Mechanistic insights into water hardness effects on AFFF foam behavior: the dominant role of Ca2+ and Mg2+

Abstract

To investigate the effects of water hardness on aqueous film-forming foam (AFFF) extinguishing agents, this study prepared AFFFs using water with varying concentrations and mixing ratios of Ca²⁺ and Mg²⁺. The effects of different hardness levels and ion compositions on conductivity, dynamic surface tension, drainage time, and initial foam height are quantitatively analyzed. The results indicate that, due to interactions between the ions and surfactants, solution conductivity initially increases and then decreases with rising ion concentration. Dynamic surface tension analysis reveals that increasing water hardness slows the adsorption of surfactants at the gas–liquid interface. Notably, when the ion concentration exceeds 60 mM, the characteristic adsorption time increases significantly, with Ca²⁺ exerting a stronger inhibitory effect than Mg²⁺. Moderate water hardness enhances foaming by shielding electrostatic repulsion. Dynamic surface tension analysis indicates that increasing water hardness slows the adsorption of surfactants at the gas–liquid interface. When the total ion concentration exceeds 60 mM, the characteristic adsorption time rises significantly, with Ca²⁺ exhibiting a stronger inhibitory effect than Mg²⁺. Moderate water hardness promotes foaming by shielding electrostatic repulsion, while extreme hardness leads to excessive ion aggregation and the formation of insoluble precipitates, which compromise foam stability and shorten drainage time. In mixed hardness systems, increasing the proportion of Mg²⁺ at the same overall hardness lessens the negative impact on AFFF performance, resulting in prolonged foam drainage time and delayed foam coarsening. This study provides valuable data support for optimizing AFFF formulations with respect to varying water hardness conditions.