158 / 2025-04-11 14:40:42

The impact of aggregation between clay and phytoplanktonic cyanobacteria on trace elemental cycling in coastal environments

clay minerals, cyanobacteria, adsorption

Theme 5: Life-Mineral Interaction and Evolutions > Session 5d: Microbe-mineral Interaction and Its Environmental Impact

Clay minerals and cyanobacteria occur ubiquitously in nature and influence trace metal cycling due to their high specific surface areas and reactivity. Although the surface reactivity of each is well-studied, the reactivity of clay-cyanobacteria aggregates has received less attention. Here, we performed potentiometric acid-base titrations and Cd adsorption experiments on various concentrations of clay-cyanobacterial aggregates using three clay minerals (kaolinite, illite, and montmorillonite) and marine phytoplanktonic cyanobacteria, Synechococcus sp. PCC 7002. A component additivity surface complexation modeling (CA-SCM) approach was applied to study the impacts of the aggregation processes on the total surface reactivity. Results demonstrate that the CA-SCM approach effectively predicted acid-base titration behavior and that the modeled Cd adsorption generally matches experimental results at low adsorbent concentrations (up to 2.5 g/L clay for proton reactivity and 0.1 g/L for Cd adsorption), indicating additive reactivity of the two components. However, at higher adsorbent concentrations (25 g/L clay for proton reactivity and 10 g/L for Cd adsorption), the CA-SCM approach overestimates the proton and Cd adsorption behaviors for aggregates, indicating significant surface site blockage during aggregation. Based on the combined results, we propose that the interaction between clays and cyanobacteria is relatively weak at low adsorbent concentrations (e.g., <10 g/L, the total concentration of clay and cyanobacteria), but that significant obstruction of surface adsorption sites occurs at high adsorbent concentrations (e.g., >10 g/L), possibly due to the encrustation of bacteria by clay minerals. The implication of this work is that where turbid rivers discharge in estuaries or coastal environments with high primary productivity, aggregation will cause the release of some previously adsorbed trace elements to seawater. Therefore, depending on whether the trace elements are nutrients (e.g., phosphorous) or toxic (e.g., cadmium) this process has the potential to impact the marine biosphere.