Influence of peptides and proteins produced by cyanobacterium Microcystis aeruginosa on the coagulation of turbid waters

https://doi.org/10.1016/j.seppur.2013.06.049Get rights and content

Highlights

  • Peptides/proteins of Microcystis aeruginosa contribute to the coagulation of kaolin.

  • Mechanisms of interactions between coagulating constituents are pH dependent.

  • The optimum pH for kaolin and peptide/protein removal by Al/Fe is 4–6.5.

  • Peptides/proteins form aggregates with kaolin even in the absence of coagulant.

Abstract

The study investigated the influence of cellular peptides and proteins derived from cyanobacterium Microcystis aeruginosa on the coagulation of kaolin particles during water treatment. To describe the coagulation mechanisms, coagulation system constituents (peptides/proteins, kaolin and coagulant) were characterized in terms of their surface charges. The removal mechanisms of peptides/proteins and kaolin were evaluated by the comparison of the coagulation tests performed with and without coagulant (ferric or aluminum sulfate). We confirmed the peptide/protein inhibiting effect on coagulation through the formation of dissolved complexes with coagulants at a pH value of about 6 for Fe and a pH value of about 6.8 for Al. On the other hand, we demonstrated that cyanobacterial peptides/proteins also have positive effects as they induce the coagulation of hydrophobic kaolin particles within the pH range 4–6 for Fe and 5–6.5 for Al. Interestingly, when peptides/proteins bear a sufficiently low amount of negative charge (pH < 4.5), they can coagulate with kaolin by means of electrostatic interactions even in the absence of a coagulant. The study showed that peptides/proteins produced by M. aeruginosa can serve as coagulation aids and contribute to the turbidity removal at pH values below neutral (pH < 6 for Fe and pH < 6.5 for Al).

Introduction

Eutrophication, followed by the growth of cyanobacteria, such as Microcystis aeruginosa, brings about several issues in drinking water treatment, especially when algal organic matter (AOM) is released into raw water [1], [2], [3]. A number of deleterious effects of AOM on drinking water treatment has been reported: reduction of coagulation efficiency resulting in a rising coagulant demand [1], [2], [3], [4], [5], increased membrane fouling [5], filter clogging, higher yield of sludge as a result of an increased coagulant dose [1] and disinfection by-product formation [6]. In addition, AOM affects the color, taste and odor of drinking water [6] and a number of cyanobacterial species also excrete toxic metabolites which can cause health problems [7]. AOM comprises extracellular organic matter (EOM) resulting from the algal metabolic activity and cellular organic matter (COM) released into water during the cell decay. In a simplified way, AOM can be divided into peptide/protein and non-peptide organic matter [8], [9], [10].

In surface water, AOM is usually accompanied by other impurities, most commonly by inorganic colloidal particles which need to be removed together with AOM during the water treatment process. Some authors have, therefore, focused on the coagulation inhibition caused by AOM, specifically on the effect of AOM on the coagulation of inorganic colloidal particles, such as quartz or kaolin [1], [8], [9]. Particular attention has been paid to COM peptides and proteins that are able to form soluble complexes with coagulants, which results in a coagulant consumption and a subsequent decrease in coagulation efficiency [1], [8], [9], [10], [11]. Moreover, AOM was also reported to interfere with the coagulation using cationic biopolymers such as chitosan and cationic starch [4]. Cationic biopolymer coagulants may interact with oppositely charged polyelectrolytes within the AOM, such as carbohydrates and proteins, which leads to the dispersion restabilization [4]. On the other hand, some studies suggested that cyanobacteria-derived organics might enhance the coagulation of other impurities under specific conditions [1], [3], similar to a range of natural polymers commonly used in water treatment (e.g. chitosan, sodium alginate, and seeds of Moringa oleifera) [12]. To achieve an efficient coagulation in a system consisting of multiple impurities such as inorganic colloidal particles and AOM, it is necessary to understand the pathways by which these impurities are removed. Although researchers have investigated the coagulation mechanisms of inorganic colloidal particles and organic matter, the majority of studies have focused separately on the interactions between the coagulant and only one of these impurities [11], [13], [14], [15], [16]. Little work has been carried out to elucidate the coagulation mechanisms when both these impurities (AOM and inorganic colloidal particles) are present in raw water [8], [9]. The highest concentrations of dissolved AOM are present in surface water during the algal bloom decay in the form of COM released from damaged cells. At that time, the most serious deterioration of coagulation process is usually observed [2]. It was reported that the major portion of cyanobacterial COM is represented by peptides and proteins [10].

Therefore, the purpose of this study was to examine the coagulation behavior of the system consisting of COM peptides/proteins produced by cyanobacterium M. aeruginosa and hydrophobic kaolin particles, which represented inorganic colloids. The study focuses on understanding the interaction mechanisms between COM peptides/proteins, kaolin particles and hydrolysis products of coagulants (Al and Fe salts). Because both AOM and kaolin removal has been reported to be highly pH-dependent [11], [13], [14], [15], [17], particular emphasis was put on the effect of pH on the coagulation efficiency.

Section snippets

Cultivation of M. aeruginosa

The inoculum of cyanobacterium M. aeruginosa was obtained from the Culture Collection of Algal Laboratory, Institute of Botany, AS CR, Czech Republic. The culture of M. aeruginosa was harvested on the 16th day of cultivation during the steady-state growth, when the concentration of chlorophyll-a was 2340 μg L−1. Methodologies of cultivation and chlorophyll-a measurements are described in the literature [10].

COM peptide/protein preparation

The M. aeruginosa cells were separated from the growth media by a 0.22 μm membrane filter

COM peptide/protein characterization

Cellular organic matter (COM) of cyanobacterium M. aeruginosa was found to comprise about 63% of peptide/protein material determined as DOCP, which is in agreement with the findings of other studies [11], [22]. The COM peptides/proteins consequently used in coagulation experiments were further characterized in terms of molecular weight (MW) distribution. Peptides/proteins of apparent MWs of approximately 1, 2.8, 4, 4.5, 5, 5.7, 6, 6.8, 8, 8.5, 12, 30, 40, 52, 106, 266, 470 and 1077 kDa were

Conclusion

The occurrence of cyanobacterial peptides/proteins in turbid waters substantially changes the optimum conditions for their treatment. The removal process is highly pH dependent since the charge of removed impurities as well as of traditional coagulants used (Al and Fe salts) changes with pH value. Though kaolin particles, which represent the clay colloids in turbid waters, are removed at pH about neutral (7–8.5 for Al and 6.4–8 for Fe) due to adsorption mechanism, the optimum pH for coagulation

Acknowledgments

The research project has been funded by the Czech Science Foundation under the Project No. P105/11/0247. The authors acknowledge the financial assistance on this project.

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