Surface modification of nanoscale zero-valent iron (nZVI) using polymer stabilizers (e.g., sodium carboxymethyl cellulose, CMC) is usually used to minimize aggregation, increase stability, and enhance transport of nZVI. We investigated the stability and dynamic aggregation of bare and CMC–nZVI as affected by variations in pH, ionic strength (IS), and nZVI particle concentration. CMC coating of nZVI resulted in smaller hydrodynamic size and larger zeta potential. The largest hydrodynamic size of nZVI was associated with bare nZVI at high IS (100 mM), pH close to the point of zero charge (PZC, 7.3–7.6), and larger particle concentration (1.0 g L⁻¹). The increase in the zeta potential of CMC–nZVI reached one- to four-fold of that for bare nZVI, and was greater at pH values close to PZC, high IS, and larger particle concentration. The stability of CMC–nZVI was increased by 61.8, 93.1, and 57.5% as compared to that of bare nZVI at IS of 1, 50 and 100 mM, respectively. Calculations of Derjaguin, Landau, Verwey and Overbeek (DLVO) interaction energy were in agreement with stability results, and showed the formation of substantial energy barriers at low IS indicating greater nZVI stability. Our results suggest that at IS above 50 mM and nZVI particle concentration larger than 0.1 g L⁻¹, the likelihood of nZVI aggregation is high. Nevertheless, CMC polymer stabilizer would enhance the stability and transport of nZVI even under these unfavorable solution chemistry conditions.

The liquid and bio-solids (landfill sediments) product from landfills has many toxic substances, which may adversely affect on the environmental health. Thus in order to have a better management of characteristics of Riyadh landfill leachates, 80 representative landfill leachate samples were collected and analyzed from February to May 2008. The analysis of landfill leachates includes pH, ECw, concentrations of soluble ions, chemical oxygen demand (COD), total suspended solids (TSS), and heavy metals contents. The obtained results showed that the landfill leachates are characterized by high concentrations of COD (13,900–22,350)mgL−1, TSS of (2280–8912)mgL−1, ECw values of (42.5–58.3)dSm−1 but the pH values ranged 5.49–6.32. Moreover, the leachates had higher concentrations of soluble Na+, K+, Ca2+, Mg2+, Cl−, HCO3- and SO42-. In addition, the leachates contained Fe, Mn, As, Ni, Cr, Zn and Cu in concentrations equivalent to: 104, 11.0, 1.4, 0.6, 0.25, 0.2 and 0.14mgL−1, respectively. While in the bio-solids (landfill sediments) the average content of Cd, Cr, Cu, Fe, Mn, Mo, Ni and V reached to 10, 242, 234, 48,608, 190, 18, 166 and 24mgkg−1. The relatively higher variations between the upper and lower limits (maximum and minimum values) of the studying parameters lead to conclude that it is essential to conduct a long-term monitoring program to obtain representative information and this fluctuation should be considered when operating the leachate treatment plant