Experimental studies on removal of microcystin-LR by peat

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Abstract

Cyanotoxins have caused worldwide concerns for their eclectic occurrence and toxic effects, which led to an intensive search of cost-effective techniques for their removal from contaminated waters. A range of biomaterials was tested for their efficacy to adsorb a potent cyanotoxin, microcystin-LR (MCLR). Among these sorbents, peat showed the maximum efficacy to sequester MCLR. The BET (Brunauer–Emmett–Teller) surface area of peat was found to be 12.134 m2/g. The pH of the reaction media played a significant role in removal of MCLR; maximum adsorption occurred at pH 3. Kinetic studies showed that the adsorption of MCLR onto peat was a rapid process. The adsorption capacity (Qmax) from the Langmuir model was found to be 255.7 μg/g at pH 3. Among various desorption media studied, strong alkali (2N NaOH) showed highest desorption (94%).

Introduction

Microcystins (MCs) are a family of cyanobacterial toxins produced by at least six genera of cyanobacteria including Microcystis, Oscillatoria, Nostoc and Anabaena, which are monocyclic heptapeptides composed of seven amino acids including an unusual amino acid (2S, 3S, 8S, 9S)-3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca-4,6-dienoic acid (Adda) that is essential for the expression of their biological activity [1], [2]. MCs are produced intra-cellularly, and their occurrence in natural water bodies has been reported worldwide [3], [4], [5]. These toxins get released into water bodies due to both natural toxin release caused by natural biological process of cell lysis and by artificial induction through cell destruction in treatment processes [6]. MCs are known for their rapid activity and acute lethal toxicity [7], and damage liver and induce tumour promoting activity through the inhibition of protein phosphatases [8]. Multiple human deaths in Brazil after haemodialysis using water containing microcystins were reported in Ref. [9]. Among 60 molecular variants of MCs isolated till date, MCLR, a most hydrophobic variant containing leucine and arginine, is considered to be the most commonly occurring and most lethal toxin [10]. A provisional safety guideline of 1.0 μg/L MCLR in drinking water was recommended by WHO [11]. Toxicity of MCLR based on intraperitoneal LD50 in laboratory mouse or rat injections was found to be 50 μg/kg [12].

Generally, MCLR is a very stable toxin in the water bodies and resistant to be removed from drinking water by traditional water treatment technology [11], [13]. Several strategies for the removal of MCLR from water have been investigated. Water treatment technologies including chemical coagulation, flocculation and sand filtration may not only be inefficient but may also enhance the release of cyanotoxins including MCLR contained within the producer-cells, thus requiring further treatment of the water for drinking purposes [14]. Chlorination has been shown in several studies to be effective in deactivating the producer-cells and partially effective in removing the toxin as such. However, chlorination has a disadvantage of generating toxic by-products of toxins such as trihalomethanes [15]. Pre-ozononation followed by rapid sand filtration showed a satisfactory removal of the toxin in a full-scale drinking water treatment [16]. The major disadvantage of this technology is that the cost-factor is too high which precludes its use on a regular basis in a huge city-water supply from a reservoir. Photo-irradiation using UV radiation proved to an effective treatment technology but its installation and maintenance costs are very high [17]. Adsorption seems to be an effective and cheaper technology, comparatively. Some researchers have reported the possibility of using activated carbon for removal of MCLR and related toxins [18], [19], [20], [21]. Even though activated carbon adsorption poses to be an attractive and cheaper technology available, the need to replenish the column with fresh carbon after its saturation makes it still economically not feasible. Thus a natural and environment-friendly sorbent could provide a cost-effective solution. There are only two reports available on successful adsorption of MCLR onto natural materials which include clay particles and pumice [22], [15].

Peat is a natural material containing lignin, cellulose and humic acid as its major constituents [23]. Peat is generally defined as young coal, organogenic sedimentary rock in the first stage of coalification and ranks as one of the lowest grades of solid carbonaceous fuels [24]. According to the incomplete list of peat resources, ∼77% of peat deposits occur in Canada and the USA and 7.5% in Scandinavia, where as in southern hemisphere, the richest deposits are in Indonesia [25]. This natural material is known to have excellent ion-exchange properties similar to natural zeolites [26]. Thus, the use of peat as a sorbent has received increasing attention. Apart from being naturally abundant and inexpensive, peat possesses several characteristics that make it effective for adsorption or ion-exchange operations [26]. To date, the use of peat as sorbent for various metal ions and dyes has been well documented [24], [26], [27]. However, its effectiveness for removal of cyanotoxins has never been evaluated.

The objective of this work was to assess the uptake capacity of various naturally occurring materials and to optimize the experimental conditions for achieving the maximum uptake of MCLR. Specifically, crab shell, sugarcane baggase, marine algae, chitin and peat were tested for their uptake efficacy of MCLR. Out of the tested materials, peat showed the best performance for the adsorptive removal of MCLR. The influence of operating parameters such as pH and contact on the adsorption capacity of peat was studied, followed by the optimization of these parameters in order to obtain a maximum removal of MCLR from the aqueous phase. The possibility of regeneration of used peat was also examined.

Section snippets

Peat preparation and characterization

Peat was collected from Sungai Sembilan peat deposit; a sub-province located 200 km away from the city of Dumai in Sumatra, Indonesia. The sub-surface layer of the peat collected from the site was dried in sunlight for 3 days and further dried in an oven at 60 °C for 2 days. The dried peat material was then ground and sieved through a 150 μm sieve, prior to its usage in experimental studies. Specific surface area and total pore size distribution were determined using the nitrogen adsorption

Screening of sorbents

We studied the MCLR sequestration efficacy of a range of natural materials (crab-shell, chitin, sugarcane bagasse, marine alga Ulva and Sargassam and peat). In addition, two ion-exchange resins (ALXD 4 and ALX 47) were also evaluated. Among the sorbents studied, peat showed the maximum sequestration of MCLR (Fig. 1). Interestingly, the sequestration efficacy of peat was even higher than the ion-exchange resins studied. This can be attributed to the inimitable nature of peat. Peat is rich in

Conclusions

Adsorption of MCLR onto peat was pH dependent and was found to be maximum at pH 3 with 90% removal efficiency in 30 min. The maximum adsorption capacity (Qmax), as predicted by Langmuir isotherm, was 255.7 μg/g. 93.7% of MCLR could be desorbed with 2N NaOH as eluting media. Based on this study, it could be concluded that peat is a promising adsorbent for removal of MCLR. Since peat is readily available, inexpensive, indigenous and environment-friendly, its use as a biosorbent would significantly

Acknowledgements

The authors gratefully acknowledge the support and contributions of this project to the Singapore-Delft Water Alliance (SDWA). The research presented in this work was carried out as part of the Singapore-Delft Water Alliance (SDWA)’s research programme (R-264-001-013-272).

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