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34Section 1. Analytical chemistry
DOI: http://dx.doi.org/10.20534/EJAAC-17-1-3-7
Gül§en Öztürk,
Dicle University Faculty of Science Department of Chemistry
Diyarbakir, Turkey E-mail: gozturk@dicle.edu.tr
Removal of Heavy Metals from Wastewater by using Dioctyldithiophosphoric Acid
Abstract: Dioctyldithiophosphoric acid was synthesised from the reaction of phosphorus pent sulphide with n-octyl alcohol. An environmental investigation was performed on the universal heavy metal contaminants such as copper, nickel, lead and zinc by using dioctyldithiophosphoric acid. Different dioctyldithiophosphoric acid solutions were used in order to remove Cu(ll), Ni(ll), Pb(ll) and Zn(ll) ions from acidic aqueous solution and original industrial wastewater samples. The optimisation for removal of the heavy metals, such as concentration of acid, pH of the aqueous media, contact time of the acid-metal ions and the effect of different organic solvents were investigated. The optimised parameters were applied to the original industrial wastewater. The results indicate that dioctyldithiophosphoric acid is an effective substance for removal of heavy metal ions from acidic aqueous media.
Keywords: Dioctyldithiophosphoric acid, extraction, heavy metal removal, wastewater.
1. Introduction pre-concentration process [4; 10]. It has been pos-
Dithiophosphorus compounds are widely sible to determine very low concentration of metals
used in chemical and industrial applications [1-3]. by this technique. For example 0.03 UgL-1 Cu(ll)
The soft and borderline element chelates of dial- and 0.004 ^gL-1 Pb(ll) can be detected by using di-
kyldithiophosphates are quite stable even in acidic thiophosphate as extractant in cloud-point extrac-
medium [4]. The alkyl groups alter the properties tion [4]. In the last decade, a few articles have been
of complexes. Due to the synthesis and purifying published on the solvent extraction of heavy metal
challenges of long chain dialkyldithiophosphoric dialkyldithiophosphates from aqueous solutions and
acids (DDTPA), their use in experiments were wastewater treatments [11; 13]. However, there are
rare, however the long chain compounds are more no any studies on removal of heavy metal from aque-
stable against hydrolysis than the short-chain ones ous solution by dioctyldithiophosphate from indus-
and more easily transferred into the organic phase trial wastewater.
[3; 5]. Organodithiophosphates have been used Heavy metal pollution is a very big concern in
widely as anti-wear and anti-corrosion agents in lu- worldwide [14; 15]. Since the heavy metals such
bricating oils throughout the world [6; 7]. Dialkyl- as Cu(ll), Ni(ll), Pb(ll) and Zn(ll) are dangerous
dithiophosphates are also used as pesticides [8] in substances for human health, their removal from
agriculture, and collectors in flotation process [9]. aqueous solutions or wastewater/municipal sludge
On the other hand, dithiophosphates has a big at- has gained importance to solve the industrial or eco-
tractivity toward the heavy metal (HM), and this logical problems [16, 17]. Various methods were de-
property has made them the wanted agents in the veloped for remove or reduce heavy metal ions from
aqueous solutions and wastewater [18-22]. According to the international standards the threshold levels of Cu(II), Ni(ll), Pb(ll) and Zn(ll) should be lower than 0,2; 0,2; 0.5 and 2 mgL-1, respectively in wastewater used for irrigation [23]. Therefore, it is necessary to remove or reduce heavy metals from contaminated waters.
In the present study, dioctyldithiophosphoric acid was used for the first time on the removal of Cu(ll), Ni(ll), Pb(ll) and Zn(ll) ions from aqueous solutions and industrial/municipal wastewater. 2. Materials and Methods The acid used in the reaction of complex formation was synthesised from the reaction of phosphorus pent sulphide with n-octyl alcohol according to our previous studies [5; 24; 25].
4C8H17OH + P2S5 ^ 2(C8H17O)2PSSH + H2S The obtained mixture was cooled and the unreact-ed 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 3M H2SO4. The free dioctyldithiophosphoric aci d(DDTPA) floating on the top of aqueous phase was separated as a yellowish-green viscous liquid with 98.5% purity.
Removal of Cu(ll), Ni(ll), Pb(ll) and Zn(ll) ions by dioctyldithiophosphoric acid from aqueous solution was investigated on a synthetic and original industrial wastewater samples at room temperature. Metal complexes of dioctyldithiophosphoric acid [M(DDTP)2] were immediately formed during mixing aqueous solution of M(II) with an adequate amount of dioctyldithiophosphoric acid solution in organic solvent. The total molar ratio of L/M should be higher than 2 for quantitative removal of heavy metals. The formed complexes were easily transferred to organic phase from aqueous phase(Scheme).
Scheme 1. Formation of M(II) dioctyldithiophosphates during the extraction process
The effect of dioctyldithiophosphoric acid concentration, influence of pH of aqueous phase, influence of contact time and different solvents were studied and optimised. 5.10-3M solutions of DDTPA in different solvents such as gasoline, kerosene, n-hexane, petroleum ether and carbon tetrachloride, and a synthetic wastewater sample containing heavy metals(HM) were prepared. In order to obtain the synthetic wastewater sample, 1.10-4 M mix stock solution of Cu(ll), Ni(ll), Pb(II) and Zn(II) in distilled water were prepared
and concentration of the metals were determined (Table 1). All reagents were of analytical grade purity and used as provided from commercial sources. Extraction experiments were carried out by stirring equal volumes(25 mL) of the organic and aqueous phases at room temperature in a separatory funnel. After separation of the phases, concentrations of the remaining metal ions in aqueous phase were determined by a Perkin Elmer Optima 2100 DV ICP-OES at the appropriate wavelengths(Table 1).
Table 1. Synthetic wastewater sample containing heavy metals and working conditions
Heavy Metals ICP-OES wavelenght(nm) Concentration(mgL 1)
Cu(ll) 234.754 6.28
Ni(ll) 221.647 5.79
Pb(ll) 220.353 20.25
Zn(ll) 213.856 6.40
The pH values were adjusted by adding sulphuric 3010 pH meter ( Jenway, London, UK). After sepa-acid or sodium hydroxide, and the pH values were ration of the phases, an aliquot of the aqueous sam-measured with a PCB501 glass electrode Jenway ple was analysed by an ICP-OES for determination
of metal ion concentrations (Fig. 1). The optimised tigated; S1) Gaziantep Küsget- Örnek industrial
condition of heavy metal removal by DDTPA from estate(battery manufacturing and related organi-
acidic aqueous solution was applied to original in- zations) wastewater, S2) Gaziantep Airport organ-
dustrial wastewater. Two types of industrial waste- ised industrial site(mineral oil manufacturing and
water samples from Gaziantep province were inves- related organizations) wastewater(Table 2).
Figurel. Removal of heavy metals from aqueous solution by dioctyldithiophosphoric acid(DDTPA); a) Effect of DDTPA concentration, b) Influence of pH of aqueous phase, c) Influence of contact time, d) Effect of solvents
Table 2. - Removal of heavy metals from wastewater by dioctyldithiophosphoric acid(DDTPA: 5.10-3M; time: 3 min; solvent: kerosene)
Wastewater Concentration of heavy metals(mg/L) in wastewater and removal ratio(%)
Samp les(S) Cu :II) Ni(ll) Pb< ;ii) Zn in)
S. No. pH (mg/L) (%) (mg/L) (%) (mg/L) (%) (mg/L) (%)
S 1 1.5 0.41 100 0.70 100 2.8 100 3.8 100
S 2 2.0 0.07 100 0.10 100 0.2 100 8.2 100
Safe Limits [25] 0.2 0.2 0.5 2.0
51) Battery manufacturing and related wastewater
52)Mineral oil manufacturing and related wastewater The recovery percentage of M(II) can be calculated as follows:
[Mn+ ] -[Mn
R(%) = '
[Mn
x 100
where [Mn+ ]o is initial concentration of the metal ion in the aqueous phase and [Mn+ ] is the metal ion concentration in the aqueous phase after extraction.
3. Results and Discussion
Extraction experiments were carried out at different conditions by stirring equal volume of the ligand containing organic phase and heavy metal containing aqueous phase. After the extraction processes, concentrations of the remaining metal ions in aqueous phase were determined, and
when the metal ions were not detected during the measurements, the extraction rates have been accepted as 100%.
The molar ratio of DDTPA/HM (L/M) has main influence in the extraction process(Fig. 1a). Cu(ll), Ni(ll), Pb(ll) and Zn(ll) were quantitatively removed from the acidic aqueous solution under the conditions that the stoichiometric ratio of DDTPA/HM was >2/1. When the experiment was performed with lower molar ratio, such as in the experiment 0.5.10-3M DDTPA Cu(ll) and Pb(ll) are quantitatively extracted, but removal ratio of Ni(ll) and Zn(ll) were found to be 13% and 0%, respectively. A favourable result was obtained, and the metal ions were quantitatively extracted from the aqueous solution to organic phase when DDTPA concentration was increased to 3.10-3M (Fig. 1a).
The pH of experiments on the extraction of Cu(II), Ni(ll), Pb(ll) and Zn(ll) show that(Fig. 1b), Cu(ll) and Pb(ll) can be quantitatively extracted at pH 0.5, while recoveries of Ni(ll) and Zn(ll) were found to be 93% and 60%, respectively. All the investigated heavy metals were removed from aqueous solution to organic phase at pH between 1-6. The contact time of metal-ligand on the removal of Cu(ll), Ni(ll), Pb(ll) and Zn(ll) were examined(Fig. 1c), and it was found that 1 minute stirring time is sufficient for quantitative removal ofCu(ll) and Pb(ll) ions, while recoveries ofNi(ll) and Zn(ll) were found to be 81% and 67%, respectively. 3 minute contact time is favourable for quantitative removal of Cu(ll), Ni(ll), Pb(ll) and Zn(ll) ions.
To investigate the effect of different solvents on removal of Cu(ll), Ni(ll), Pb(ll) and Zn(ll) ions from aqueous solution, 5.10-3M solutions of DDT-PA in different solvents such as gasoline, kerosene, n-hexane, petroleum ether and carbon tetrachloride and 1.10-4 M solution of Cu(ll), Ni(ll), Pb(ll) and Zn(ll) in distilled water were mixed at room temperature. The results indicated that all the investigated organic solvents are suitable for the removal of Cu(ll), Ni(ll), Pb(ll) and Zn(ll) from acidic aqueous solution (Fig. 1 d).
In the present study, the best results were obtained when the experiments were performed under the following conditions; stoichiometric ratio
of DDTPA/HM >2/1, pH range 1 to 6 and contact time ofligand — metal 3 min. The optimised conditions were also applied to the original industrial/municipal wastewater (Table 2). The samples S1 and S2 are typical industrial area effluents and first one belong to Gaziantep Kusget-Ornek industrial estate(battery manufacturing and related organisations) wastewater and the second, effluent ofAirport organised industrial site(mineral oil manufacturing and related organisations). It was found that the heavy metal concentrations of the battery manufacturing site wastewater are higher than the effluents of motor oil industrial sample (Table 2). The pH values of the untreated samples S1 and S2 were measured as 1.5 and 2.0 at the source and the experiments was performed on the original samples without changing pH values under the optimised conditions (DDTPA 5.10-3M; time 3 min and solvent kerosene). Table 2 show that when the experiments were performed under the optimised conditions, the investigated metal ions were quantitatively removed from the original industrial wastewater samples.
4. Conclusions
O, O'-Dioctyldithiophosphoric acid reacts with heavy metal ions forming M(DDTP)2 complexes at room temperature and the complexes are insoluble in water, but very soluble in organic solvents. Cu(ll), Ni(ll), Pb(ll) and Zn(ll) were quantitatively extracted from aqueous solutions in the pH range of 1 < pH < 6, when the concentrations ratio adjusted as DDTPA/HM >2/1. Only a few minutes are sufficient for removal Cu(ll), Ni(ll), Pb(ll) and Zn(ll) from aqueous solution by DDTPA/any organic solvent such as gasoline, kerosene, n-hexane, petroleum ether and carbon tetrachloride. The heavy metal extractability order was found to be Cu(ll) > Pb(ll) >>Ni(ll) >Zn(ll). According to the present results, Cu(ll) and Pb(ll) ions are easily separated from the other metal ions and it is possible to use this information for analytical purposes. The experiments on original industrial/municipal wastewater without changing pH values showed that Cu(ll), Pb(ll), Ni(ll) and Zn(ll) ions were easily removed from the wastewater under the optimised conditions; DDTPA 5.10-3 M; pH 1-6, time 3 min and kerosene as solvent.
It is possible to indicate that Cu(ll) and Pb(ll) can be removed by DDTPA from industrial wastewater or municipal sludge into organic phase by a counter current process. There is a great convenience for using DDTPA in the removal of heavy metal ions from aqueous solution, althouh having some difficulties in sythesis and purification.
In conclusion, dioctyldithiophosphoric acid is an effective ligand for removal of Cu(ll), Ni(ll), Pb(ll) and Zn(ll) from industrial-municipal wastewater without any pH change, and very convenient reagent for determination of trace amounts of heavy metals via pre-concentration procedure.
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