Transport of carboxymethyl cellulose-coated zerovalent iron nanoparticles in a sand tank: Effects of sand grain size, nanoparticle concentration and injection velocity

Abstract

The transport of nanoscale zerovalent iron (NZVI) particles colloidally stabilized with 70,000 Da carboxymethyl cellulose (CMC), through sands with mean grain diameters of 180, 340 and 1140 μm (referred to as fine, intermediate and coarse-sized sand, respectively) was investigated in a 70-cm long, two-dimensional tank. The effect of NZVI concentrations (1 and 3 g-Fe L−1) and CMC concentrations (1 and 2 g L−1) and injection velocities (0.96 and 0.40 cm min−1) on particle transport were also evaluated with the intermediate sand. The overall NZVI mass fractions eluted from the tank were 36%, 25% and 16% in the coarse, intermediate and fine sands, respectively, when injected with 1 g L−1 NZVI stabilized in 1 g L−1 CMC. However, the mass fraction eluted reduced to 2.33% when the injection velocity was reduced from 0.96 to 0.40 cm min−1 in the intermediate-sized sand. Maximum transport efficiency (38% NZVI mass eluted) in the intermediate-sized sand was achieved with 3 g L−1 NZVI suspended in 2 g L−1 CMC at an injection velocity of 0.96 cm min−1. The transport efficiency was substantially decreased (11% NZVI mass eluted) when 3 g L−1 NZVI was stabilized with only 1 g L−1 CMC. The NZVI mean particle diameters in the porewaters remained unchanged at different locations in the tank suggesting that straining or gravity settling did not influence NZVI deposition. After NZVI injection, the hydraulic conductivity in the tank reduced by 80%–96%, depending on the CMC concentration and injection velocity.