Advanced Oxidation of Tris-2-Chloroethyl Phosphate (TCEP) in Water

Tris-2-chloroethyl phosphate (TCEP) is a biorecalcitrant flame retardant and plasticizer. It is a toxic and carcinogenic compound that is frequently detected in wastewater effluents and water bodies, including those used for drinking water, around the world. Due to the incomplete removal of TCEP from current wastewater and drinking water treatment facilities, effective treatment processes must be identified that will remove TCEP from water. This research investigated the effectiveness of Fenton’s oxidation for removing TCEP from water. In laboratory scale experiments, batch reactions of aqueous TCEP solutions were conducted at different pH conditions and dosages of H2O2:Fe 2+ :TCEP molar ratios. Samples were tested at pH values ranging from 2.5 to 4.5 and H2O2:Fe 2+ :TCEP molar ratios from 5.0:0.5:1 to 200:5.0:1. TCEP concentrations were quantified by solid phase micro-extraction (SPME) followed by gas chromatography (GC). Complete removal of TCEP was achieved at equilibrium conditions with a molar ratio dose of 150:5:1 H2O2:Fe 2+ :TCEP. The kinetics of TCEP removal were also investigated. Pseudo-first order rate constants for TCEP removal were obtained. The effectiveness of ozonation was also investigated in this research. Batch reactions of aqueous TCEP at different doses of ozone were conducted at pH 7.0 ± 0.1. Ozonation was found to have no effect on the degradation of TCEP. Chapter 1: Introduction Organophosphorus compounds (OPs) consist predominantly of arylated and alkylated phosphates that are used as pesticides, plasticizers, flame retardants, antifoaming agents in hydraulic fluids, and extraction solvents [1,2]. Most of these compounds are produced in high quantities, on the order of ten thousand tons per annum [3]. Similar to many additives, OPs are usually mixed with and not chemically bonded to the host materials; therefore, they can be released into the surrounding environment [4]. In fact, they have been detected in several matrices including air [3, 2], sludge from wastewater treatment plants [5], and water samples [1, 6 9]. Detection of OPs in water is a particular concern due to the high mobility of the most polar species (i.e., chlorinated flame retardants) [4]. These high mobility polar species can pass through conventional wastewater treatment plants (WWTPs) and water treatment plants without significant removal. The presence of OPs in the aquatic environment may be a concern due to the potential human health and ecosystem effects, which are unknown at this time. Some of these compounds could be neurotoxic, e.g., tri-phenyl-phosphate (TPP) and tri-n-butyl-phosphates (TnBP), while others like tris-(1,3-dichloroisopropyl)-phosphate (TDCP) and tris-(2chloroethyl)-phosphate (TCEP) are considered carcinogenic [11 14]. TCEP is a flame retardant and plasticizer used predominantly in rigid polyurethane foam. This flame retardant is one of the most frequently detected compounds in wastewater effluents and water bodies, including those used for drinking water, in the U.S., Europe, Canada, and Asia [7, 9, 11, 15 18]. TCEP does not meet the European Union (EU) PBT criteria (Persistence, Bioaccumulation, and Toxicity) as it is not bioaccumulative. According to the 2009 EU Risk Assessment Report [13], there is no need for further studies or reduction measures of TCEP in regards to the environment and human health. In spite of this risk assessment, TCEP is recognized to be carcinogenic, highly toxic, and persistent in the environment. Studies on rats indicate that this compound caused kidney and liver cancers as well as damage to testes and impairment to the reproductive system [11,19]. It has also been shown that TCEP is genotoxic, neurotoxic, and possibly mutagenic [8, 12]. In addition to being a health hazard, TCEP is a biorecalcitrant compound that is extremely resistant to degradation in conventional biological treatment processes [8, 14, 17]. Hence, TCEP has been found in WWTP effluents and finished drinking water at concentrations of up to 557 ng/L [8] and 99 ng/L [18], respectively. Due to the incomplete removal of TCEP from current wastewater and drinking water treatment processes, assessment of alternative water treatment technologies have been conducted. Using adsorptive processes (metal salt coagulation, lime softening, powdered activated carbon) and oxidative processes (chlorine and ozone) in three different drinking water supplies, Westerhoff et al. [20] found that only powdered activated carbon was able to remove > 20 % of TCEP. Removal of TCEP using chlorine and ozone oxidation were 90% of TCEP was removed with membranes regardless of feed water conditions. While nanofiltration membranes [21] has proven to effectively remove TCEP from water, advanced oxidation processes (AOPs) are treatment processes that can completely destroy the contaminant instead of transferring it to another phase. AOPs aim to mineralize organic contaminants by converting the organic contaminants to carbon dioxide and water by reaction with hydroxyl radicals (HO•). Hydroxyl radicals are a highly reactive oxidant that is often selected for treatment of waters contaminated with anthropogenic organic compounds. Fenton’s oxidation is an AOP that produces HO• through reaction of hydrogen peroxide (H2O2) with ferrous iron (Fe 2+ ) in aqueous solution. Fenton oxidation has been used to degrade a number of contaminants, including biorecalcitrant compounds. Pignatello [22] used Fe(II)/H2O2 for the degradation of herbicides 2,4-dichlorophenoxyacetic acid, 2,4,5-trichloroohenoxyacetic acid, and atrazine in aqueous solution. The destruction of phenolic compounds in water using Fe(II)/H2O2 was reported by Vella and Munder [23]. Perchloroethylene (PCE) and polychlorinated biphenyls (PCBs) adsorbed on sand were also effectively removed with Fenton reagent [24]. Reactions in these studies were pH sensitive, and acidic conditions were needed for effective treatment. This research investigated the effectiveness of Fenton’s oxidation for treating water contaminated with TCEP. It was hypothesized that production of hydroxyl radicals in Fenton’s oxidation removes TCEP from water. Thus, the following objectives were developed:  determine the influence of different operating parameters (e.g., pH, Fe and H2O2 dosage)  determine whether the oxidation process produces by-products  determine the kinetic rate constants In addition to Fenton’s oxidation, ozone oxidation for removing TCEP was conducted at neutral