عنوان مقاله [English]
Developing chemical remediation techniques has become an increasingly important research topic due to the growth in the use of organic dyes which can be harmful to nature and human health. Catalytic reduction reaction of organic pollutant dyes (OPDs) in the presence of metal nanoparticles (MNPs) such as copper, silver and palladium is an efficient and simple method to treat polluted industrial waters. However, easily agglomeration of the MNPs impacts their efficiency and re-usability. Therefore, we used chicken eggshells as a natural resource to make hydroxyapatite (HAP) as a heterogeneous bed with high contact surface by friendly hydrothermal and environmentally method. The HAP was coated with silver nanoparticles (AgNPs@HAP) and was used as an efficient and recyclable catalyst for the reduction of OPDs (Methylene blue (MB), 4-Nitrophenol (4-NP) and 4-Nitroaniline (4-NA)) in the presence of sodium borohydride. The reduction reaction was performed in less than 10 minutes with excellent efficiency (approximately 100%) and the reaction kinetics were investigated by change of the absorption intensity using a two-beam UV-Vis spectrophotometer. The physical and chemical properties of AgNPs@HAP were evaluated and the prepared solid heterogeneous catalyst was characterized via FTIR, XRD, BET, TEM, FE-SEM/EDS and elemental mapping analysis. This study demonstrates the potential use of HAP derived from biowaste eggshell with high efficiency for the degradation of OPDs under mild reaction conditions at room temperature. In addition, AgNPs@HAP showed high stability and reusability for four cycles with almost constant catalytic performance.
Large volumes of effluent are produced daily by industries such as textiles, cosmetics, printing and dye production. If these polluted effluents are not treated, they can enter surface and groundwater resources that in turn can have devastating and irreversible effects on human health and the ecosystem. Therefore, water and wastewater treatment has become one of the major global concerns.
Aromatic organic dyes and pigments are the most common toxic pollutants among industrial effluents, which are often not biodegradable. For example, 4-NP is a hazardous and carcinogenic organic dye that is very stable in water and difficult to decompose environmentally . In addition to irritating the eye, this aromatic colour pollutant can cause severe kidney and liver dysfunction, which has led the US Environmental Protection Agency to blacklist it.
Today, various methods such as physical, chemical and biological treatment are used to remove hazardous and toxic pollutants from polluted effluents and waters. Examples include membrane filtration, oxidation, light or chemical degradation, coagulation, flocculation, biodegradation, and catalytic reduction. The catalytic reduction method is an alternative method that has been reported to be a cheap, fast and more efficient method for the treating of water containing OPDs.
However chemical compounds such as hydrazine and are among the catalysts that can reduce OPDs such as 4-NP, 4-NA and MB, but these substances are hazardous and produce secondary pollutants that can be harmful to the environment. On the other hand, although reduce agents such as sodium borohydride (NaBH4) alone is unable to reduce the mentioned pollutant compounds, but it can perform the reduction process in the presence of metal catalysts completely. Up to now, some of the metal catalysts such as silver, copper and palladium nanoparticles were applied to reduce OPDs with acceptable efficiency and kinetics.
However MNPs can be used for the preparation of catalysts to increase significantly their catalytic activity but these particles have high tendency to accumulate and aggregate. Therefore, they are often dispersed and stabilized on heterogeneous substrates to prevent agglomeration. Today, the development of solid substrates has been considered especially by researchers, industrial chemical production and environmentalists. Solid substrates should have characteristics such as high thermal stability and good access to active sites. Numerous heterogeneous substrates, especially environmentally friendly substrates, have been proposed and used by researchers to stabilize MNPs.
In recent years, to prevent the production of secondary pollutants, researchers have especially used biowastes such as lignin, animal bones and eggshells to the preparation of solid substrates such as hydroxyapatite (HAP) in the catalytic reactions. HAP is a bioceramic material with the molecular formula of Ca10(PO4)6(OH)2. In addition to being biocompatible, HAP has a high porosity, surface contact, mechanical, chemical and thermal stability. So far, it has been widely used in various medical and chemical fields such as bone reconstruction, tooth restoration and suitable and recyclable substrate for the preparation of heterogeneous adsorbents and catalysts. Up to now, various methods such as hydrothermal method, sol-gel process, mechanical method, microwave synthesis and wet chemical deposition method have been proposed to produce HAP. Annually millions of eggshells are dumped or buried as waste in the environment that can be used as a cheap and available precursor to the preparation of hydroxyapatite, which can be a "trash to treasure" process. Therefore, using eggshell as a raw material is one of the most common methods for the preparation of HAP.
In this study, eggshell derived HAP was prepared via a simple and green process and used as a solid substrate for the stabilization of silver nanoparticles. Finally, eggshell derived hydroxyapatite coated by silver nanoparticles (AgNPs@HAP) was applied as a catalyst in reduction reaction of some OPDs (4-NP, 4-NA and MB) in the presence of NaBH4 at room temperature. The obtained results show that OPDs can be reduced in the presence of AgNPs@HAP in high yield along a short time.
HAP was prepared from biowaste eggshells based on the reported method by Shi et al. with minor modifications (Scheme 1). At first, eggshells were collected from locally restaurants and confectioneries in Sirjan City which produce daily a large number of eggs to make omelets and sweets. To remove organic matter, membranes and excess particles, the collected eggshells are boiled and washed for 2 hours in water and hexane solvents under reflux conditions and dried for 5 hours at 90 ° C. Dried eggshells were then grounded in a mortar and a white powder of eggshells was obtained (ES1). To obtain an equal molar ratio of calcium and phosphorus, 5 g white powder ES1 and 6 mL phosphoric acid (85 %) were added to 50 mL deionized water and was exposed to ultrasonic for 30 minutes at 50 °C. By performing the reaction, the releasing of carbon dioxide gas from the reaction medium is easily visible in this step of the procedure. Finally, the obtained suspension of calcium phosphate (CaHPO4) was centrifuged (4000 rpm). Obtained calcium phosphate was added to 50 mL deionized water, then was stirred 30 min (500 rpm) to prepare a white suspension (ES2).