Humic acid degradation by the synthesized flower-like Ag/ZnO nanostructure as an efficient photocatalyst
© Ghaneian et al.; licensee BioMed Central Ltd. 2014
Received: 9 June 2014
Accepted: 29 October 2014
Published: 9 December 2014
Nano-sized flower-like Ag/ZnO was synthesized by a simple method using zinc acetate and silver acetate under hydrothermal condition. Powder X-ray diffraction (PXRD) and transmission electron microscopy (TEM) were used to characterize the structure and morphology of the synthesized powder. Nano flower-like Ag/ZnO was used as a photocatalyst for degradation of humic acid in aqueous solution. The disappearance of HA was analyzed by measuring the absorbance of sample at special wavelength (254 nm). The effects of various parameters such as amount of photocatalyst, pH, initial humic acid concentration and irradiation time on degradation rate were systematically investigated. Photodegradation efficiency was small when the photolysis was carried out in the absence of Ag/ZnO and it was also negligible in the absence of light. Approximately 70% of humic acid (50 mg/L) has been eliminated after 40 minutes in the presences of catalyst (catalyst dose o.6 g/L and pH =7) and UVA irradiation. While, 100% of humic acid has been eliminated with solar irradiation.
Natural organic matters or NOMs, refers to a group of carbon-based compounds that are found in the surface water and some groundwater supplies. Humic substances represent a major fraction of natural organic matters. Humic substances are divided to three fractions: insoluble components in all pH values (humin); humic acid which is soluble in water at pH >2 and fulvic acids (FA), which are soluble in aqueous solutions at all pH values ,. Macromolecus ofhumic acid have a backbone of aromatic and aliphatic residues with numerous substituent such as phenolic, ketenes, amino acids and carboxylic groups and carrying negative charges in natural waters -. Removal of humic acids is necessary before drinking water chlorination, due to the reaction between chlorine and humic acids in water treatment cause generating carcinogenic substances, such as, trihalomethanes (THMs) and haloacetic acids (HAAs) ,. Furthermore, the presence of macromolecular dissolved organic materials may reduce the adsorption rates and equilibrium capacities of employed membranes or micro-porous adsorbents for water treatment processes -. For the treatment of humic acid-containing wastewater, various biological, physical and chemical methods such as microbial biodegradation, coagulation, filtration, ozonation, adsorption and heterogeneous photocatalysis have been used -.
All purchased chemicals were of reagent grade and used without further purification. Humic Acids (HA) was purchased from Sigma-Aldrich (Switzerland) and used without further purification.
A Shimadzu Model 160-A UV–vis spectrophotometer with 10 mm quartz cells was used to make absorbance measurements. A Wegtech pH meter (Mi 151 22, England) was used for pH controlling was used for stirring the solutions. X-ray powder diffraction (XRD) measurements were performed using a Bruker, Advance D8 with Cu Kα (λ = 1.5406 Å) incident radiation. The size distribution and morphology of the synthesized catalyst was analyzed by scanning electron microscopy (SEM, Philips XL30) and transmission electron microscopy (Zeiss - EM10C - 80 KV).
Preparation of Ag/ZnOnano structure
Zinc acetate (2 g) and silver acetate (0.2 g) were dissolved in water (80 mL). The pH value of the mixture was adjusted to 9 by triethyl amine. The reaction mixture was stirred for 30 min at room temperature. Then the reaction mixture was placed in a Parr-Teflon lined stainless steel vessel. It was sealed and heated at 130°C for 28 h. The reaction mixture was gradually cooled to room temperature. The resulting precipitate was filtered and washed using double distilled water.
Stock solution of the humic acid (50 mgL−1) was prepared by dissolving of 50 mg of humic acid in appropriate amount of NaOH 0.1 N and diluting to 1000 mL. Working solutions were prepared daily by diluting the stock solution with water.
where C0 is the initial concentration of HA, and C the concentration of HA at time t. The light source emitted light just above the sample. The intensity level of light is controlled by fixing the distance between the source of light and the sample.
Results and discussion
Characterization of photocatalyst
At first, experiments concerning the decomposition of HA (50 and 25 mgL-1) were performed, in the presence of Ag/ZnO semiconductor (0.5 g/L), UVA and solar illumination, two blanks experiment in the absence of semiconductor and one blank in the absence of light. Results show that degradation of HA in the presence of the photocatalyst and irradiation could lead to the disappearance approximately 74% of HA under UVA and 82% of HA under solar illumination after 40 min. Blank experiments in the absence of photocatalyst or light demonstrated no noticeable changes in the solution absorbance during of stirring of HA.
Effect of pH
Effect of catalyst concentration
Effect of concentration of humic acid
For HA solution of 10 and 25 mg/L, more than 90% of pollution was degraded in the presence of solar light within 40 min and in case of 50 mg/L, almost 70% of degradation occurred within 70.
UV–vis spectra changes
Nano-sized flower-like Ag/ZnO was synthesized and its photocatalytic effect on the degradation of humic acid under UVA and solar irradiation was investigated. Flower-like Ag/ZnO nano structure is an efficient catalyst for the degradation rate of HA and degradation rate increased under solar light. Approximately 100% of HA has been eliminated after 40 minutes in the presences of catalyst and solar irradiation and without additional oxidation agent.
This research was supported by Shahid Sadoughi University of Medical Sciences and the Islamic Azad University, Yazd Branch.
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