Galvanochemical Oxidation of Thiocyanates - Springer Link

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Abstract—Method for galvanochemical purification of thiocyanate-containing wastewater, with hydrogen peroxide as oxidizing agent, was suggested.
ISSN 1070-4272, Russian Journal of Applied Chemistry, 2010, Vol. 83, No. 11, pp. 1948−1951. © Pleiades Publishing, Ltd., 2010. Original Russian Text © A.A. Batoeva, B.A. Tsybikova, 2010, published in Zhurnal Prikladnoi Khimii, 2010, Vol. 83, No. 11, pp. 1816−1819.

APPLIED ELECTROCHEMISTRY AND CORROSION PROTECTION OF METALS

Galvanochemical Oxidation of Thiocyanates A. A. Batoeva and B. A. Tsybikova Baikal Institute of Nature Management, Siberian Branch, Russian Academy of Sciences, Ulan-Ude, Buryat Republic, Russia Received February 27, 2010

Abstract—Method for galvanochemical purification of thiocyanate-containing wastewater, with hydrogen peroxide as oxidizing agent, was suggested. The optimal conditions were found and kinetic parameters of the process were calculated. DOI: 10.1134/S1070427210110108

One of sources from which thiocyanates come into wastewater and return water are gold-recovery plants using cyanide solutions for hydrometallurgical extraction of gold from ore slurries or flotation concentrates [1–4]. Thiocyanates are formed in reactions of cyanides with sulfides present in water in processing of refractory ores. Thiocyanates are no less toxic than cyanides, with their maximum permissible concentration (MPC) being 0.1 mg l–1 [5]. In addition, presence of thiocyanates in return water adversely affects basic processes employed in recovery of noble metals. Therefore, purification of thiocyanate-containing wastewater or return water is a topical task. Thiocyanate-containing wastewater and return water are frequently purified using biological, chemical, physicochemical, and combined methods [1, 4, 6–8]. It is noteworthy that advanced oxidation processes based on oxidative destruction reactions initiated by hydroxy radicals, which are being intensively developed, are best studied for neutralization of bioresistant pollutants of organic nature [9–11]. To promising combined methods can also be attributed the galvanocoagulation technique for wastewater purification with hydrogen peroxide, named galvanochemical in what follows. The method employs the effect of formation of a multitude of shortcircuited cells that appear as water being treated and air are passed through an active charge. Fe–Cu, Fe–C (coke), Mn–C, and Al–C are most frequently used as charge components [12–14]. The method is rather resource-saving and is characterized by low specific energy expenditure because the process occurs when

charge components come in contact with a solution being processed in the absence of an external current source. Few reports are known, devoted to galvanochemical purification of wastewater containing various organic impurities (phenols, chlorophenols, dyes) [15–17]. Our study is concerned with fundamental aspects of the galvanochemical oxidation (GCO) of thiocyanatecontaining solutions. EXPERIMENTAL The study was carried out in a static glass reactor containing charge components and a certain amount of a solution under study and the oxidizing agent. The GCO method is the most effective in the presence of a considerable amount of dissolved oxygen in a solution being processed, and, therefore, good aeration is a necessary condition for effective purification. We used iron shavings and coke as the active charge. It should be noted that materials serving as the charge are industrial wastes. Model thiocyanate solutions with concentrations in the range 0.86–8.6 mM were used. We examined the effect of various parameters: pH of the reaction medium, molar ratio between the oxidizing agent and a catalyst, and initial concentrations of thiocyanates, on the efficiency of the GCO of thiocyanates. The content of thiocyanates in solution was monitored by spectrophotometry [18]. The concentration of iron ions was determined by photometry of a corresponding complex [19].

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GALVANOCHEMICAL OXIDATION OF THIOCYANATES

The mechanism of the galvanochemical purification of wastewater is determined by processes occurring when water being purified and atmospheric oxygen are brought in contact with charge components having different electrochemical potentials [13]. As a result of operation of a short-circuited galvanic cell, Fe–C (coke) in the given case, there occur numerous redox reactions, including dissolution of iron shavings to give Fe2+ ions and intense oxidation of these ions to Fe3+. Addition of hydrogen peroxide to the reaction mass creates conditions for existence of the Raff–Fenton systems H2O2–Fe2+ (Fe3+) [20, 21] and for catalytic oxidation of thiocyanates. It should be noted that, in the case of galvanochemical purification, the acidity of the medium (pH) is an important parameter for performing processes in the 3+ system H2O2–Fe2+ GCO (Fe GCO)–SCN–. The oxidation of thiocyanates under the action of the Fenton (Raff) reagent is the most effective in an acid medium, at pH 2–2.5 (Table 1), with the 100% oxidation efficiency. The observed dependence may be due to the effect of the acidity of the medium on the process of generation of 3+ iron ions Fe2+ GCO (Fe GCO) in the system and, consequently, on the rate of hydrogen peroxide disproportionation to give hydroxy and peroxy radicals, which are strong oxidation agents for toxic compounds. The conversion of thiocyanates nearly linearly grows with decreasing pH of the reaction medium. For example, at a pH 6.8, the initial reaction rate is 5.33 × 10–4 mM s–1, whereas in a more acidic medium (pH < 3.0), the initial reaction rates W0 are an order of magnitude higher. At pH 2–2.5, in the range in which full conversion of thiocyanates is observed, W0 = (5.03–5.2) × 10–3 mM s–1. In this Table 1. Effect of the pH of the reaction medium on the initial reaction rate W0 and conversion S of thiocyanates. GCO duration 60 min. c0SCN– = 1.72 mM, T = 20°C

6.8

S, % relative to the initial value 10

5.1

21

1.16

4.0

25

3.5

рНin

W0 × 103, mM s–1 0.53

1949

case, the concentration of iron ions generated in GCO 3+ (Fe2+ GCO, Fe GCO) in the reaction medium is 0.4–1.6 mM, depending on the pH of the medium. Further studies were performed at pH 2.0–2.5. With the initial concentrations of thiocyanates in the GCO process varied at the optimal pH values, the conversion of SCN– decreases because, as the initial 3+ – values of [SCN–] in the H2O2–Fe Fe2+ GCO (Fe GCO)–SCN system increase, the number of iron ions generated in the system at pH 2–2.5 is insufficient (Table 2). The full conversion of SCN is achieved at the optimal molar ratio [SCN–] : [FeGCO] = 1 : (0.6–0.69). Consequently, the method is the most efficient for detoxication of solutions with thiocyanate concentrations c ≤ 100 mg l–1. When studying the effect of the hydrogen peroxide concentration on the GCO of thiocyanates, we carried out experiments with [H2O2] of 1.72 to 13.77 mM (Table 3). According to the stoichiometry, the dose of H2O2 per mole of SCN– is 3 mol. The initial rate of the reaction of GCO of thiocyanates grows by nearly a factor of 2.7 as the concentration of the oxidizing agent is raised from 1.72 to 3.44 mM (67% relative to the stoichiometry), with the purification efficiency increasing from 40 to 90%. The result obtained can serve as indirect evidence of additional generation of hydrogen peroxide via reduction of dissolved oxygen, in agreement with published data on indirect electrochemical oxidation of thiocyanates [22, 23]. It was found that, for deep oxidation of thiocyanates, the concentration of iron ions should satisfy the condition [H2O2] : [FeGCO] ≈ 1 : (0.2–0.23). Comparative experiments on purification of real thiocyanate-containing wastewater in various iron– peroxide systems demonstrated that, all other conditions being the same, the efficiency of SCN– GCO is, on average, 1.3–1.7 times higher than that in homogeneous Table 2. Effect of the initial concentration of SCN– on the initial reaction rate W0 and conversion S of thiocyanates. GCO duration 60 min, T = 20°C, pHin 2.5

1.77

[SCN–]0, mM

S, % relative to the initial value

W0 × 103, mM s–1

49

2.40

0.86

98.9

2.53

3.1

77

3.67

1.72

96.6

9.56

2.5

100

5.03

3.44

83.2

16.3

2.0

100

5.20

8.60

60.8

17.1

RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 83 No. 11 2010

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BATOEVA, TSYBIKOVA

S, %

τ, s Conversion S of thiocyanates in various iron–peroxide systems vs. time τ. pHin 2.5; c0SCN– = 1.72 mM, c0HP = 3.44 mM, c0Fe3+ = c0Fe2+ = 1.34 mM; T = 20°C. (1) FeGCO–H2O2, (2) Fe3+–H2O2, and (3) Fe2+–H2O2.

Raff–Fenton systems (see figure). Based on the results obtained, we developed a method for purification of thiocyanate-containing wastewater and return water. The method consists in that water with preliminary introduced hydrogen peroxide is passed through a galvanocoagulation charge, which is a mixture of equal volume parts of iron shavings and coke, with simultaneous delivery of atmospheric oxygen [24]. The mechanism of GCO of thiocyanates by hydrogen peroxide in an acid medium is determined by specific features of the interaction between SCN– and H2O2 in the presence of iron ions. In the course of GCO in a reaction medium containing thiocyanates and an active charge, with H2O2 added and atmospheric oxygen delivered, the solution is intensely colored, which indicates that 3+ Fe2+ GCO (Fe GCO) ions are generated and thiocyanate complexes of the type [FeIII(SCN)n] are formed. Further Table 3. Effect of the content of H2O2 on the initial reaction rate W0 and conversion S of thiocyanates GCO duration 60 min. c0SCN– = 1.72 mM, T = 20°C [H2O2]0, mM

S, % relative to the W0 × 103, mM s–1 initial value

1.72

40

1.50

3.44

90

4.00

5.16

100

4.63

6.88

100

5.26

13.77

100

5.63

treatment results in that solutions are decolorized via oxidation of thiocyanate ions by hydroxy radicals formed in disproportionation of hydrogen peroxide by intermediate thiocyanate complexes of iron. Analysis of the results obtained indicates that the method of galvanochemical purification with hydrogen peroxide and simultaneous dispersal of atmospheric oxygen enables a rather effective oxidation of thiocyanates. On the whole, the method of galvanochemical purification of industrial wastewater and return water to remove thiocyanates has a number of advantages over a number of destructive methods (purification with chlorinated lime, ozonation). First, the GCO method is instrumentally simple. Second, the technique we developed makes it possible to substantially reduce the operating costs and to diminish the purification cost by avoiding additional introduction of the catalyst, smaller expenditure of the oxidizing agent, and use of industrial waste as the active charge. CONCLUSIONS (1) Fundamental aspects of the process of galvanochemical oxidation of thiocyanates in aqueous solutions were studied. The optimal conditions of effective galvanochemical destruction of thiocyanates were found: pH 2–2.5, [H2O2] : [FeGCO] ≈ 1 : (0.2–0.23), [SCN–] : [FeGCO] ≈ 1 : (0.6–0.69). (2) The results of the study were used to develop a method for purification of thiocyanate-containing wastewater or return water. REFERENCES 1. Milovanov, L.V., Ochistka i ispol’zovanie stochnykh vod predpriyatii tsvetnoi metallurgii (Purification and Use of Wastewater by Nonferrous Metallurgy Plants), Moscow: Metallurgiya, 1971. 2. Marsden, J. and House, I., The Chemistry of Gold Extraction, New York: Ellis Horwood, 1992. 3. Tsar’kov, V.A. and Dobroskin, V.V., Abstracts of Papers, III kongress obogatitelei SNG, 20–23 marta 2001 g (III Congress of CIS Dressers, March 20–23, 2001), Moscow: Al’teks, 2001, pp. 212–213. 4. Treatment of Cyanide Heap Leaches and Tailings, Washington: Technical report U.S. EPA., 1994. 5. Lur’e, Yu.Yu., Analiticheskaya khimiya promyshlennykh ctochnykh vod (Analytical Chemistry of Industrial Wastewater), Moscow: Khimiya, 1984.

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