Mechanisms and Factors Affecting Sorption of Microcystins onto Natural Sediments

Abstract

The sorption of microcystins (MCs) to fifteen lake sediments and four clay minerals was studied as a function of sediment/clay properties, temperature, and pH through well−controlled batch sorption experiments. All sorption data for both sediments and clays are well described by a nonlinear Freundlich model (n f varies between 0.49 and 1.03). The sorption process for MCs exhibited different adsorptive mechanisms in different lake sediments mainly dependent on the sediment organic matter (OM). For sediments with lower OM (i.e., less than 8%), the sorption of MCs decreases with increasing OM and is dominated by the competition for adsorption sites between MCs and OM. In contrast, MC sorption to organic-rich (i.e., more than 8%) sediments increases with increasing OM and is dominated by the interaction between MCs and adsorbed OM. The sorption thermodynamics of MCs onto sediments showed that MC sorption is a spontaneous physisorption process with two different mechanisms. One mechanism is an exothermic process for sediment with lower OM, and the other is an endothermic process for sediment with higher OM. Furthermore, the sorption of MCs onto sediments is pH dependent (sorption decreased with increasing pH). These results provide valuable informations for a better understanding of the natural abiotic attenuation mechanisms for MCs in aquatic ecosystems.

Optimal strategies for determination of free/extractable and total microcystins in lake sediment

“…These findings suggest that the recovery of MCs was primarily affected by the type of sediments and MC variants, which agreed with the results reported by Babica et al [17]. The results of a previous study suggested that the sorption of MCRR, which has one more arginine residue than MCLR, can lead to the formation of cation bridges and more hydrogen bonds between MCs and the sediment surface [12]. This would explain why the extraction yields for MCRR were very lower than those for MCLR.…”

“…The increase in extraction efficiencies of MC with the decrease of the pH under acid conditions maybe resulted from: (1) the enhancement of the protonation of the two carboxylic acid groups in the MC molecules at lower pH (i.e., R-(COOH) 2 [29]) or (2) competition between the abundant H + and MCs for sorption sites on the sediment surface, and correspondingly, the tendency of the MCs to be extracted with less polar solvents such as methanol [17,28]. However, the recoveries of MCs from sediments extracted using 50% (v/v) methanol solutions under weakly acidic conditions were also much lower than those obtained under neutral conditions, which might have been caused by the strong adsorption of MCs in sediments under low pH solutions [12]. These findings suggest that a relatively higher recovery of MCs could only occur during the extraction of MCs in 50% (v/v) methanol solutions under neutral or very strong acid conditions.…”

“…Nevertheless, sediment, which often serves as one of the most important sources and sinks of environmental pollutants, may also play an important role in the fate of MCs. In recent years, numerous studies have reported that MCs can be strongly adsorbed [10][11][12] and favor biotic degradation [10,13,14] in sediments. However, in these studies, the sorption and biodegradation were calculated based on variations in the MCs concentration in the aqueous phase, the bioavailability, toxicity, and migratory activity of MCs in the sediment phase are still unclear, especially the level of bound residues resulting from chemical interactions.…”

Mechanisms and Factors Influencing Adsorption of Microcystin-LR on Biochars

“…Numerous studies have reported that clay minerals effectively adsorb MCs through combining with their ionizable (carboxylic and/or amino) groups (Mohamed et al 2007; Morris et al 2000). Additionally, Wu et al (2011) found that sediment organic matters (SOMs) serve a dual function in adsorbing MCs. (i) SOMs inhibit MC sorption on organic-poor sediment (SOMs % <8 %) by competing for surface adsorption sites on sediment.…”

“…Thus, the dominant mechanism between MCLR molecule and sediment may be primarily attributed to hydrophilic interaction rather than hydrophobic binding. These discussions are based on a large amount of experimental data, where the sorption coefficient (K d ) of MCLR to sediments only ranged from several liters per kilogram to dozens of liters per kilogram (Wu et al 2011;Mohamed et al 2007).…”