Quantifying and Elucidating the Role of Ionic Strength in the Inhibitory Effect of Alginic Acid on Struvite Crystallization

As a substitute for extracellular polymeric substance from waste activated sludge, alginic acid (AA) has been proven to exert a significant inhibitory effect on struvite crystallization for phosphorus recovery from the sludge. The inhibitory effect is highly dependent on ionic strength in the sludge supernatant, especially in terms of seawater and brine as the magnesium source. However, the role of ionic strength in the inhibitory effect has remained elusive. The purpose of the study was, therefore, to comprehensively quantify and elucidate the role of ionic strength in the effect of AA on struvite crystallization. With ionic strength increasing from 0.001 to 0.2 mol/L NaCl, the induction time with 100 mg/L AA significantly increases from (626 ± 29) to (3602 ± 37) s. Meanwhile, the struvite growth rate at 100 mg/L AA decreases from 2.3 × 10–5 to 4.8 × 10–6 mol/min. It was revealed that the increase in ionic strength strengthened the inhibitory effect. Moreover, trimodal particle size distribution was proposed to well identify the impact effect of AA and ionic strength on the nucleation, growth, and aggregation of struvite crystals. Furthermore, the ionic strength-dependent effect is mainly dominated by the AA adsorption on struvite crystals, likely associated with struvite growth rate, electronic repulsive force, and chemical bonds. Interestingly, the chemical bonds were revealed to mainly involve hydrogen bonding as well as the surface complex between struvite and AA. The findings herein shed light on the role of ionic strength in the inhibitory effect of AA on struvite crystallization and the underlying mechanism thereof, which makes it applicable to the cases related to the synergetic effect of organic matter and ionic strength on mineral crystallization.