Spectroscopic and Thermogravimetric Investigation of Cd(II) Dinonyldithiophosphate: Removal of Cadmium from Aqueous Solutions
Dinonyldithiophosphoric acid (HDDTP) was synthesised from the reaction of phosphorus pentasulphide and nonyl alcohol. Dinonyldithiophosphate complex of cadmium [Cd(DDTP)2] was prepared by mixing solutions of Cd(II) with HDDTP in ethanol at room temperature. The acid and its complex were characterised by elemental analysis and spectroscopy. The thermal behaviour of Cd(DNDTP)2 was investigated by thermogravimetric analysis. Removal of Cd(II) from aqueous media by HDDTP solution was also studied. The optimum conditions for removal of Cd(II) were investigated for effects of solvent, pH, contact time, concentration, and inorganic anions. Cd(II) was quantitatively removed from aqueous solutions at the pH range of , under the conditions that the stoichiometric ratio of HDDTP/Cd(II) ≥2/1. It can be stated that contact of the Cd(II) with HDDTP was sufficient for quantitative removing of cadmium from acidic aqueous solutions.
Organodithio compounds are widely used in chemical and industrial applications [1–3]. Due to the synthesis and purifying challenges of long chains dialkyldithiophosphoric acids, their use in experiments was rare [2, 4], although long chains dialkyldithiophosphoric acids are more easily transferred into the organic phase  and more stable against hydrolysis than the short-chain ones . The well-known lubricant zinc dialkyldithiophosphate (ZDDTP) has been extensively investigated , but only a few articles have been published on the thermal properties of the other heavy metal dialkyldithiophosphates [8, 9]. Similarly, there are only a few studies on Cd(II) dialkyldithiophosphates for its removal from aqueous solutions [10–14], but there are no articles on Cd(II) dinonyldithiophosphate.
Cadmium is one of the most toxic heavy metals for living organisms. One of the important natural heavy metal pollution sources is phosphate rocks/phosphorus fertilizers. During the production of phosphorus fertilizers, dangerous amounts of heavy metals pollute the environment. The main source of cadmium accumulation in the agricultural lands is the phosphorus fertilizers and its restriction is almost impossible . Thus, cadmium is one of the common environmental contaminants and widely distributed around the world.
The other important pollution sources are different industrial activities such as plating of metal, mining, pigments, polyvinyl chloride plastics, petrochemicals, alloy and steel industries, and municipal discharge-wastewater . According to the international standards the threshold level of Cd(II) concentration should be lower than 0.01 mg/L in wastewater used for irrigation . Therefore it is necessary to eliminate Cd(II) from contaminated waters.
Because of its high mobility in soil and plant system and easy accumulation, cadmium may cause toxicities such as cancer (lung and prostate), kidney damage, and bone diseases . To reduce or remove Cd(II) from wastewater or contaminated waters, various methods have been developed including adsorption [19–22], biosorption [23, 24], ion exchange , hydrogel , and chemical precipitation-extraction .
In the present study, dinonyldithiophosphoric acid and its complex of Cd(II) were prepared and characterised by spectroscopic techniques and thermal analysis. The removal of Cd(II) from aqueous solutions by using dinonyldithiophosphoric acid was investigated.
2. Materials and Methods
2.1. Preparation and Characterisation of HDDTP and Cd(DDTP)2
HDDTP was prepared from the reaction of phosphorus pentasulphide and nonyl alcohol by using a microwave oven (CEM-MDS 2000, Matthews, USA) according to a previously published method  and, to make a sufficiently pure compound, the method described in the literature  was modified. The obtained mixture was cooled and the unreacted solid part was filtered. The oily acid was converted to calcium salt by neutralisation with lime slurry at room temperature. The salt was separated and washed with hexane and acidified with 3 M H2SO4. The free dinonyldithiophosphoric acid floating on top of aqueous phase was separated as a yellowish-green viscous liquid with 98% purity:An adequate amount of 10−2 M solutions of viscous oil HDDTP and Cd(II) in ethanol is mixed at room temperature, and Cd(DDTP)2 was obtained as white precipitate with melting point 28.5°C (see Scheme 1).
The prepared acid and its complex were characterised by elemental analysis (EA), ultraviolet-visible (UV-vis), infrared (IR) spectrophotometers, and inductively coupled plasma-optical emission spectrometry (ICP-OES), and the thermal behaviour of Cd(DDTP)2 was investigated by thermogravimetric analysis (TGA). Elemental analysis was carried out using a CHNS-O elemental analyser (Carlo Erba EA 1108, Milan, Italy). UV-vis spectra of the samples dissolved in CCl4 were recorded in the range 200–900 nm by a UV/visible spectrophotometer (Unicam-UV2-100, Cambridge, UK) using a 1 cm quartz cell. The IR spectra were recorded on a Fourier transform IR (FTIR) spectrometer (Midac Co. M Series, Costa Mesa, CA, USA) as neat liquid, using a NaCl cell in the range 400–4000 cm−1. The concentrations of Cadmium were determined by an ICP-OES (Perkin Elmer 2100 DV, Massachusetts, USA) at 228.802 nm. The TG measurement was performed using a thermogravimetric analyser (Shimadzu TGA-50, Kyoto, Japan) with a temperature range from room to 800°C at the rate of 20°C min−1 under nitrogen (20 mL min−1) atmosphere.
2.2. Experiments on Cd(II) Removal from Aqueous Solutions
We have prepared solutions of HDDTP (2·10−2 M) in the different solvents such as kerosene, benzene, -hexane, petroleum ether, and carbon tetrachloride. The stock solution of cadmium (1·10−2 M) was also prepared in distilled water. All of the chemicals were of analytical reagent grade. Removal of Cd(II) from aqueous solutions to the organic phase was investigated as a function of pH for various anions, solvents, extractant concentration, and extraction times. Extraction or contact experiments related effect of time and pH were performed between 1 and 5 minutes and pH: 0.5 and 6.0, respectively. Removal of Cd(II) using HDDTP was carried out from different 0.1 M aqueous solution of NaClO4, NaCl, NaBr, NaNO3, and CH3COONa salts by stirring equal volumes (25 mL) of 10−3 M organic and aqueous phases at room temperature in a conventional separation funnel at different times (min). The pH values were adjusted by adding sulphuric acid or sodium hydroxide, and the pH values were measured with a pH meter (Jenway 3010, London, UK). After separation of the phases, an aliquot of the aqueous part was analysed by an ICP-OES for the particular Cd(II) ion concentrations at 228.802 nm.
The recovery percentage () of Cd(II) can be calculated as follows:where is initial concentration of the Cd(II) in the aqueous phase and is the concentration in the aqueous phase after extraction.
3. Result and Discussion
3.1. Characterisation of HDDTP and Cd(DDTP)2
Potentiometric titration of the acid with 0.1 M NaOH showed that the product contained 98% HDDTP, which is a yellowish-green viscous oil. The elemental analysis results for (C9H19O)2PSSH; Analytical calculated (%): C, 56.5; H, 10.2; S, 16.7, and found values (%): C, 56.8; H, 10.3; S, 16.5. Elemental analysis of the recovered white solid Cd(DDTP)2 (melting point 28.5°C) was C, 49.3; H, 8.7; S, 14.6; P, 7.1; and Cd, 12.8 for analytical calculations and the found values were found to be C, 49.5; H, 8.2; S, 14.4; P, 7.0; and Cd, 12.6, and the results are in agreement with each other.
The UV-vis absorption spectrum of dinonyldithiophosphoric acid in CCl4 using a 1 cm quartz cell showed a peak at 267 nm can be attributed to electronic transition related >P(=S)SH group (Table 1). The UV-vis absorption spectrum of Cd(DDTP)2 was obtained at the same conditions and relevant band was found 294 nm (Table 1) and can be attributed to the metal-ligand electronic transition.
The recorded IR data of the acid and complex (Table 1) showed a good agreement with earlier studies carried out on other dialkyldithiophosphates [1, 2]. The IR spectral data of cadmium dinonyldithiophosphate have been recorded at the range of 4000–400 cm−1 (Table 1) and it was observed that the characteristic band of the dinonyldithiophosphoric acid at 2550 cm−1 related to (P)S–H stretching vibration was vanished with the formation of Cd(DDTP)2, and there was no absorption record in the region. Classically, the strong intensity band present in the region 740–660 cm−1 has been attributed to P=S double bond and the absorption peak at ~540 cm−1 has been assigned to P–S single bond (Table 1). By formation of the complex, the absorption values of the bands were significantly shifted, and P=S peak was shifted down, while P–S peak was shifted up. Thus, P=S double bond strength was decreased, while P–S single bond strength was increased. This result was in agreement with the previous studies on the other dialkyldithiophosphates [9, 10, 13]. This approach can be easily acceptable for simple M+ salt of dithiophosphoric acids; for example, these two bands (P=S and P–S) of potassium butyldithiophosphate can be seen at 703 cm−1 and 550 cm−1, respectively .
According to the obtained thermogram (Figure 1), Cd(DDTP)2 is stable at temperatures up to ~207°C and does not contain water. As shown in Figure 1, Cd(DDTP)2 decomposes in three steps, and the main weight loss of the complex takes place between 207 and 332°C because of the decomposition of the organic part. The total weight loss percentage of the Cd(DDTP)2 is 70.4% up to the temperature 499°C. The weight change is lower than 3% over 332°C temperature. The results are in good agreement with our earlier studies on heavy metal dialkyldithiophosphate complexes .
3.2. Cd(II) Removal from Aqueous Solutions
The molar ratio of dinonyldithiophosphoric acid/cadmium has main influence in the extraction process (Table 2(a)). Cd(II) was quantitatively recovered from the acidic aqueous solution under the conditions that the stoichiometric ratio of HDDTP/Cd(II) was >2/1. When the experiment was performed with lower molar ratio, such as equal volumes of 1·10−3 M HDDTP, and 1·10−3 M metal solution was mixed, the removal ratio of Cd(II) was found to be 60%. A favourable result was obtained, and Cd(II) was quantitatively extracted from the aqueous solution to organic phase when molar ratio of HDDTP/Cd(II) increased to 3 (Table 2(a)). The pH experiments on the extraction of Cd(II) showed that (Table 2(b)) it is possible to suggest Cd(II) can be quantitatively extracted between pH 0.5 and 6. The contact time of metal-ligand on the removal of Cd(II) was examined (Table 2(c)), and it was found that 1-minute stirring time can supply quantitative removal of cadmium.
The experiment conducted on different solvents such as kerosene, benzene, -hexane, petroleum ether, and CCl4 implies that there is no difference between solvents used (Table 2(d)). Therefore, all the investigated solvents can be used in the removal of Cd(II). We have also investigated the effect of common inorganic anions such as NaClO4, NaCl, NaBr, NaNO3, and CH3COONa on the extractions of Cd(II) from aqueous solution. We have found that there were no differences except for CH3COONa salt (Table 2(e)). Cadmium acetate was also a stable complex and its solubility in water was very high. In addition, our primary results showed that the Cd(II) could not be recovered back by 1–5 M H2SO4 from organic to aqueous phase.
Our results were in agreement with the studies related to extraction of Cd(II) by diethy-, dipropyl-, and dibutyl dithiophosphates [10–14]. In these studies they showed that pH 3–5 is suitable for Cd(II) removal, and they form different Cd(II) complexes in the basic pH in order to regenerate from organic phase to aqueous phase. The compounds containing long chain alkyl or more carbon are easily soluble in organic solvents and their solubility in aqueous media decreases. If the results were compared with our findings, the effect of dinonyl group can easily be seen. Cd(DDTP)2 with long nonyl group is insoluble in water and very soluble in organic solvents and 1 minute is sufficient for quantitatively removing of cadmium from aqueous media. There was an acceptable agreement between the previous results and our findings on removing Cd(II) from aqueous solutions using alkyldithiophosphates as extractants.
Present results showed that Cd(II) could be easily removed from the acidic aqueous solutions by HDDTP/any organic solvent.
Dinonyldithiophosphoric acid instantly reacts with Cd(II) to form a white solid complex at room temperature. The Cd(DDTP)2 is stable up to ~207°C and insoluble in water but very soluble in organic solvents such as kerosene, benzene, -hexane, petroleum ether, and carbon tetrachloride and these are suitable solvents for using HDDTP or its salt as extractants.
The experiments concerning the influence of the concentration of HDDTP/any solvent on removal of Cd(II) from the acidic aqueous solutions show that cadmium was quantitatively removed, under the conditions that the stoichiometric ratio of HDDTP/Cd(II) ≥2/1. The experiments related with pH of aqueous media show that Cd(II) can be removed completely from the acidic solutions at pH range of 0.5 < pH < 6. When the technique is applied on any acidic wastewater or sludge, it does not require any pH adjustment and the method can supply low costs. Experiments related to effect of contact time for removal of Cd(II) showed that contact of the HDDTP with Cd(II) was sufficient for its quantitative removing. Cd ions were quantitatively transferred to organic phase after 1 min. mixing of Cd(II) and HDDTP. Because the potential ecological risk of cadmium is relatively high, it is necessary to remove or reduce Cd(II) levels in the municipal or industrials discharges. It can be concluded that HDDTP is an appropriate substance for removal of Cd(II) from acidic aqueous solutions.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
J. C. Ramos, A. J. Curtius, and D. L. G. Borges, “Diethyldithiophosphate (DDTP): a review on properties, general applications, and use in analytical spectrometry,” Applied Spectroscopy Reviews, vol. 47, no. 8, pp. 583–619, 2012.View at: Publisher Site | Google Scholar
A. M. Barnes, K. D. Bartle, and V. R. A. Thibon, “A review of zinc dialkyldithiophosphates (ZDDPS): characterisation and role in the lubricating oil,” Tribology International, vol. 34, no. 6, pp. 389–395, 2001.View at: Publisher Site | Google Scholar
I. Haiduc, “Thiophosphorus and related ligands in coordination, organometallic and supramolecular chemistry. A personal account,” Journal of Organometallic Chemistry, vol. 623, no. 1-2, pp. 29–42, 2001.View at: Publisher Site | Google Scholar
T. H. Handley and J. A. Dean, “O,-dialkyl phosphorodithioic acids as extractants for metals,” Analytical Chemistry, vol. 34, no. 10, pp. 1312–1315, 1962.View at: Publisher Site | Google Scholar
B. Gümgüm, N. Biricik, A. Baysal, O. Akba, and G. Öztürk, “Preparation and solvent extraction of nickel complex of O,-dinonyldithiophosphate and its application to spectrophotometric determination of nickel in sediment samples,” Annali di Chimica, vol. 96, no. 11-12, pp. 681–688, 2006.View at: Publisher Site | Google Scholar
V. F. Toropova, A. R. Garifzyanov, and I. E. Panfilova, “Steric and hydrophobic effects of substituents in extraction of metal complexes with O,O-dialkyldithiophosphoric acids,” Talanta, vol. 34, no. 1, pp. 211–214, 1987.View at: Publisher Site | Google Scholar
K.-H. Ohrbach, G. Matuschek, A. Kettrup, and A. Joachim, “Simultaneous thermal analysis-mass spectrometry on lubricant systems and additives,” Thermochimica Acta, vol. 166, pp. 277–289, 1990.View at: Publisher Site | Google Scholar
S. Y. Wu and B. Xie, “Synthesis and spectroscopic properties of (tetraazacyclotetradeca) center dot bis(O,O′-bi(1-naphthyl) dithiophosphate) nickel or copper complexes,” Chinese Journal of Inorganic Chemistry, vol. 15, no. 2, pp. 267–271, 1999.View at: Google Scholar
N. Biricik and B. Gümgüm, “The preparation and thermo-analytical characterization of heavy metal complexes of O,O′-dinonyldithiophosphate,” Thermochimica Acta, vol. 417, no. 1, pp. 43–45, 2004.View at: Publisher Site | Google Scholar
X. Ying and Z. Fang, “Experimental research on heavy metal wastewater treatment with dipropyl dithiophosphate,” Journal of Hazardous Materials, vol. B137, no. 3, pp. 1636–1642, 2006.View at: Publisher Site | Google Scholar
J. S. Carletto, R. M. Luciano, G. C. Bedendo, and E. Carasek, “Simple hollow fiber renewal liquid membrane extraction method for pre-concentration of Cd(II) in environmental samples and detection by Flame Atomic Absorption Spectrometry,” Analytica Chimica Acta, vol. 638, no. 1, pp. 45–50, 2009.View at: Publisher Site | Google Scholar
R. M. Luciano, G. C. Bedendo, J. S. Carletto, and E. Carasek, “Isolation and preconcentration of Cd(II) from environmental samples using polypropylene porous membrane in a hollow fiber renewal liquid membrane extraction procedure and determination by FAAS,” Journal of Hazardous Materials, vol. 177, no. 1–3, pp. 567–572, 2010.View at: Publisher Site | Google Scholar
Y. Xu, Z. Xie, and L. Xue, “Chelation of heavy metals by potassium butyl dithiophosphate,” Journal of Environmental Sciences, vol. 23, no. 5, pp. 778–783, 2011.View at: Publisher Site | Google Scholar
Y. Xu, Y. Chen, and Y. Feng, “Stabilization treatment of the heavy metals in fly ash from municipal solid waste incineration using diisopropyl dithiophosphate potassium,” Environmental Technology, vol. 34, no. 11, pp. 1411–1419, 2013.View at: Publisher Site | Google Scholar
A. W. Al-Shawi and R. Dahl, “The determination of cadmium and six other heavy metals in nitrate/phosphate fertilizer solution by ion chromatography,” Analytica Chimica Acta, vol. 391, no. 1, pp. 35–42, 1999.View at: Publisher Site | Google Scholar
J. Duan and B. Su, “Removal characteristics of Cd(II) from acidic aqueous solution by modified steel-making slag,” Chemical Engineering Journal, vol. 246, pp. 160–167, 2014.View at: Publisher Site | Google Scholar
N. Gupta, D. K. Khan, and S. C. Santra, “An assessment of heavy metal contamination in vegetables grown in wastewater-irrigated areas of Titagarh, West Bengal, India,” Bulletin of Environmental Contamination and Toxicology, vol. 80, no. 2, pp. 115–118, 2008.View at: Publisher Site | Google Scholar
K. J. Yost, L. J. Miles, and R. A. Greenkorn, “Cadmium: simulation of environmental control strategies to reduce exposure,” Environmental Management, vol. 5, no. 4, pp. 341–352, 1981.View at: Publisher Site | Google Scholar
G. Yang, L. Tang, X. Lei et al., “Cd(II) removal from aqueous solution by adsorption on α-ketoglutaric acid-modified magnetic chitosan,” Applied Surface Science, vol. 292, pp. 710–716, 2014.View at: Publisher Site | Google Scholar
C. Cheng, J. Wang, X. Yang, A. Li, and C. Philippe, “Adsorption of Ni(II) and Cd(II) from water by novel chelating sponge and the effect of alkali-earth metal ions on the adsorption,” Journal of Hazardous Materials, vol. 264, pp. 332–341, 2014.View at: Publisher Site | Google Scholar
M. Xu, Q. Chang, W. Zhang, and Q. Du, “Removal of cadmium (II) from aqueous solution using chitosan modified with thioglycolic acid,” Fresenius Environmental Bulletin, vol. 22, no. 1A, pp. 163–170, 2013.View at: Google Scholar
D. Bingöl, “Removal of cadmium (II) from aqueous solutions using a central composite design,” Fresenius Environmental Bulletin, vol. 20, no. 10, pp. 2704–2709, 2011.View at: Google Scholar
G. Edris, Y. Alhamed, and A. Alzahrani, “Biosorption of cadmium and lead from aqueous solutions by Chlorella vulgaris biomass: equilibrium and kinetic study,” Arabian Journal for Science and Engineering, vol. 39, no. 1, pp. 87–93, 2014.View at: Publisher Site | Google Scholar
C. G. Rocha, D. A. M. Zaia, R. V. D. S. Alfaya, and A. A. D. S. Alfaya, “Use of rice straw as biosorbent for removal of Cu(II), Zn(II), Cd(II) and Hg(II) ions in industrial effluents,” Journal of Hazardous Materials, vol. 166, no. 1, pp. 383–388, 2009.View at: Publisher Site | Google Scholar
Y. Zheng, C. Xiong, C. Yao et al., “Adsorption performance and mechanism for removal of Cd(II) from aqueous solutions by D001 cation-exchange resin,” Water Science & Technology, vol. 69, no. 4, pp. 833–839, 2014.View at: Publisher Site | Google Scholar
A. M. Atta, H. S. Ismail, and A. M. Elsaaed, “Application of anionic acrylamide-based hydrogels in the removal of heavy metals from waste water,” Journal of Applied Polymer Science, vol. 123, no. 4, pp. 2500–2510, 2012.View at: Publisher Site | Google Scholar
Y. Shengke and C. Yuyun, “Synergistic effects of chelating precipitation and flocculation on removal of cadmium amino-complex from wastewater,” Fresenius Environmental Bulletin, vol. 20, no. 12, pp. 3235–3240, 2011.View at: Google Scholar