Removal of microcystin-LR from drinking water using adsorption and membrane processes.

摘要

Blooms of cyanobacteria are of emerging concern in the United States as well as other parts of the world. These cyanobacteria can make drinking water smell and taste poorly, and the cyanotoxins released from harmful cyanobacteria may cause mass mortalities of wild and domestic animals and result in human sickness. Microcystins are well known to be one of the most dangerous and most commonly occurring classes of cyanotoxins in the drinking water supplies. When consumed or in contact with skin, microcystins can lead to skin irritation or liver and kidney damage as well as may initiate liver cancer. Due to adverse health effects, the World Health Organization (WHO) set a guideline level of 1 part per billion (ppb) for microcystin. However, current water treatment facilities may not specifically treat drinking water for microcystin.;The overall goal of this research was to develop an advanced and effective process for the removal of microcystins from drinking water. To achieve this goal, powdered activated carbon (PAC), iron oxide nanoparticles, and ultrafiltration (UF) membranes were explored as promising treatment technologies. Laboratory-scale experiments were performed to examine the effectiveness of each treatment process, determine the optimum operational conditions, and explore the mechanisms controlling toxin removal.;The use of ultrafiltration (UF) was investigated for the rejection of microcystin-LR from drinking water. Adsorption dominated rejection for most UF membranes, at least at early filtration times, while both size exclusion and adsorption were important in removing microcystin-LR by the tight thin-film (TF) membranes with a molecular weight cutoff (MWCO) of 1-4KDa. The extent of membrane adsorption was generally related to membrane hydrophobicity. The initial feed concentration had a significant influence on the adsorption capacity of TF membranes for microcystin-LR, resulting in a linear adsorption isotherm. Higher permeate flux resulting from increasing water recovery or operating pressure, led to greater adsorption of microcystin-LR on the polyethersulfone and thin-film membranes and a decrease in size exclusion.;The application of ultrafiltration coupled with powdered activated carbon (PAC-UF) was also investigated as a drinking water treatment process for microcystin-LR removal. The influence of different operating factors such as activated carbon type and dosage, membrane composition, and mixing time was examined to define optimum operational conditions for effective removal of microcystins from drinking water. Of the two different PAC materials, wood-based activated carbon was more effective at removing microcystin-LR than coconut-based carbon due to greater mesopore volume. The PAC-UF system had the highest removal efficiency among the three processes (i.e., PAC adsorption, ultrafiltration, and PAC-UF) for both hydrophobic polyethersulfone (PES) and hydrophilic cellulose acetate (CA) membranes. When PAC was coupled to UF using PES membranes, greater removal of microcystin-LR occurred compared to when CA membranes were used, due to sorption of the toxin to the PES membrane surface.;In further studies, Suwannee River Fulvic Acid (SRFA) was used to examine the effect of natural organic matter (NOM) on the removal of microcystin-LR during ultrafiltration, either as a stand-alone process or in combination with PAC. When PES membranes were previously fouled by SRFA, increased size exclusion and reduced adsorption of microcystin-LR were observed, probably due to pore blockage and fewer available adsorption sites as a result of SRFA sorption. However, simultaneous addition of both microcystin and SRFA resulted in no change in microcystin-LR adsorption since microcystin molecules are apparently able to adsorb before significant amounts of SRFA associated with the PES membrane. The presence of SRFA reduced microcystin-LR removal by PAC-UF, primarily due to competition between SRFA and microcystin-LR for adsorption sites on the PAC surface.;Finally, an adsorption study was performed on microcystin-LR using iron oxide (maghemite) nanoparticles. Factors influencing the sorption behavior examined included microcystin-LR and maghemite concentration, pH, ionic strength, and the presence of SRFA. The results indicated that adsorption was primarily attributed to electrostatic interactions, although hydrophobic interactions may also play a role. The adsorption of microcystin-LR decreased with increasing pH, primarily due to a decrease in surface charges of maghemite and subsequently, reduced electrostatic attraction. The ionic strength (i.e. NaCl concentration) affected microcystin adsorption by screening the electrostatic interactions. The presence of SRFA strongly influenced microcystin adsorption; the extent of microcystin-LR adsorption decreased with increasing SRFA concentration (above 2.5 mg/L) due to the preferential adsorption of SRFA over microcystin-LR. This laboratory-scale work is an initial step in developing an advanced treatment system that could be easily incorporated into drinking water treatment facilities. It is expected that this research can provide both practical and fundamental information for more efficient process design, leading to effective removal of harmful cyanotoxins and improved water quality and safety.