Transport and retention behaviors of titanium dioxide nanoparticles in iron oxide-coated quartz sand: Effects of pH, ionic strength, and humic acid

The transport and retention behaviors of titanium dioxide nanoparticles (nTiO2) in iron oxide-coated sand were examined in water-saturated sand columns as functions of pH, ionic strength, and humic acid. Experimental breakthrough curves and retention profiles indicated that the transport and retention of nTiO2 in the absence of Suwannee River humic acid (SRHA) were dominated by strong attractive electrostatic forces at both pH 5 and 7 due to patchy coating of iron oxide on the surfaces of bare quartz sand. Owing to the existence of attractive electrostatic force and rougher collector surfaces, pH and ionic strength have minimal influence on the transport and retention of nTiO2 in iron oxide-coated sand in the absence of SRHA. Although the presence of 1 mg L−1 SRHA in solutions insignificantly increased nTiO2 transport in iron oxide-coated sand, non-monotonic retained profiles were found with 1 mg L−1 SRHA in contrast to log-linear retained profiles without SRHA. Unlike the negligible effect of 1 mg L−1 SRHA on the breakthrough curves of nTiO2, increasing the SRHA concentration to 10 mg L−1 dramatically enhanced the breakthrough curves of nTiO2 in iron oxide-coated quartz sand. Moreover, the maximum retained concentrations of nTiO2 particles with 10 mg L−1 SRHA shifted to near to the column inlets. The site competition between the suspended SRHA that did not adsorb onto nTiO2 surfaces and the nTiO2 particles was found to be the main mechanism governing the transport and retention of nTiO2 in iron oxide-coated sand when SRHA was present in suspensions.