The Effect of Particle Concentration and Sand Grain Size on Transport Breakthrough Curves and Retention Profiles of CMC-nZVI Particles in Saturated Media

nZVI particles are strong reducing agents, capable of degradation and detoxification of a wide range of organic and inorganic pollutants in contaminated aquifers. Understanding the transport and retention of these nanoparticles in subsurface environments is required for treatment systems and in situ groundwater remediation. During the last decade, several studies have been conducted to investigate the effect of different physicochemical conditions on the transport and retention of nZVI in saturated porous media. This study aimed to evaluate the effect of sand grain size and nanoparticle concentration on fundamental processes governing CMC-nZVI nanoparticle transport and retention in saturated porous media.

nZVI (NANOFER STAR, NANOIRON, s.r.o. Czech Republic was employed in this study. To prepare CMC-nZVI, nanoparticle, suspension and polymer solution was added by the relative dose of CMC to nZVI mass 1:2, in a 250-ml flask reactor. pH was fixed at 9.5 by NaOH and the solution was mixed for 144 h under ambient temperature condition at the absence of oxygen. Quartz sands with ∼ 99.38% SiO2 and 0.27 Fe2O3 based on XRF analysis, was used as the porous medium. The experiments were conducted using a cylindrical Plexiglas column 30 cm in length and 2.5 cm in inner diameter. In order to capture the effect of particle concentration and grain size, 12 tests were conducted with four different concentrations (C = 10, 200, 3000, 10000 mg/l) and three sizes of grain (dc = 0.297–0.5 mm, 0.5–1 mm, 1–2 mm). In each test, ∼4 PVs of nZVI suspension were introduced into the columns and to complete the test, ∼6 PVs of deionized water were flushed. The column effluent was collected every 2 min and analyzed for total Fe using UV-Vis. The normalized effluent iron concentration (C/C0) for each transport test was plotted as a function of pore volumes. The spatial distributions of retained CMC-nZVI in the sand columns were determined to right after the breakthrough experiment. The quartz sand in each column was carefully excavated in ~3 cm increments, transferred into 50 mL vials and analyzed for total Fe. The concentration of retained CMC-nZVI in all the sand columns was also plotted as a function of travel distance.

The breakthrough curves indicate that both grain size and nanoparticle concentration had a relevant impact on CMC-nZVI mobility, even if the influence of nanoparticle concentration was more evident. In all experimental conditions, the BTCs were not symmetrical, which indicates that attachment and detachment phenomena occurred in different modes. The breakthrough curves can be interpreted in two steps: injection and flushing times. The maximum relative concentration (C/C0) decreased, during injection time, for three different grain sizes while influent concentration increased from 10 to 10,000 mg/L, which can be attributed to the increase in particle-particle interaction (aggregation) and particle-sand interaction (attachment). The breakthrough curves, after the initial increase, showed a strong decline, which is a clear indication of the ripening phenomenon. This phenomenon affected the porous medium properties such as porosity and hydraulic conductivity. Moreover, At higher influent CMC-nZVI concentration, Na+ ion and subsequently ionic strength increases because of higher doses of Na-CMC. As a result, aggregation and deposition will occur under a shallower secondary energy minimum well, that they are reversible. At the same nZVI concentrations, the breakthrough curve decreased by a decrease in grain size. Decreases in grain size can lead to an increase in surface area, decrease in pore throat size; and consequently, retention of nanoparticles by straining phenomena. However, another behavior was governed during the flushing time. During the flushing time, a narrow sharp increase in C/C0 was observed called flushing peak. In this study, CMC-nZVI aggregates deposited onto surfaces of sands due to secondary energy minimum were eventually released during the flushing period of the column with DI water. The results suggest that the grain size and particle concentration can have a positive effect on this peak. The results of retention profiles demonstrate that the CMC-nZVI retention in low concentration (10 mg/L) is consistent with filtration theory; whiles the highly concentrated polymer-modified nZVI dispersions (especially 3000 and 10000 mg/l) contradicts filtration theory. Based on filtration theory (Elimelech et al. 1995; Tufenkji et al. 2004), if all factors affecting the transport of colloids are kept constant, grain size increase can lead to a decrease in surface area and attachment efficiency (α). The contradiction at high concentration can be explained by considering the effect of hydrodynamic forces (especially fluid shear) on agglomeration and disagglomeration and deposition and detachment. The size of stable aggregates formed in the pores of finer sands is smaller than when they are formed in the pores of larger sands because the magnitude of local shear is higher for narrower pores. This led to decreased retention in finer sand.

The results of this research show that during the injection time, ripening and straining phenomena are key retention mechanisms of nanoparticles by decreasing the sand size and increasing particle concentration. While during the flushing time, secondary energy minima and hydrodynamic forces play critical roles in the deposition and transport mechanisms of CMC-nZVI. At high particle concentration (3000 and 10000 mg/l), these factors can lead to an increase in nanoparticle mobility by decreasing sand size. However, the results of retention profiles were consistent with colloid filtration theory at low particle concentration (10 and 200 mg/l).