Investigating the effect of hardness cations on coagulation: The aspect of neutralisation through Al(III)-dissolved organic matter (DOM) binding

Hardness cations are ubiquitous and abundant in source water, while the effect of hardness on the performance of coagulation for dissolved organic matter (DOM) removal in water treatment remains unclear due to the limitation of methods that can characterise the subtle interactions between DOM, coagulant and hardness cations. This work quantified the competition between coagulant Al³⁺ and hardness cations to bind onto DOM using absorbance spectroscopy acquired at different Al³⁺ concentrations in the absence and presence of Ca²⁺ or Mg²⁺. The results indicate that, in the presence of either Mg²⁺ or Ca²⁺, an increasing depression of the binding of Al³⁺-DOM could be observed in the differential spectra of DOM with the increasing of Mg²⁺ or Ca²⁺ at a level of 10, 100 and 1000 μM, with the observation being more significant at higher pH from 6.5 to 8.5. The results of zeta potentials of DOM indicate that the competition of hardness cations results in the negative DOM being less efficiently neutralised by Al³⁺. This study demonstrates that the removal of DOM by coagulation would significantly deteriorate with the presence of hardness cations, which would compete with coagulant Al³⁺ to neutralise the unsaturated sites in DOM.