Characterization of wildfire-impacted dissolved organic matter in drinking water and its influence on cylindrospermopsin removal by powdered activated carbon

Abstract

Wildfires are increasingly altering the chemistry of surface waters used for drinking water supply in the western United States. When vegetation burns, thermally modified organic compounds are released and transported into streams and reservoirs, increasing dissolved organic carbon (DOC) concentrations and changing its composition. This wildfire-impacted dissolved organic matter (DOM) interacts with drinking water treatment processes, particularly adsorption onto powdered activated carbon (PAC). PAC is a key control for dissolved organic contaminants such as cyanobacterial toxins. However, the extent to which fire-impacted organic matter affects adsorption performance and treatment reliability remains poorly understood. This study examined how DOM impacted by wildfire ash influences the removal of the cyanobacterial toxin cylindrospermopsin (CYN) by two PACs with differing pore structures. DOM was leached from wildfire ashes and unburned materials into synthetic surface water and characterized using ultraviolet and fluorescence spectroscopy to understand aromaticity, molecular weight, and source indicators. Adsorption experiments measured CYN and DOC removal across a range of PAC doses and contact times. Equilibrium, non-equilibrium and kinetic adsorption models, including the Freundlich isotherm and the Ideal Adsorbed Solution Theory-Equivalent Background Compound (IAST-EBC) framework, were applied to quantify the competitive effects of DOM on CYN adsorption. Wildfire-impacted DOM exhibited higher aromaticity than unburned DOM, consistent with the enrichment of humic-like fluorescent compounds following combustion. These DOM properties corresponded to stronger competition with CYN for adsorption sites and lower overall CYN removal efficiency. The magnitude of DOM competition depended strongly on PAC pore structure: the mesoporous carbon showed greater resistance to DOM competition and higher CYN removal while the microporous carbon was more strongly inhibited by the presence of DOM. Modeling results confirmed that wildfire-impacted DOM increased the competition with both PACs, with the effect amplified for the microporous adsorbent. The findings demonstrate that wildfire-impacted organic matter can significantly reduce PAC performance for CYN removal and that adsorbent porosity plays a critical role in mitigating competitive effects. Integrating optical property DOM characterization with competitive adsorption modeling improves understanding of post-fire water-quality risks and supports proactive treatment strategies to ensure safe and reliable drinking water in fire-affected watersheds.