Dimethyl phthalate treatment with zero-valent iron based advanced oxidation process

During the past few decades, water resources consumption and unrestrained wastewater discharges into the receiving environment significantly increased which resulted in several environmental, social, and economical problems. From the environmental aspects, limited amount of freshwater resources is one of the biggest concerns of this century. As a result, the water resources should be protected considering also the water scarcity problem. Due to the above-mentioned environmental problems, stricter control mechanisms have been started to be implemented by governments on effluent wastewater discharge limits. According to the latest researches, it was observed that conventional water and wastewater treatment systems have limited treatment efficiency on removal of complex organic compounds. Thus, the concentration of these pollutants started to increase in receiving water bodies. There are also numbers of chemicals that could be hormonally active detected in human and wildlife as well as in environmental samples. Endocrine disruptor compounds (EDCs) are identified as hormonally active chemicals that are widely generated from industrial activities directly as a product or byproduct. The list of these chemicals is diverse and comprehensive including different chemical families such as alkylphenols, phthalates, phenols, and natural and synthetic estrogens. Because of the adverse effects of EDCs on both wildlife and human health, EDCs have been investigated intensely. These chemicals could be present in our environment that have ability to interfere our food and drink through different industries. Therefore, EDCs became a common concern of the public in recent years. There are extreme diversity of EDCs identification due to complex assessment procedures. Accordingly, researches on these chemicals should be widen as much as possible to collect sufficient information for future generations. In recent years, phthalic acid esters (PAEs) have gained increasing attention by public considering their worldwide use as plasticizers and additives in numerous industries. These toxic, recalcitrant, and potentially EDCs are expansive in environments. Dimethyl phthalate (DMP) is one of the most frequently used PAE and suspected to be an EDC. DMP has been extensively used as additive in cosmetics, varnishes, insecticide, and cellulose acetate film to improve flexibility. Dimethyl phthalate (dimethyl 1,2-benzenedicarboxylate, dimethyl benzeneortho dicarboxylate; DMP) is short-chained, low molecular weight phthalate. DMP is industrially significant chemical widely used as plasticizers and additives which improve the mechanical properties and flexibility of several products such as plastics, paints, and synthetic fibers. Due to the wide range of DMP usage in the industry and high consumption of DMP-containing products, DMP is one of the most frequently identified phthalate in different environmental samples that include surface water, freshwater, marine water, and sediments. Therefore, DMP removal especially from water sources gained specific importance. As a consequence, DMP have attracted enormous public and academic interest lately. The major concern on DMP is causing reproductivity and developmental effects that act as an endocrine disrupting compound (EDC). Since, DMP is considered as an EDC and has particular attention due to its possible accumulation in different environmental compartments. It has been already specified that the accumulation of DMP could result in potential harmful effects on human health and wildlife. DMP has been determined often in wastewater effluents and river water due to its high mobility in the aquatic environment. Since, DMP is an aqueous pollutant, this recalcitrant chemical is of particular concern in drinking water and wastewater treatment systems. Phthalates cannot be removed effectively from the water environment (wastewater and drinking water) by conventional treatment processes that require long retention time. In addition, these systems cannot achieve complete mineralization of DMP that is not beneficial to deal with toxic DMP. In comparison, the advanced oxidation processes (AOPs) have been used widely for the removal of recalcitrant organic chemicals from industrial and municipal wastewater. Specifically, AOPs have been used for the degradation of phthalates due to the high efficiency and easy operation. The main mechanism of the AOPs function is based on generation of highly reactive free radicals. Lately, rapid and highly effective treatment processes have been improved successfully for DMP degradation through different AOPs. More recently, heterogeneous Fenton systems, ferrate (Fe4+), and zero-valent iron (ZVI; Fe0) application have been developed to overcome the limitation of other treatments systems. ZVI based treatments systems exhibit a capacity to abate organic pollutants under acidic conditions. ZVI can activate oxidants to generate highly reactive free radicals that can degrade organic compounds. Thus, in the presence of oxygen and ZVI/oxidant combinations including hydrogen peroxide (H2O2; HP) and persulfate (S2O82-; PS) showed elevated oxidation capacity as a result of high stability of aqua-complexed Fe3+ that leads to generate OH• and SO4•- free radicals through electron transfer mechanism from ZVI to oxidant. In this study, the removal of commercially significant DMP was investigated via nanoscale zero-valent iron (nZVI)-activated HP and PS oxidation from aqueous solution. Eventually, it is also important to follow the changes in toxicity while applying selected AOPs. Accordingly, two sets of biotoxicity tests were conducted using test organisms from two different trophic levels namely; the photobacterium Vibrio fischeri (V. fischeri) and the freshwater microalgae Pseudokrichneriella subcapitata (P. subcapitata) to designate the ecotoxicological effects of DMP and its oxidation products. In the first part of this study, DMP degradation in the absence of oxidants was investigated via different initial nZVI concentrations and varying pH values. The removal performances were maintained in distilled water (DW) to estimate DMP degradation as well as its TOC content. Additionally, the changes in the pH values were also followed. During the experiments conducted in the absence of oxidants to designate the most proper pH value, the highest DMP removal was obtained as 65% (DMP= 20 mg/L; nZVI= 1g/L; at 40 min) at pH 9 where the highest TOC removal was recorded at pH 5 as 10% (DMP= 20 mg/L; nZVI= 1 g/L; at 60 min). Thereafter, additional experiments were conducted in the absence of oxidants for determining the most proper initial nZVI concentration where the experimental conditions were maintained as follow; 20 mg/L DMP concentration and pH at 5. Considering the results of these sets of experiments, DMP removal was limited between 16% - 58%. Throughout these experiments, limited TOC removal was observed. According to the results of these experiments, an initial nZVI concentration and pH values were detected (DMP= 20 mg/L; nZVI= 1 g/L; pH=3 for HP and pH=5 for PS). Thereafter, the degradation of DMP was investigated with nZVI/HP and nZVI/PS combinations. Complete DMP degradation was achieved with nZVI/HP combination during the first minute of the experiments (DMP= 20 mg/L; nZVI= 1 g/L; pH=3; HP= 1, 2.5, and 5 mM) and 80% of DMP removal was achieved with nZVI/PS combination (DMP= 20 mg/L; nZVI= 1 g/L; pH=5; PS=1, 2.5, and 5 mM). On the other hand, TOC removal was achieved with nZVI/HP combination between 33% - 61% where the limited TOC removal (25%) was achieved with nZVI/PS combination. During the second part of this study, toxicity tests were carried out utilizing two battery test organisms that selected from different trophic levels. The experiments were conducted for 120 min where the conditions were selected referring previous sections considering optimum DMP and TOC removal (DMP= 20 mg/L; HP= 2.5 mM; nZVI= 1 g/L; and pH=3). The results revealed that the treated nZVI/HP samples taken during the experiments have inhibitory effects on both V. fischeri and P. subcapitata. As it was expected for the experiments conducted, the complete DMP degradation was achieved during the first minutes of the experiments however, limited TOC removal was observed. The percent relative inhibition rate of V. fischeri increased from 44% (initial DMP solution) to 47% after 120 min. On the other hand, according to the toxicity test conducted with P. subcapitata, the initial percent relative inhibition value was recorded as 75% that reduced to 42% after 120 min. Considering the toxicity test results obtained during nZVI/HP treatment of DMP in distilled water, it could be concluded that the sensitivity of P. subcapitata is higher than V. fischeri maintaned in the present work. In addition to the inhibitory effect of DMP, the battery test results were also indicating the inhibitory effect of oxidation products of DMP.