Adsorbents Used for Microcystin Removal from Water Sources: Current Knowledge and Future Prospects

Abstract

The increasing occurrence of toxic cyanobacteria in water sources, driven by climate change and eutrophication, is of great concern worldwide today. Cyanobacterial blooms can negatively affect water bodies and generate harmful secondary metabolites, namely microcystins (MCs), which significantly impair water quality. Various adsorbents used for MC removal from water sources were assessed in this investigation. Activated carbon constitutes the most widely used adsorbent for treating contaminated waters due to its high affinity for adsorbing MCs. Alternative adsorbents have also been proposed and reported to provide higher efficiency, but the studies carried out so far in this regard are still insufficient. The mechanisms implicated in MC adsorption upon different adsorbents should be further detailed for a better optimization of the adsorption process. Certainly, adsorbent characteristics, water pH and temperature are the main factors influencing the adsorption of MCs. In this context, optimization studies must be performed considering the effectiveness, economic aspects associated with each adsorbent. This review provides guidelines for more practical field applications of the adsorption in the treatment of waters actually contaminated with MCs.

Influence of biochar on the removal of Microcystin-LR and Saxitoxin from aqueous solutions

“…This consequently facilitates charge attraction or repulsion between the toxin and adsorbent. The MCLR molecule contains carboxylic acid groups and amino acid groups, imparting a negative charge at pH values 3 and 12 69 . This explains the higher removal efficiency at lower pH, as electrostatic attraction aids adsorption, while at higher pH, repulsion ensues between the toxins and biochar, limiting uptake.…”

“…As for the hydrogen bonding, the biochar contains substantial amount of acidic oxygen containing function groups of 941.33 ± 0.86 µmol/g (Table S1 ) which are favorable for chemisorption. These chemical interactions may arise via the polar or polarizable MCLR molecules and the oxygen containing functional groups present on the biochar 69 , 82 . Hydrophobic interactions play interesting roles for the uptake of MCLR onto biochar.…”

Evaluating the Application of Chitosan-Based Sorbents for the Solid-Phase Adsorption Toxin Tracking of Microcystins in Irrigation Water

“…All of this has a direct impact on the electrostatic interactions. According to El Bouaidi et al [28], the adsorption of MC-LR has been demonstrated to increase with a decrease in pH (from 8 to 2.5). Under acidic conditions, the weak acidic functional groups of MCs are presumed to become available for adsorption [28], and this probably explains the significantly higher adsorption of the Diaion ® HP-20 at site H2, which had waters with slightly neutral pH values.…”

“…According to El Bouaidi et al [28], the adsorption of MC-LR has been demonstrated to increase with a decrease in pH (from 8 to 2.5). Under acidic conditions, the weak acidic functional groups of MCs are presumed to become available for adsorption [28], and this probably explains the significantly higher adsorption of the Diaion ® HP-20 at site H2, which had waters with slightly neutral pH values. With regards to the chitosan-based substrates, the amino groups in the chitosan polymer are known to protonate as -NH 3+ in neutral conditions, and the MC-LR has two carboxylate (-COO − ) groups under such conditions for electrostatic interactions to take place.…”

“…However it is worth noting the unavailability of studies that look into the effects of pH on the adsorption of other congeners of MCs by chitosan and by other substrates. Notes: * Department of Water Affairs and Forestry [28]; ** Food and Agriculture Organization (FAO) guidelines of 1985 [29]. n.a.= no applicable standards.…”