Effect of humic acid, oxalate and phosphate on Fenton-like oxidation of microcystin-LR by nanoscale zero-valent iron
Introduction
Toxic cyanobacterial blooms are a growing environmental health concern. Cyanobacteria can produce a range of potent toxins, including hepatotoxins and tumour-promoters, neurotoxins, and skin irritants [1], [2]. Microcystin-LR (MC-LR) is the most abundant microcystins, which the World Health Organization has set a provisional guideline value of 1.0 μg/L as the highest acceptable concentration for MC-LR in drinking water [2], [3], [4], [5], [6]. Cyanobacteria present water-borne hazards to health via drinking water and recreational water, so the water from the water supply need to purification. Due to the cyclic structure and presence of novel amino acids, the microcystins are stable and resistant to chemical hydrolysis or oxidation. A variety of traditional methods have been examined to assess the removal of cyanotoxins, including coagulation/sedimentation, activated carbon adsorption and membrane separation. However, to date these methods have only achieved limited success [7], [8], [9].
Research in the past two decades mainly focused on finding appropriate treatment technologies for the purification treatment of microcystins contaminated water [10], [11], [12]. Among them, advanced oxidation processes (AOPs) have received significant attention because they generate hydroxyl radicals and demonstrate potential for the degradation of biorefractory organic compounds. However, the obtained electrophiles react rapidly and non-selectively with organic molecules. The rate constants are in the order of 106–109 L mol−1 s−1 [13]. Fenton reagent, an advanced oxidation technology (AOT), has been applied to detoxify the microcystins, and the toxin’s decomposition is complete in 30 min [14]. However, the homogeneous Fenton process has limitations, including: firstly, the need for additional processing to deal with the dissolved iron ions and sludge during the sewage treatment process; secondly, the acidification of effluents before decontamination to avoid the formation and subsequent precipitation of iron oxyhydroxides; and thirdly, further treatments for neutralization [15], [16], [17], [18].
Recently, zero-valent iron (Fe0) nanoparticles (nZVI) have been investigated as potential catalysts in the heterogeneous Fenton-like oxidation of organic contaminants [19], [20], [21], [22], [23], [24], as nZVI are corroded in acidic solution and generate ferrous ions, leading to Fenton oxidation of organic contaminants in the presence of hydrogen peroxide. However, waste water and ground water contain many dissolved electron acceptors (e.g., nitrate or sulfate) that can react with nZVI surface and lead to surface passivation [25]. The effect of both contaminant and solute concentrations on the performance of nZVI are still unclear. Recently, anions such as phosphate, silicate, sulfate, and nitrite have been observed to adsorb to the surface of ion-based nanoparticles and hence affect the fate of contaminants [26]. In addition, natural organic matters (NOM), such as humic acids, are ubiquitous in the environment and can perform as electron shuttles. They tend to adsorb onto iron oxides and enhance electron transfer between nZVI and contaminants [26]. However, it is still not clear how different NOM, complexing agents and inorganic anions affect the heterogeneous Fenton-like oxidation [19], [20]. Therefore, in this study we addressed this issue by investigating the impact of NOM (humic acid), compelxing agent (oxalate) and anion (phosphate) on heterogeneous Fenton oxidation of MC-LR using nZVI as the catalyst. The objectives here were to: (1) understand the effect of humic acid, oxalate and phosphate as a function of concentration on the Fenton-like oxidation of MC-LR; (2) analyze the degradation kinetics of MC-LR to understand both oxidation and adsorption in Fenton-like systems; and (3) characterize nZVI before and after reaction with MC-LR to determine surface changes using various techniques.
Section snippets
Materials and chemicals
All chemical reagents used in this study were of analytical reagent grade. FeCl3·6H2O, NaBH4, Na3PO4·12H2O, anhydrous ethanol were obtained from Xilong Chemical Co., Ltd. (China) without further purification. Oxalic acid and humic acid were purchased from Qvzhou Chemical Co., Ltd. (China) and Jvfeng Chemical Co., Ltd. (China), respectively. MC-LR was obtained from the Institute of Aquatic Organisms (purity > 95%, Wuhan, China), Chinese Academy of Sciences and stored at −25 °C.
Synthesis of iron nanoparticles
nZVI were prepared
Effect of humic acid on the Fenton-like oxidation of MC-LR
Humic acid contains various types of carboxylate, phenolic, and carbonyl functional groups, which can complex with iron and iron oxide. It can increase the degradation efficiency of nitro aromatic or halogenated compounds as an electron-transfer media [29]. In addition, humic acid adsorbs more strongly to the active sites of the iron surface than the contaminants, leading to reduced Fenton-like oxidation efficiency [29]. In this study, three concentrations of humic acid (0.04 mM, 0.4 mM and 4 mM)
Conclusions
A heterogeneous Fenton-like system with nanoparticulate zero-valent iron as a running water treatment technologies to remove the MC-LR and the effects of humic acid, oxalate and phosphate have been investigated in this study. At low concentrations, humic acid, oxalate and phosphate all compete with MC-LR and adsorb strongly on the active surface sites of the nZVI catalyst, consequently inhibit the Fenton-like oxidation of MC-LR. At high concentrations, for humic acid and oxalate the formation
Acknowledgement
The work was supported by the Natural Science Foundation of Fujian Province (2015J05082). Key Program of Natural Science Foundation of Fujian Province (2014Y0055). Program of Industry-University Collaboration (2016Y4002), China.
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