessments of Molybdenum (+VI) to Daphnia Magna

Chemosphere 147 (2016) 82e87

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Aquatic acute toxicity assessments of molybdenum (þVI) to Daphnia magna

* Chi-Wei Wang, Chenju Liang , Hui-Ju Yeh

Department of Environmental Engineering, National Chung Hsing University, 250 Kuo-kuang Road, Taichung 402, Taiwan highlights graphical abstract

þ Impact of Mo6 in aquatic system via D. magna acute toxicity bioassay was studied. The acute toxicity increased in the

order: Na2MoO4‧ 2H2O < MoO3 < (NH4)6Mo7O24‧4H2O. The level of Mo toxicity is highly dependent on the form of molybdenum salts.

LC50 determined for Mo can be used to establish toxicity values for Mo in aquatic systems. article info abstract

Article history: Generally, molybdenum (Mo) metals in the environment are very rare, but wastewater discharges from Received 2 October 2015 industrial processes may contain high concentrations of Mo, which has the potential to contaminate Received in revised form water or soil if not handled properly. In this study, the impact of three common compounds of hexavalent 1 December 2015 Mo (sodium molybdate (Na MoO ‧2H O), ammonium molybdate ((NH ) Mo O ‧4H O) and molybde- Accepted 17 December 2015 2 4 2 4 6 7 24 2 num trioxide (MoO3)) in an aquatic system were assessed based on 48-h exposure acute toxicity to Available online xxx þ 2 Daphnia magna (D. magna). The LC50 toxicities for associated conjugate ions including Na ,Cl ,SO4 , and þ Handling Editor: Jim Lazorchak NH4 were determined. Furthermore, the LC50 values for the three forms of hexavalent Mo were determined, and the acute toxicities of the Mo forms were found to follow the order: (NH4)6Mo7O24‧ 1 Keywords: 4H2O > MoO3 > Na2MoO4‧2H2O in solution. (NH4)6Mo7O24‧4H2O exhibited the lowest LC50 of 43.3 mg L Industrial wastewater (corresponding to 23.5 mg Mo L 1) among the three molybdenum salts. The research conﬁrmed that the Semiconductor toxicity of molybdenum in the aquatic system is highly dependent on the form of molybdenum salts Daphnia magna used, and is also associated with the inﬂuence of the background water quality. Ammonium molybdate © 2015 Published by Elsevier Ltd. Median lethal concentration

1. Introduction traditional toxic metals (such as hexavalent chromium) in many industrial processes (Shields, 2013; Heijerick et al., 2012). Mo is Molybdenum (Mo), with exceptional thermal conductivity and now used prevalently in the semiconductor and thin-ﬁlm- electrical conductivity, low thermal expansion coefﬁcient, and good transistor liquid-crystal display industries. Typically, the concen- corrosion resistance, was gradually adopted as a replacement of the tration of Mo in natural environments is very low, but discharges from industrial processes may contain high concentrations of Mo, which poses the potential to contaminate water or soil if releases to the environment occur. Mo has a wide range of oxidation states, * Corresponding author. þ þ þ E-mail address: [email protected] (C. Liang). i.e., 2to 6; however, the most stable valences are 4 and 6 http://dx.doi.org/10.1016/j.chemosphere.2015.12.052 0045-6535/© 2015 Published by Elsevier Ltd. C.-W. Wang et al. / Chemosphere 147 (2016) 82e87 83

(Huang et al., 2012). Among them, molybdate (þVI), an oxyanion predict the potential impact of new chemicals, or whole effluents, with hexavalent Mo, is the most soluble Mo salt compound. on the aquatic environment (Barmentlo et al., 2015; Stanley et al., In the organism, Mo is one of the essential dietary micro- 2013; Yim et al., 2006). This research was conducted to explore nutrients (Atia, 2008; Smedley et al., 2014), but excessive ingestion the impact of Mo (þVI) from different molybdate salts in aquatic of Mo could result in anemia, gastrointestinal disturbances, hypo- system via D. magna acute toxicity bioassay. There are seven thyroidism, bone and joint deformities, sterility, liver and kidney commercially available molybdenum salts, including ammonium abnormalities, etc. (Shields, 2013; Diamantino et al., 2000). In view molybdate, potassium molybdate, sodium molybdate, phosphor of this, the World Health Organization recommends a limit of molybdic acid, molybdenum disulphide, molybdenum trioxide, and 70 mgL 1 Mo in drinking water (WHO, 2011). Moreover, Taiwan, a molybdic acid (Sajan overseas Ltd., 2015). Heijerick et al. (2012) 2 high-tech industry export-oriented country, formulated the current indicated that the simple MoO4 is most likely to be formed from limit of 0.6 mg L 1 Mo for industrial effluents. Generally, the reg- different molybdenum containing substances under common ulatory concentration or standard for any controlled substance environmental conditions. The predominating ionic species of Mo 6þ would be low compared to the LC50 in order to ensure that the toxic (þVI) reported to be present in solution at pH > 2, were Mo7O24 2 substance produces no observed adverse effects to the organism (pH 2e7) and MoO4 (pH > 4) (Xiong et al., 2011). Molybdenum when exposure occurs in the environment. It has been reported trioxide (MoO3) is produced on the largest scale of any molybde- that the concentration of Mo is low in natural environments, e.g., num compound (US Research Nanomaterials Inc., 2015). Therefore, 6þ 2 around 5 nM in river waters (Martin and Meyback, 1979). However, Mo7O24 , MoO4 and MoO3 were selected as reference test sub- Mo in industrial wastewater could be high as 4e145 mg L 1 (Shan stances for the evaluation of Mo (þVI) compounds. As a first step, et al., 2012). this study carried out a thorough LC50 evaluation of several aquatic However, even though a maximum concentration level was impact factors including pH, conductivity, and background aquatic regulated, several different Mo ions and/or compounds (e.g., ions associated with Mo (þVI) salts. Thereafter, LC50 values for different valences and associated conjugate cations or anions) in three different sources of molybdate (i.e., sodium molybdate water may present different biological toxicities. For example, the (Na2MoO4), ammonium molybdate ((NH4)6Mo7O24), and molyb- median lethal concentrations (LC50) for hexavalent Mo, sodium denum trioxide (MoO3) were determined. Data from this study molybdate and ammonium molybdate, were reported to be addresses the critical data gaps concerning the acute toxicity of Mo 800 mg L 1 (rainbow trout, 96 h exposure) (Sigma-Aldrich, 2015a, associated with releases to the environment from industrial b, c) and 420 mg L 1 (rainbow trout, 96 h exposure) (Sigma-Aldrich, processes. 2015a, b, c), respectively, which are equivalent to 373 and 228 mg L 1 Mo. It can be seen that with the same test toxicity organism, ammonium molybdate presents a higher toxicity with 2. Materials and methods respect to Mo than that induced by sodium molybdate. Hence, the toxicity of Mo in aquatic systems is also dependent on the contri- 2.1. Chemicals bution of its associated conjugate cations. Bioassays, which rely on measuring the response of organisms All reagents were of analytical grade and used without further fi ‧ exposed to contaminants, relative to a control, are the most widely puri cation. Sodium molybdate dihydrate (Na2MoO4 2H2O, min ‧ used test methods for the toxicity assessment of chemical com- 99%), ammonium molybdate tetrahydrate ((NH4)6Mo7O24 4H2O, pounds and effluents. Daphnia magna (D. magna)(Fig. 1) is one of min 99%), molybdenum trioxide (MoO3, min 99%), sulfuric acid the biological organisms allowed in Taiwan's biological acute (H2SO4, min 95%), potassium chloride (KCl, min 99.5%), sodium toxicity testing for the Water Pollution Control Act. D. magna is bicarbonate (NaHCO3, min 99.7%), sodium hydroxide (NaOH, min sensitive enough to distinguish between the toxicity to the several 99%), sodium chloride (NaCl, min 99.8%), nitric acid (HNO3, min different Mo compounds and is therefore a good test organism to 65%), ammonium hydroxide solution (NH4OH, min 28%), ammonium sulfate ((NH4)2SO4, min 99%) and sodium sulfate (Na2SO4,

Fig. 1. (a) side view and (b) dorsal view of D. magna. 84 C.-W. Wang et al. / Chemosphere 147 (2016) 82e87 min 99%) were purchased from SigmaeAldrich (St. Louis, MO, USA). 9146, Woonsocket, RI, USA) and a pH/conductivity meter (Eutech Magnesium sulfate (MgSO4‧7H2O, 100.4%) and acetone ((CH3)2CO, PC 5000, Singapore) equipped with a Fisher Scientific Accumet min 99.5%) were purchased from J.T. Baker (Center Valley, PA, USA). four-cell conductivity probe, respectively. Calcium sulfate dehydrate (CaSO4‧2H2O, min 99.0%), and hydro- chloric acid (HCl, min 37.6%) were purchased from Merck 3. Results and discussion (Drmstadt, Germany). The water used was purified using a reverse osmosis (RO) purification system. 3.1. Background toxicity

2.2. D. magna stock and culture conditions The LC50 values determined for the individual compounds of NaCl, H2SO4,Na2SO4,NH4OH, NaOH and (NH4)2SO4 are presented in D. magna were obtained from Chi Mei Inspection Tech Co., Ltd. Table 1 and the associated variations of pH, conductivity, and sur- (Taichung, Taiwan), and acclimatized at least 21 days in borosilicate vival rate in different solutions during a 48 h period are shown in beakers (2 L) to assess whether they were healthy enough to be Fig. 2. A reference toxicity test was conducted using the toxicant used in the bioassay experiments. The organisms were cultured and NaCl to demonstrate the sensitivity of the test organisms prior to handled according to the procedures outlined in the Taiwan Na- performing further toxicity tests. The result of the NaCl reference 1 tional Institute of Environmental Analysis (NIEA) method (NIEA, toxicant test indicated the LC50 to be 3597 ± 248 mg L (61.6 mM) 2013). Culture water for D. magna was reconstituted hard water (see Table 1), and the coefficient of variation (CV) obtained was 1 1 1 (MgSO4 246 mg L , NaHCO3 192 mg L , CaSO4$H2O 120 mg L 6.9%, which is within 50% of the recommended value by Taiwan and KCl 8 mg L 1), with a hardness of 170 ± 10 mg L 1, and a dis- NIEA method (NIEA, 2013). Therefore, the acute toxicity test pro- solved oxygen (DO) concentration of greater than 5 mg L 1. During cedure used in this study exhibits a high reliability. the period of cultivation, D. magna were fed on wheat germ powder, When comparing the pH and conductivities in NaCl test solu- yeast powder and commercially available fish feed mixture a tions (see Fig. 2), pHs are similar at approximately neutral pH minimum of once per day. A 16 h photo-period and a temperature conditions (pH 6.5e8.5), but the conductivities are much higher in of 25 ± 2 C (controlled by the air conditioner), were maintained. the 100% test solution than that in the 20% test solution (e.g., Furthermore, before carrying out the experiment, the test D. magna 15,500 mScm1 for 145.30 mM (100%) NaCl solution versus were quarantined for 2 h without feeding. D. magna neonates 4000 mScm 1 for 29.06 mM (20%) NaCl solution). There was 100% (younger than 24 h) were used in the toxicity test bioassay. mortality of the D. magna in the 145.30 mM NaCl test solution, and the survival rate of D. magna was 72% in the 29.06 mM NaCl test 2.3. Experimental design solution. Note that a survival rate of 100% was observed in the blank control solution with the conductivity of 1000 mScm 1. Therefore, it Initial experiments were designed to determine the LC50 for can be preliminarily concluded that a near neutral pH (6.5e8.5) and þ þ þ 2 m 1 Na ,H ,NH4 ,SO4 and OH as toxicity baselines in the aquatic a conductivity of less than 4000 Scm might not cause D. magna system. The concentration ranges associated with the toxicity of mortality. þ these reagents were unknown prior to the experiment. Therefore, Moreover, the effects of H and OH on D. magna survial were þ several preliminary range-finding tests were conducted using evaluated to determine their LC50 concentrations. The LC50 for H 1 groups of 5 test organisms, which were exposed to several widely- was 0.24 ± 0.04 mg L (0.12 mM carried out using H2SO4). The LC50 spaced sample dilutions to estimate the toxicity range (note that concentrations for OH were determined using NH4OH and NaOH, 1 these results exhibit no statistical significance and are not re- and were found to be 12.9 ± 1.5 mg L (0.76 mM using NH4OH) and ported.). Afterward, in the second step of the experiments, the 53.3 ± 3.8 mg L 1 (3.14 mM using NaOH) (see Table 1). Note that the 1 toxicities of different Mo sources (i.e., sodium molybdate, ammo- LC50 values of Na2SO4 (18.5 ± 2.8 mg L (0.13 mM)) and (NH4)2SO4 nium molybdate, and molybdenum trioxide) were evaluated. (26.6 ± 3.2 mg L 1 (0.76 mM)) were evaluated as reference toxic- The 48 h acute toxicity tests were conducted in accordance with ities for H2SO4,NH4OH, and NaOH. The LC50 for Na2SO4 is associated þ 1 2 the Taiwan NIEA method (NIEA, 2013). The tests were conducted with Na concentration of 5.9 mg L and a SO4 concentration of 1 þ under static non-renewal conditions, using D. magna at 25 ± 2 Cin 12.3 mg L (pH ~7.5). The LC50 for NaCl is associated with a Na a temperature controlled room, and a 16 h light and 8 h dark concentration of 1416 mg L 1, which is much higher than the 1 photocycle were maintained. One set of experiments was con- 5.9 mg L determined from the Na2SO4 test. Therefore, it can be þ ducted using serial dilutions of the test solution (i.e., 20%, 40%, 60%, assumed that the Na concentration did not cause the toxicity in 2 80%, and 100%). Each concentration level test was carried out in the Na2SO4 test, and that the toxicity was the result of the SO4 four replicates; each replicate was in a 40 mL vial (EPA volatile concentration. Further evaluation of the results of the H2SO4 test 2 1 organic compound sample vial) containing 25 mL of test solution found the corresponding LC50 for SO4 to be 11.6 mg L . Based on fi 2 and ve D. magna neonates of less than 24 h old, for a period of 48 h. the similar LC50 for SO4 obtained from H2SO4 and Na2SO4 tests, it 2 Each concentration level test was repeated three times. Therefore, a can be deduced that the SO4 concentration of approximately total of sixty D. magna were used for one concentration level test. 12 mg L 1 may be acutely toxic to D. magna. Nevertheless, even Control tests in culture water (artificial hard water) were also though pH in the H2SO4 solution was at near neutral condition conducted side by side. No food was given during the course of all (except for the 0.2 mM solution (100%), which had a pH of 4), the þ 2 experiments. H in the solution may have added to the toxicity of SO4 . It should be noted that the conductivity of the solution (<600 mScm 1)was 2.4. Analysis much smaller than the 4000 mScm 1 conductivity measured in the NaCl test, and therefore conductivity in this test likely did not The Mo ions concentration was determined using an inductively contribute to mortality in any significant measure. þ coupled plasma atomic emission spectrometer (VARIAN 710 ES ICP- The LC50 for NaOH is associated with a Na concentration of þ OES, USA). A portable pH/ORP meter (TS-100, Suntex) equipped 72.1 mg L 1 and OH of 53.3 mg L 1. The Na concentration is much with a pH electrode and a temperature probe was used to monitor less than that determined in the NaCl test and therefore would not the solution pH and temperature in this study. DO and conductivity be responsible for acute toxicity in the NaOH solutions. Therefore, were measured periodically using a portable DO meter (Hanna HI the D. magna mortality is perhaps due to the increased OH ion C.-W. Wang et al. / Chemosphere 147 (2016) 82e87 85

Table 1

48 h LC50 values of acute toxicity bioassays with D. magna for background ions in water.

Species Initial Conc. LC50 mM mM mg L 1 (species)

þ NaCl 145.30 61.6 ± 4.3 (NaCl) (Na ) (Cl ) 3597 ± 248 1416 ± 98 2182 ± 151 ± þ 2 H2SO4 0.20 0.12 0.02 (H2SO4)(H) (SO4 ) 11.8 ± 1.6 0.24 ± 0.04 11.6 ± 1.8 ± þ 2 Na2SO4 0.20 0.13 0.02 (Na2SO4) (Na ) (SO4 ) 18.5 ± 2.8 5.92 ± 1.1 12.3 ± 2.2 þ NH4OH 1.02 0.76 ± 0.09 (NH4OH) (NH4 ) (OH ) 26.6 ± 3.2 13.6 ± 1.6 12.9 ± 1.5 þ NaOH 5.00 3.14 ± 0.22 (NaOH) (Na ) (OH ) 125.6 ± 8.8 72.1 ± 5.1 53.3 ± 3.8 ± þ 2 (NH4)2SO4 0.50 0.31 0.004 ((NH4)2SO4) (NH4 ) (SO4 ) 41.0 ± 0.5 11.0 ± 1.3 11.7 ± 1.6

100 (a) 100 (b) (c) (d) (e) (f) 0 hr 48 hr 80 80

60 60

40 40

20 20 Survival rate (%)

15000 800 12000 600 9000 400 6000

3000 200 Cond. (us/cm)

0 12 12

10 10

8 8

6 6 pH 4 4

2 2 1.0 2.0 3.0 4.0 5.0 0.1 0.2 0.3 0.4 0.5 0.04 0.08 0.12 0.16 0.20 0.08 0.20 0.04 0.16 0.12 0.204 0.408 29.06 58.12 87.18 0.816 0.612 1.020 Blank 116.24 145.30 NaCl (mM) H SO (mM) Na SO (mM) NH OH (mM) NaOH (mM) (NH ) SO (mM) 2 4 2 4 4 4 2 4

Fig. 2. Variations of pH, conductivity, and survival rate in (a) NaCl, (b) H2SO4, (c) Na2SO4, (d) NH4OH, (e) NaOH, and (f) (NH4)2SO4 solutions of acute toxicity tests. concentration (e.g., pH ~11 for 5.0 mM NaOH (100%) and pH ~10.5 threshold levels determined for pH and conductivity, it can be þ 1 2 1 for 4.0 mM NaOH (80%)). Furthermore, the LC50 for NH4OH corre- further concluded that NH4 (11 mg L ) and SO4 (12 mg L ) may þ 1 sponds to an NH4 concentration of 13.6 mg L and OH of pose toxicity to D. magna in water bodies. 12.9 mg L 1 (pH ~ 9.5 to 10.0). Although the concentration of OH in the NH OH solution was lower than that in the NaOH solution, the 4 3.2. Toxicity of Mo ion (þVI) and associated salts in the aquatic pH in all dilutions of the NH OH solution exceeded the pH safety 4 system threshold (pH ¼ 6.5e8.5). Based on this, it appears that the þ simultaneous presence of both NH and OH may result in greater 4 Different sources of Mo ions (e.g., different valences or bonded toxicity than each individual ion. On the other hand, the results of with conjugate cations or anions) in water may result in different the (NH4)2SO4 test indicated that the LC50 of (NH4)2SO4 is associ- þ 1 2 biological toxicities. Therefore, various species of Mo salts (i.e., ated with an NH4 concentration of 11.0 mg L and SO4 of 1 þ Na2MoO4‧2H2O, (NH4)6Mo7O24‧4H2O and MoO3) were tested for 11.7 mg L . The NH4 value from the (NH4)2SO4 test was compa- þ 1 their biological acute toxicities. The variations of pH and conduc- rable to that from the NH4OH test (NH4 LC50 ¼ 13.6 mg L ), and the 2 tivity, and the resulting survival rate of D. magna in solution during SO4 value from the (NH4)2SO4 test was also similar to that from 2 1 a 48 h period, are shown in Fig. 3. From the bioassay test using D. the Na SO test (SO LC ¼ 12.3 mg L ). Moreover, the trend of 2 4 4 50 magna, the LC values were 367.8, 43.3 and 89.2 mg L 1 for survival rates from the different diluted solutions was similar to the 50 Na2MoO4‧2H2O, (NH4)6Mo7O24‧4H2O and MoO3, respectively NH4OH and Na2SO4 tests. Hence, in addition to the previous þ (Fig. 4). In the Na2MoO4‧2H2O test, the LC50 is associated with a Na 86 C.-W. Wang et al. / Chemosphere 147 (2016) 82e87

100 (a) (b) (c) 0 hr )

% 80 48 hr ( e t a

r 60 l a v i 40 v r u

S 20

1000

800

600

400

Cond. (us/cm) 200

pH 4

2 3.20 2.08 0.64 1.28 1.92 2.56 0.080 0.048 0.832 1.248 1.664 0.016 0.032 0.416 0.064 Blank Na MoO 2H O (mM) (NH ) Mo O 4H O (mM) MoO (mM) 2 4Ʉ 2 4 6 7 24Ʉ 2 3

Fig. 3. Variations of pH, conductivity, and survival rate in (a) Na2MoO4‧2H2O, (b) (NH4)6Mo7O24‧4H2O, and (c) MoO3 solutions of acute toxicity tests.

6þ Fig. 4. The LC50 values of Mo obtained from different salts: Na2MoO4‧2H2O, (NH4)6Mo7O24‧4H2O, and MoO3.

þ concentration of 35.01 mg L 1 and Mo6 of 145.8 mg L 1 lower than those reference toxicity levels determined earlier. 2 1 (pH ¼ 7.5e8.5). Additionally, the LC50 of (NH4)6Mo7O24‧4H2Ois Therefore, the MoO4 LC50 concentration of 243.1 mg L and þ 1 6þ 6 1 associated with a NH4 concentration of 3.8 mg L and Mo of Mo7O24 LC50 concentration of 37.0 mg L appear to indicate that 1 þ 23.5 mg L (pH ¼ 7.0e8.0). It can be seen that both the LC50 of Na these compounds can be acutely toxic to D. magna. þ and NH4 , and pH and conductivity in the Na2MoO4‧2H2O and Molybdenum trioxide, in the form of the rare mineral molyb- 1 (NH4)6Mo7O24‧4H2O tests are within the threshold levels or much date, dissolves slightly in water (e.g., 1.0 g MoO3 L at 20 C) C.-W. Wang et al. / Chemosphere 147 (2016) 82e87 87

(Sigma-Aldrich, 2015a, b, c). However, the dissolved MoO3 would Technology of Taiwan under Project No. 103-2221-E-005 -010-MY3. - 2 react with OH to form HeOeMoO3 and subsequently MoO4 (Eqs. (1) and (2)), thereby resulting in the decrease of solution pH (Krishnan et al., 2009). Hence, gradual decreases in pH were observed when the MoO3 concentration increased (see Fig. 3) References þ / e e Atia, A.A., 2008. Adsorption of chromate and molybdate by cetylpyridinium MoO3 OH H O MoO3 (1) bentonite. Appl. Clay Sci. 41, 73e84. Barmentlo, S.H., Stel, J.M., Doorn, M., Eschauzier, C., Voogt, P., Kraak, M.H.S., 2015. 2 Acute and chronic toxicity of short chained perfluoroalkyl substances to OH þ HeOeMoO /MoO þ H2O (2) 3 4 Daphnia magna. Environ. Pollut. 198, 47e53. Diamantino, T.C., Guilhermino, L., Almeida, E., Soares, A.M.V.M., 2000. Toxicity of The LC50 for the MoO3 is associated with Mo concentration of 1 e sodium molybdate and sodium dichromate to Daphnia magna straus evaluated 59.5 mg L and pH is in the range of 3.0 8.0. The D. magna mor- in acute, chronic, and acetylcholinesterase inhibition tests. Ecotox. Environ. Safe talities in all of the diluted solutions exceeded 70% except for the 45, 253e259. 20% diluted solution, in which the pH was approximately 8.0. Heijerick, D.G., Regoli, L., Stubblefield, W., 2012. The chronic toxicity of molybdate to marine organisms. I. Generating reliable effects data. Sci. Total Environ. 430, Therefore, it can be deduced that the presence of Mo at acidic pH 260e269. caused the D. magna mortality. Note that aqueous conductivity is Huang, Y.H., Tang, C., Zeng, H., 2012. Removing molybdate from water using a hy- far less than the toxicity threshold level. After the toxicity test, the bridized zero-valent iron/magnetite/Fe(II) treatment system. Chem. Eng. 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According to the results obtained in this study, the acute toxicity Shan, W.-J., Fang, D.-W., Shuang, Y., Kong, Y.-X., Zhao, Z.-Y., Xing, Z.-Q., Biswas, B.-K., for different Mo compounds within a 48 h exposure increased in Xiong, Y., 2012. Equilibrium, kinetics, and thermodynamics studies on the re- ‧ < < ‧ covery of Rhenium(VII) and Molybdenum(VI) from industrial wastewater by the following order: Na2MoO4 2H2O MoO3 (NH4)6Mo7O24 4H2O chemically modified waste paper gel. J. Chem. Eng. Data 57, 290e297. 6 in solutions. The data showed that Mo7O24 presents the most toxic Shields Jr., J.A., 2013. Applications of Molybdenum Metal and its Alloys. Interna- 1 tional Molybdenum Association, London, UK. dissolved species (i.e., with the lowest LC50 of 37.0 mg L among Sigma-Aldrich, 2015a. Material Safety Data Sheet of Ammonium Molybdate. http:// the three molybdenum salts). In terms of acute toxicity caused by www.sigmaaldrich.com/taiwan.html. Mo alone, the use of (NH4)6Mo7O24‧4H2O reveals the lowest LC50 of Sigma-Aldrich, 2015b. Material Safety Data Sheet of Molybdenum(VI) Oxide. http:// 1 www.sigmaaldrich.com/taiwan.html. 23.5 mg Mo L and the Na2MoO4‧2H2O is the least toxic substance. Sigma-Aldrich, 2015c. Material Safety Data Sheet of Sodium Molybdate. http:// The pH and conductivity may also contribute biological toxicity and www.sigmaaldrich.com/taiwan.html. hence should be limited to pH in the range of 6.5e8.5 and a con- Smedley, P.L., Cooper, D.M., Lapworth, D.J., 2014. Molybdenum distributions and ductivity of less than 4000 mScm 1. As for the effects of Mo salts variability in drinking water from England and Wales. Environ. Monit. Assess. 186, 6403e6416. and associated conjugate cations or anions, it can be suggested that Stanley, J.K., Perkins, E.J., Habib, T., Sims, J.G., Chappell, P., Escalon, B.L., Wilbanks, M., þ < 1 2 the following concentrations, NH4 of 11 mg L ,SO4 of Garcia-Reyero, N., 2013. The good, the bad, and the toxic: approaching hormesis þ <12 mg L 1 and Na of <1416 mg L 1 be considered when evalu- in Daphnia magna exposed to an energetic compound. Environ. Sci. Technol. 47, e ating the potential toxicity to D. magna in aquatic systems. The data 9424 9433. US Research Nanomaterials Inc, 2015. Molybdenum Oxide Nanopowder Nano- showed that the level of molybdenum toxicity in aquatic systems is particles. http://www.us-nano.com. highly dependent on the form of molybdenum salts used and also World Health Organization (WHO), 2011. Molybdenum in Drinking Water. Back- on the background water quality and associated conjugate ion ground Document for Development of WHO Guidelines for Drinking-water Quality. species. Xiong, Y., Chen, C., Gu, X., Biswas, B.K., Shan, W., Lou, Z., Fang, D., Zang, S., 2011. Investigation on the removal of Mo(VI) from MoeRe containing wastewater by Acknowledgment chemically modified persimmon residua. Bioresour. Technol. 102, 6857e6862. Yim, J.H., Kim, K.W., Kim, S.D., 2006. Effect of hardness on acute toxicity of metal mixtures using Daphnia magna: prediction of acid mine drainage toxicity. This study was funded by the Ministry of Science and J. Hazard. Mater 138, 16e21.