Size Distribution and Phosphate Removal Capacity of Nano Zero-Valent Iron (nZVI): Influence of pH and Ionic Strength

Nano zero-valent iron (nZVI) has been considered as a promising material for groundwater remediation in the past few decades. The size distribution of nZVI is one of the main factors that influences its transport capability and remediation capacity. However, studies on the size distribution of nZVI under different environmental conditions are still limited. In this study, the influence of the pH (pH = 5, 7, 9) and ionic strength (IS = 0, 15, 30, 45 mM) on the size distribution of nZVI are investigated. The dynamic light scattering (DLS) method is used to study the variation of the size distribution of nZVI aggregate with time, and batch tests are performed to evaluate the efficiency of phosphate removal. Meanwhile, the phosphate removal capacity of nZVI with different size distribution was examined. Experimental results show that under low IS and high pH conditions, nZVI aggregate exhibited a stable, narrow and one-peak size distribution. By contrast, under high IS and low pH conditions, nZVI exhibited a wide and complicated size distribution with multiple peak values. This different pattern in size distribution was further explained by the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. The phosphate removal rate of nZVI under acidic and neutral conditions is higher than 98% but is only 68% under alkaline conditions. The phosphate removal capacity is insensitive to the variation of IS since the removal rate is higher than 97% for different IS conditions. Favorable environmental conditions for colloidal stability and removal capacity of nZVI can be different, which needs comprehensive consideration in the application