This work is dedicated to the removal of free cyanide from aqueous solution through oxidation with hydrogen peroxide H2O2 catalyzed by copper oxide nanoparticles. Effects of initial molar ratio [H2O2]0/[CN−]0, catalyst dose, temperature, pH and the catalyst stability on cyanide removal have been investigated. The use of copper oxide has improved the reaction rate showing catalytic activity. The cyanide removal efficiency was increased from 60% to 94% by increasing in the dose of catalyst from 0.5 g/L to 5.0 g/L. Increasing the temperature from 20 °C to 35 °C promotes cyanide removal and the four successive times re-use of catalyst shows good stability. Kinetics of cyanide removal was found to be of pseudo-first-order with respect to cyanide. The rate constants have been determined.
Liquid effluents containing cyanides are generated in some industrial operations such as gold and silver extraction, production of paint, adhesives, surface finishing, plating and electroplating, petro-chemical applications, photography industry, rocket propellant, pharmaceuticals, coal carbonization (Dash et al. 2009; Kuyucak & Akcil 2013). Plants, microorganisms and insects can also naturally produce cyanide. Cyanides in all their forms, such as free cyanide, cyanide salts, or metal-cyanide complexes are potentially toxic for aquatic life and human beings (Dash et al. 2009; Shen et al. 2014). Short-term exposure can cause rapid breathing, tremors, and other neurological effects. Long-term exposure can cause weight loss, thyroid deficiency, and nerve damage. Their toxicity is mainly due to their aptitude to release free cyanides (Kuyucak & Akcil 2013; Uppal et al. 2016). It is therefore absolutely essential to remove or reduce the concentration of these cyanides' harmful compounds in wastewater at their point source before they are released into the environment.
For the cyanide removal from solution or slurries, different processes are proposed and/or applied (Kuyucak & Akcil 2013). Physicochemical methods such as alkaline chlorination oxidation (Kuyucak & Akcil 2013), oxidation by ozone (Cui et al. 2014), oxidation by Caro's acid (Teixeira et al. 2013; Hewitt et al. 2015), oxidation by oxygen and by hydrogen peroxide (Yeddou et al. 2010; Tian et al. 2016; Yubo et al. 2018), photocatalytic degradation (Raisa et al. 2018), electrochemical oxidation (Berenguer et al. 2017), adsorption (Arif et al. 2019) and biological degradation at low concentrations (Singh & Balomajumder 2016). The efficiency of these processes depends on the type and concentration of the cyanide species present (Yazici et al. 2006).
Considering the disadvantages of conventional chemical treatment methods, the oxidation by hydrogen peroxide can be a good treatment alternative for the complete destruction of cyanide and transformation to a not dangerous form. In presence of hydrogen peroxide, the cyanides are oxidized into cyanates (CNO−) or into (CO32−), when the excess of H2O2 is used (Yazici et al. 2006). This process can remove the totality of free and non-stable metal complex (WADs) cyanides, it does not enrich the medium with salt and does not produce dangerous products (Yeddou et al. 2011). In opposition, this process needs use of a catalyst to increase the rate of cyanides removal.
Soluble copper is frequently used as a catalyst for the oxidation of cyanides by hydrogen peroxide because of its high efficiency. However, it cannot be recovered and it can therefore cause environmental problems as it is a heavy metal. For these reasons, some authors have used copper in a heterogeneous form (Kitis et al. 2005; Yeddou et al. 2011). Yeddou et al. (2011) used copper impregnated carbon and suggest that the catalytic action would be due to the CuO that formed on the surface of the activated carbon.
CuO is well known as excellent adsorbent of several compounds (Zhu et al. 2013) and active catalyst in oxidative reactions (Suramwar et al. 2012; Li et al. 2014). The CuO catalyst may play the critical role of decomposing H2O2 and producing free radical species as HO• and HOO•, who have a strong potential of oxidation (Liou & Chen 2009). Copper oxide has been used as adsorbent for adsorption of hydrogen cyanide (HCN) (Yang et al. 2015). However, to our knowledge, oxidation of cyanides by hydrogen peroxide in the presence of copper oxide has not been yet investigated.
The present work is carried out to investigate the use of copper (II) oxide nanoparticles for cyanide oxidation in solution by hydrogen peroxide and to evaluate its catalytic activity. Effects of initial molar ratio [H2O2]0/[CN−]0, CuO dose, pH, temperature and catalytic stability of CuO on cyanide removal have been analyzed and studied.
MATERIAL AND METHODS
Chemicals and reagents
Copper (II) oxide nanoparticles (CuO) (Sigma-Aldrich) are used as a catalyst in this work. It has been initially repeatedly washed with boiling bidistilled water and dried for few hours at 105 °C. The point of zero charge (pHzc) is determined by the mass titration method (Noh & Schwarz 1990). All other used chemicals are analytical reagent grade (Merck, Darmstadt, Germany). Cyanide solutions are prepared with potassium cyanide (KCN) (99%). Hydrogen peroxide H2O2 (50 wt.%) is used for their oxidation into cyanates. The adjustment of the pH is carried out by the addition of NaOH (1 N) and HCl (1 N).
The X-ray diffraction (XRD) patterns of CuO are obtained using a diffractometer (Inel cps 120) which is equipped with an iron anticathode (k = 1.936 A) and an ethane ionization curved detector allowing to read angles in the range 2θ = 5–125°. The surface morphologies of the copper oxide are characterized using a scanning electron microscope (SEM) (Philips ESEM XL 30).
Cyanide concentration is analyzed by potentiometric method with a cyanide-specific electrode (Orion 96-06) (Clesceri et al. 1998). Cyanate concentration is found out by hydrolyzing it to ammonia at acidic pH (1.5–2.0) and ammonia is measured by the potentiometric method (Clesceri et al. 1998). The pH is measured using a standard pH-meter (HI 221, Hanna Instruments, Tanneries, France).
Experimental procedure of cyanide removal
The experiments are performed in a double-walled glass reactor in batch mode with 1 L of cyanide solutions. The reactor is covered with aluminum foils to prevent contact with light, and kept closed with PTFE lid to avoid air penetration. The solution is stirred with a magnetic stirrer. The required amount of copper oxide nanoparticles (CuO) is added. The beginning of the test is considered to occur simultaneously with the injection of hydrogen peroxide. The concentration of cyanide is determined versus time while the tests are performed. The amount of cyanate is analyzed in the end of tests. The tested parameters are the initial molar ratio [H2O2]0/[CN]0, the CuO dose, the temperature and the catalyst stability. In all experiments, the initial concentration of cyanide and the pH are maintained at 260 mg/L and 11.0, respectively, to be in conditions close to those existing in some local cyanided industrial discharges. Maintaining the pH at the value of 11 also allows avoiding formation of very volatile and dangerous HCN.
RESULTS AND DISCUSSION
Characterization of copper oxide nanoparticles
The pH of zero charge (pHzc) for copper oxide (CuO) is found to be 6.1. The XRD patterns of CuO (Figure 1) show that the CuO material used has a crystalline structure. It is also illustrated that CuO did not change after four consecutive tests of oxidation with hydrogen peroxide. The SEM image of the CuO (Figure 2) shows a rough surface with pores.
Effect of initial molar ratio [H2O2]0/[CN−]0
The oxidation ability of hydrogen peroxide is examined for initial molar ratios [H2O2]0/[CN−]0 of 1, 3, 5 and 10. (Figure 3) shows the decrease of cyanide concentration caused by the hydrogen peroxide. Initial preliminary tests are carried out with hydrogen peroxide in absence of catalyst ([CuO] = 0) and also without hydrogen peroxide ([H2O2]0 = 0) in presence of catalyst.
In presence of CuO without peroxide hydrogen and after 40 min, the cyanide concentration decreases from the initial value of 260 mg/L to around 135 mg/L (elimination of 45% of cyanides). The cyanide removal is due to the adsorption on CuO. The amount of removed cyanide by 1 g of copper oxide is 125 mg. Although the adsorption capacity found in our work is very interesting compared to other works (Dash et al. 2009), it separates only 52% of the initial cyanides.
In the absence of CuO, the oxidation of cyanide by hydrogen peroxide is very slow compared to the oxidation in the presence of catalyst. For initial molar ratio of 3, in the absence of CuO, only 15% of cyanides are removed during 40 min whereas in the presence of CuO and presence of CuO, more than 63% of cyanides are removed after 15 min. The rate of cyanide removal rises with the increase of molar ratio [H2O2]0/[CN-]0. For molar ratios of 1.0, 3.0, 5.0, and 10.0, the cyanide concentrations reached after 12 min are 164.5, 135.2, 26 and 5.2 mg/L, respectively (corresponding to the removal percentages of 36.7%, 48.0%, 90.0% and 98.0%). These values are interesting compared with those found in other works about oxidation of cyanide by hydrogen peroxide (Kitis et al. 2005; Yeddou et al. 2010; Chergui et al. 2015). The positive effect of increasing the amount of H2O2 can be explained by the strong oxidant hydroxyl radicals, which are generated on the surface of the CuO (Suramwar et al. 2012; Zhu et al. 2013; Li et al. 2014).
At the end of the oxidation tests, the concentration of cyanate found shows that 90–95% of cyanides were converted into cyanate which are much less toxic (Kuyucak & Akcil 2013).
Effect of copper oxide dose
In a previous study (Kitis et al. 2005; Yeddou et al. 2010; Chergui et al. 2015), authors observed that the increase of the amount of catalyst improve the kinetics of cyanides oxidation by hydrogen peroxide. Yeddou et al. (2011) observe, for an initial molar ratio of 3, 260 mg/L of initial cyanide concentration and 10 g/L of copper impregnated activated carbon catalyst, an elimination of more than 90% of cyanides. Chergui et al. (2015) removed, in the same conditions, 81% of cyanides after 450 min of oxidation catalyzed by alumina. Kitis et al. (2005) showed that more than 90% of cyanides were removed in 4 h in presence of copper impregnated pumice catalyst with a dose of 3 g/L for a concentration of hydrogen peroxide and cyanide of 150 and 100 mg/L, respectively.
Stability of CuO
Controlling the stability of a catalyst is of crucial importance for retaining process of its properties and activity. This parameter is studied by the re-use of the same sample of CuO four successive times. Before each re-use, the CuO is washed with distilled water and dried for few hours at 80 °C. The results (Figure 5) show that the rate of cyanide removal decreases slightly after each re-use, showing a good stability of catalyst. The same RDX patterns (Figure 1) for fresh CuO and CuO, used four consecutive times, confirm this stability. The stability of catalyst was also studied by the determination the amount of copper leached during the cyanide oxidation. Less than 5.0 mg/L of copper concentration in the solution was found after total elimination of cyanide at pH 11 with catalyst dose of 10 g/L, cyanide concentration of 260 mg/L and initial molar ratios [H2O2]0/[CN−]0 of 10.
Kinetics of removal of cyanide in oxidation by hydrogen peroxide
The kinetic data of cyanide oxidation by hydrogen peroxide catalyzed by CuO are analyzed using the pseudo-first-order (Equations (1) and (2)) and the pseudo-second-order (Equations (3) and (4)) models with respect to cyanide.
The graphs of and versus time (figures not shown) show that the kinetics data of cyanide oxidation are well fitted by the pseudo-first-order model with high coefficients of determination R2 values (>95%). The calculated constants of the two kinetic models along with coefficients of determination R2 values are presented in Table 1. It is observed that the apparent rate constants increase with the increase in the concentration of the hydrogen peroxide and the dose of the CuO catalyst. Figure 6 represents the evolution of Kapp as a function of the initial molar ratios [H2O2]0/[CN−]0; it is shown as a non-linear trend. The reaction mechanism is probably very complex in this case and no precise order with respect to the hydrogen peroxide can be determined, thus confirming a complexity of the oxidation mechanism in the presence of CuO catalyst. This complexity was mentioned by Yeddou et al. (2010) and Chergui et al. (2015) when using the heterogeneous catalysts copper impregnated activated carbon and activated alumina, respectively.
|[CuO] (g/L) .||[H2O2]0/[CN−]0 .||Temperature (°C) .||Pseudo-first-order|
|k1 (min−1) .||R2 .||K2 (L/mmol.min) .||R2 .|
|[CuO] (g/L) .||[H2O2]0/[CN−]0 .||Temperature (°C) .||Pseudo-first-order|
|k1 (min−1) .||R2 .||K2 (L/mmol.min) .||R2 .|
Effect of temperature and determination of activation energy
The effect of temperature (Figure 7) is studied for the values of temperatures of 20, 25, 30 and 35 °C, for initial molar ratios [H2O2]0/[CN−]0 equal to 3, for catalyst dose of 1 g/L and an initial cyanide concentration of 260 mg/L. It is observed that the initial rate and the extent of cyanide removal are improved with the increase of temperature. This may be due to the fact that the conversion of H2O2 into hydroxyl radicals is accelerated with the increase of temperature.
After plotting lnKapp as a function of 1/T (Figure 8), the value of the apparent activation energy is determined from the slope of regression line. The apparent activation energy is 32.2 kJ/mol for the oxidation in presence of 1 g/L of copper oxide (CuO). In the previous study, Yeddou et al. (2010) found values of 82.7 and 46.2 kJ/mol for the apparent activation energy in the oxidation of 260 mg/L of cyanide by hydrogen peroxide with initial molar ratio [H2O2]0/[CN−]0 equal to 5 in the absence and in the presence of activated carbon catalyst (10 g/L), respectively. Chergui et al. (2015) found a value of 55.7 kJ/mol in presence of 10 g/L of alumina catalyst (Al2O3). The CuO, with dose of 1 g/L, reduces significantly the apparent energy, confirming thus its role of catalyst.
In this study, the oxidation of free cyanide by hydrogen peroxide catalyzed with the copper oxide nanoparticles is discussed. The copper oxide has significantly increased the reaction rate, showing a catalytic activity. This study showed the following conclusions.
The rate of cyanide removal increases with the initial molar ratio [H2O2]0/[CN−]0 between 1.0 and 10. In all cases, more than 90% of cyanides are removed in 30 min.
The increase in the concentration of the copper oxide from 1.0 to 10 g/L has a beneficial effect.
The cyanide kinetics improves with the increase of the temperature between 20 and 35 °C. The apparent activation energy of the cyanide removal by hydrogen peroxide is 32.2 kJ/mol.
The four successive reuses of catalyst shows a good stability.
The kinetic study showed that the pseudo-first-order model responds better than the pseudo-second-order for cyanides removal by oxidation with hydrogen peroxide catalyzed by CuO with respect to cyanide.
The proposed CuO catalyst can be a good alternative to the use of homogeneous catalysts, the removal of which after treatment can cause a problem. This catalyst allows removing cyanide with fast kinetic and, at the same time, consumes only hydrogen peroxide as chemical product. Interesting rates of cyanide removal can be obtained even with small initial molar ratios [H2O2]0/[CN−]0.