Effect of organic matter on cyanide removal by illuminated titanium dioxide or zinc oxide nanoparticles
© Farrokhi et al.; licensee BioMed Central Ltd. 2013
Received: 5 December 2012
Accepted: 26 June 2013
Published: 2 August 2013
Effect of different type of organic compounds (humic acid, oxalate, ethylenediaminetetraacetic acid, nitrilotriacetic acid, phenol) on the photocatalytic removal of cyanide with TiO2 or ZnO was studied in this work with variation of the solution pH, contact time, initial cyanide concentration and type of organic compounds. Photocatalytic oxidation efficiency of cyanide with TiO2 was greatly affected by the solution pH. It increased as the solution pH decreased. Also maximum removal of cyanide by ZnO was observed near at neutral pH because of the reduced photocatalytic activity of ZnO at exceedingly low and high pH values originated from either acidic/photochemical corrosion of the catalyst and/or surface passivation with Zn(OH)2. Removal efficiency of cyanide greatly decreased in the presence of humic acid, ethylenediaminetetraacetic acid, nitrilotriacetic acid compared to that without presence of organic compound because of the competitive oxidation as well as surface blocking by relatively large organic compounds. The oxidation pattern of cyanide was better described by first-order kinetic model. Finally photocatalytic reaction with TiO2 or ZnO can be effectively applied to treat synthetic wastewater contaminated with cyanide.
Cyanide (CN) is a highly toxic component, which is generated from several industrial activities such as gas production, pharmaceutical, mining, electroplating processes and coal gasification [1, 2]. It is known that wastewater generated from electroplating industry contains high concentration of cyanide . Cyanides emissions are increasing at a fast rate and cause a serious concern since they are toxic to living organisms even at very low concentrations [1, 4]. Physicochemical methods such as chlorination, electrolytic oxidation, ozonation, etc. and biological methods have been applied to remove cyanides. The generally accepted physicochemical technique for the treatment for the industrial waste containing cyanide compounds is alkaline chlorination [5–10]. The first reaction product generated from chlorination is cyanogen chloride (CNCl), a highly toxic gas having limited solubility. The toxicity of CNCl may exceed that of equal concentrations of cyanide. At an alkaline pH, CNCl hydrolyzed into the cyanate ion (CNO–), which has only limited toxicity [1, 2].
To conquer these problems, advanced oxidation processes (AOP) have been studied and are recommended as talented techniques. Overall, AOPs use hydroxyl free radical (HO.) as a strong oxidant to destroy inorganic compounds that cannot be oxidized by conventional oxidants such as oxygen, ozone and chlorine. The hydroxyl radical can be generated in aqueous solutions using UV/O3, UV/H2O2, Fe(II)/H2O2 and UV/TiO2 [11, 12]. Among these methods, photocatalytic reaction using UV/TiO2 can treat inorganic compounds and heavy metals at the same time through oxidation and adsorption process. Consequently, this method can be used as pre- or post-treatment methods to extra wastewater treatment methods because it is suitable to fit and inexpensive process in the application of wastewater treatment. TiO2 has been widely used as the form of suspension or as a thin film in water treatment [6, 8, 13]. Also ZnO has much attention because electron in valence band of ZnO can be excited at room temperature under low excitation energy. The greatest advantage of ZnO is that it absorbs over a larger fraction of the solar spectrum than that of TiO2 . The surfaces of ZnO support strong chemisorption of oxygen and are sensitive to ultraviolet (UV) light [14, 15]. The most important properties of ZnO are non-toxic in itself, providing attractive photocatalytic efficiency . Photocatalytic reaction using ZnO/UV can simultaneously treat organic compounds and metallic elements in addition to change non-biodegradable to biodegradable organic compounds [17, 18]. ZnO is used as an effective, inexpensive and nontoxic photocatalyst for the degradation of synthetic dyes, oxidation cyanide and reduction of Cr(VI) [17, 19, 20].
As wastewater generated from electroplating industry generally contains high concentration of cyanide, synthetic wastewater containing cyanide concentration ranging from 50 to 200 mg/L was used in this work . The present study investigated the effect of different type of organic compounds (humic acid (HA), oxalate, ethylenediaminetetraacetic acid (EDTA), nitrolotriacetic acid (NTA), phenol) on the photocatalytic removal of cyanide by illuminated titanium dioxide or zinc oxide nanoparticles with variation of solution pH, contact time and initial cyanide concentration. In addition, kinetic parameters were obtained by application of zero, first and second-order equations.
Material and methods
Physicochemical properties of TiO 2 and ZnO
Specific surface area (BET) (m2/g)
50 ± 15
Average primary particle size (nm)
Ignition loss 2 hours at 1000°C, based on material dried for 2 hours at 105°C (wt.%)
where A and B is volume (mL) of standard AgNO3 of sample and of blank, respectively.
All experiments were repeated three times and the average values with error percents were reported.
Results and discussion
Removal of cyanide at different solution pH
Removal of cyanide at different concentrations cyanide
In the presence of photocatalysts, it can be seen that percent cyanide removal decreased as the initial cyanide concentration increased. The presumed reason is that when the initial cyanide concentration increased, more cyanide molecules can be removed on the surface of TiO2 or ZnO. The large amount of removed cyanide might have an inhibitive effect on the further photocatalytic reaction of cyanide because of the decreased adsorption sites on the TiO2 or ZnO as well as the limited oxidants on the surface of TiO2 or ZnO [25, 26].
Effect of type of organic compound
The removed amount of cyanide in the presence of oxalate and phenol gradually increased and was greater than that in the absence of any organic additives over the entire reaction time. Removal efficiency of 100 mg/L cyanide without presence of organic compound increased from 28.9% at 15 min to 73% at 120 min in UV/TiO2 system and increased from 38.67% at 15 min to 70.32% at 120 min in UV/ZnO system. The removal efficiency of cyanide in the presence of oxalate increased from 88.2% at 15 min to 99.3% at 120 min in UV/TiO2 system and increased from 76.6% at 15 min to 92.3% at 120 min in UV/ZnO system, showing greatly different removal efficiency over the entire reaction time.
The residual cyanide concentration after 120 min of photocatalytic reaction with illuminated TiO2 in the presence of oxalate was below the discharge standard of cyanide in USA (1.2 ppm as a maximum value in a day) or Korea (1.0 ppm). It was difficult to explain the enhanced cyanide removal in the presence of oxalate and phenol. Removal efficiency of cyanide greatly decreased in the presence of HA, EDTA and NTA compared to that without presence of organic compound because of the competitive oxidation as well as surface blocking by relatively large organic compounds. Osathaphan et al. (2008) investigated kinetics of the photocatalytic oxidation of cyanide in aqueous TiO2 suspensions in the presence of EDTA (0.4-40 mM) at pH 13.0 . They reported a retardation of the cyanide removal rate in the presence of EDTA, indicating that EDTA successfully competed with cyanide for oxidizing species during the photocatalytic processes.
Kinetic parameters for the photocatalytic removal of cyanide by UV/TiO 2 and UV/ZnO at different initial cyanide concentration (catalyst dosage = 1g/L, pH=7)
UV/TiO 2CN (mg/L)
k 0(mol L -1min -1)
k 1(min -1)
k 2(L mol -1min -1)
UV/ZnO CN (mg/L)
k 0 (mol L -1 min -1 )
k 1 (min -1 )
k 2 (L mol -1 min -1 )
Kinetic parameters for the photocatalytic removal of cyanide by UV/ TiO 2 and UV/ZnO with organic compounds (catalyst dosage= 1g/L, pH=7, cyanide= 100 mg/L, organic compounds= 100 mg/L)
k 0(mol L -1min -1)
k 1(min -1)
k 2(L mol -1min -1)
k 0 (mol L -1 min -1 )
k 1 (min -1 )
k 2 (L mol -1 min -1 )
The major findings of this study are as follow:
Removal of cyanide by UV/TiO2 or UV/ZnO increased through photocatalytic removal in the presence of organic compounds, especially oxalate. The cyanide removal decreased at higher pH because of the decreased potential difference between the conduction band of TiO2 and cyanide and the anionic-type adsorption of cyanide onto the surface of TiO2. Maximum cyanide removal was observed near at neutral pH because of the reduced photocatalytic activity of ZnO at exceedingly low and high pH values. Photocatalytic oxidation rate of cyanide was well described by the first-order kinetics. Photocatalytic reaction with UV/TiO2 and UV/ZnO in the presence oxalate can be effectively applied to treat industrial wastewater contaminated with cyanide.
This work was supported by Department of Environmental Health Engineering, School of Health, Guilan University of Medical Sciences, Rasht, Iran in 2013 and was also partially supported by a Research Grant of Kwangwoon University in 2013.
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