Effect of Na-Hexametaphosphate on the Hydraulic Conductivity of Kaolinite-Sand Mixtures

Abstract

Abstract--Mixtures of kaolinite clay and quartz sand were packed in columns and leached with NaCICaC12 solutions of different concentrations (ranging from 0.5 M to distilled water) having sodium adsorption ratios (SAR) of 5, 10 and 20. Sodium hexametaphosphate (NaHMP) (10 g/m s) was added to each solution. Changes in the hydraulic conductivity (HC) and clay concentration in the effluent were measured, and the dispersion of the clay was evaluated. In the absence of NaHMP, using distilled water instead of 0.01 M solutions caused some reduction in the HC; the reduction became more severe as the SAR increased. At SAR 20 the HC dropped to 73% of that obtained when the 0.5 M salt solution was used. This reduction in HC was related to increases in the pH of the effluent and partial dispersion of the clay. On adding the NaHMP to the leaching solutions, a marked decrease in the HC and severe clay dispersion were observed at all SAR levels, if the kaolinite-sand mixtures were leached with dilute solutions and distilled water. The HC dropped to 40% of that obtained when leaching with 0.5 M salt solution, and dispersed day in the effluent peaked at about l~ in the SAR 20 treatment following leaching with distilled water. The observed reduction in the HC of the kaolinite-sand mixture when NaHMP was added to the leaching solutions was due to the dispersive effect of NaHMP and sodium on the kaolinite. The polyanion NaHMP reduced the edge-to-face particle interactions, thereby enabling the diffuse doublelayer repulsion forces to predominate. Following addition of the polyphosphate, the response of kaolinite to sodium and electrolyte concentration was similar to that reported in the literature for smectite.

Evaluation of the particle sizes of four clay minerals

“…Moreover, ultrasound dispersion generates more <1 µm particles, while chemical dispersion yields more >1 µm particles. The chemical dispersant, NaHMP, may alter the particle sizes in two ways: (1) it can partly reverse the clay mineral's positive edge charges by (PO 3 -) 6 and hence reduce the tendency of aggregation via edge-face interactions (Levy et al, 1991) and (2) the added Na + can exchange with the interlayer Ca 2+ , resulting in a change of Ca 2+ -Mt to Na + -Mt that can swell further and then exfoliate into smaller particles (as discussed previously). Therefore, adding the dispersant, NaHMP, to Ca 2+ -Mt may generate misleading PSD results because of the partial conversion of Ca 2+ -Mt to Na + -Mt.…”

Clay Dispersion and Hydraulic Conductivity of Clay-Sand Mixtures as Affected by the Addition of Various Anions

“…Frenkel et al (1992) showed that addition of small amounts of the anions to suspensions containing reference clays increased the critical flocculation concentration (CFC) of Na-montmoriUonite five to six times, and that of Na-kaolinite by an order of mag-515 nitude. Levy et al (1991) added a very small amount (10 g m -3) of hexametaphosphate to a kaolinite-sand mixture and noted that it caused a sharp decrease in the kaolinite's HC.…”