Adsorption of Microcystin LR by Activated Carbon Fibers

Abstract

Blooms of toxic microorganisms are commonly found in freshwater in several areas of the world. Freshwater poisonings are mainly caused by species of cyanobacteria, also known as blue-green algae. They develop in eutrophic water wherever proper conditions for their growth are found, including a mild wind, a water temperature between 15 and 30 C, a neutral or alkaline pH and a rather high level of mineral nutrients. As a consequence of the extended use of nitrates and phosphates, blooms occur more and more frequently. There is a risk that these blooms contaminate water supplies that are used as recreational areas or as drinking water reservoirs. This is a serious water quality problem because many of the cyanobaterial species are able to produce potent toxins. The most frequently reported toxins are hepatotoxins. They are classified as microcystins and nodularins, the former representing the largest group. All microcystins have a common cyclic heptapeptide structure consisting of (-DAla-L-X-D-MeAsp-L-Z-Adda-D-Glu-Mdha), where MeAsp stands for erythro-β-methylaspartic acid, Mdha for Nmethyldehydroalanine, Adda for 3-amino-9-methoxy-2,6,8trimethyl-10-phenyldeca-4,6-dienoic acid, and X and Z for the variable amino acids that give its name to the molecule. For example, microcystin LR has X = Leusine and Z = Arginine, microcystin RR has X = Arginine and Z = Arginine. Microcystins can be synthesized by various genera of cyanobacteria such as Microcystis, Oscillatoria, Anabaena or Nostoc. They have been responsible for the poisoning of fish, birds, wild and domestic animals in many countries. Adverse effects on human health have also been recognized. Recent researches have actually shown that microcystins act as type 1, 2A and 3 protein phosphatase inhibitors, as well as tumor promoters when present in nanomolar concentrations. Nowadays, WHO guidelines have been established for the monitoring of microcystins in water. Maximal values of 0.1 μg/L in drinking water for a long term exposure or 1 μg/L for a short term exposure have been proposed on the basis of laboratory experiments of toxicity on mice and pigs. Blooms of blue-green algae, which have been increasing in recent years, have the potential to cause problems for the water supply industry. Recent occurrences of toxic algal blooms have highlighted the potential risk of toxins entering drinking water supplies, and posing a threat to public health. This necessitates a need for research to determine the effectiveness of different water treatment processes for removing algal toxins. At present, slow sand filtration has shown some potential to remove toxins by biodegradation. However, not all types of toxin were successfully removed. The limited amount of work on ozonation has shown that it is an effective process for removing toxins, even with relatively low ozone doses. Ozonation does produce by-products, but these have been shown to be non-toxic. Activated carbon adsorption has been shown to be capable of effectively removing different toxins. With powdered activated carbon (PAC), high PAC doses were generally required. However, most of the tests were conducted with higher toxin concentrations than would probably be found in raw waters. Granular activated carbon (GAC) can effectively remove toxins, but the studies have not fully assessed the most suitable GAC. In addition, the PAC doses required may be impractical to dose at a treatment works. For GAC adsorption to be effective, contact times of more than 15 minutes should be considered, assuming that microcystin does not biodegrade on the GAC. Activated Carbon Fiber (ACF) is a recently developed adsorbent which has more superior adsorbing capacity to active carbons. Recently, Chungbuk National University (Chungju, South Korea) has developed a manufacturing process of ACF by using various precursors of fabrics, cottons, viscous rayons and mixed fabrics etc. ACF made by fabrics and cottons has never been used to remove microcystins in water sample. There is no published data available that has investigated water treatment processes using ACF made by fabrics and cottons, In this study we aimed to obtain basic data on the adsorption of the toxic peptide, microcystin LR to the ACF that was prepared in Chungbuk National University. Our future goal would be to identify and develop ACF treatment processes suitable for removal of algal toxins and to provide a basis for estimating potential water treatment usage.