Mobilization and Deposition of Variably Charged Soil Colloids in Saturated Porous Media

1. Deposited colloid (mg, colloid/g, sand) Relative Concentration (C/C0) Relative Concentration (C/C0) Relative Concentration (C/C0) Relative Concentration (C/C0) Saitama University Relative Concentration 1L of ARW 26.3cm, 26.9 cm Soil + ARW Shake 24 hrs, 25oC Let it stand 20 hrs, 25oC Filtration 1μm Soil (125g of VAS) or (10g of RYS) Relative Concentration (C/C0) Cumulative volume of particle (%) RYS-WDC & Narita sand ζ Potential (mV) Depth (cm) VAS-WDC & Toyoura sand Mobilization and Deposition of Variably Charged Soil Colloids in Saturated Porous Media Time (hrs) Nakijinson (Okinawa) (A) RYS-WDC (B) VAS-WDC Pore Volumes pH RYS-Br- ASA-CSSA-SSSA 2009 International annual Meeting November 1-5, 2009, Pittsburgh, PA Pore Volumes Pore Volumes Data logger Pore Volumes • Hilly site • Red-yellow soil (Kunigami mahji) LC_HF RYS-WDC VAS- Br- VAS-WDC Particle size (µm) HC_HF LC_HF Pore Volumes LC_HF HF_HC LF_HC Anu Sharma1, Chamindu Deepagoda T.K.K.2, Ken Kawamoto1, Per Moldrup2, Toshiko Komatsu1 L=10cm dia = 4.91cm CS_Low pH CS_Natural pH Nishi-Tokyo (Tokyo) Computer Bromide 1Graduate School of Science and Engineering, Saitama University, JAPAN (anoozworld@yahoo.com)2Department of Biotechnology, Chemistry and Environmental Engineering, Aalborg University, DENMARK • Pasture / Agricultural land • Volcanic ash soil (Tachikawa loam) ARW + CS, Br- Pressure transducer Nylon Mesh 105 µm Fraction collector ARW 3PVs Colloidal Solution 10PVs ARW 7PVs Effect of pH COLUMN EXPERIMENTS ABSTRACT • Solution Application Understanding colloid mobilization and retention in subsurface is important for predicting colloid-facilitated transport of contaminants and developing remedial strategies. The behavior and transport of colloids in varying physical and chemical conditions is yet to be fully understood. This study investigated transport behavior of water dispersible colloids (WDC) with different surface charges, extracted from volcanic ash soil (VAS) from Nishi-Tokyo, Japan and Red-yellow soil (RYS) from Okinawa, Japan. WDC solutions containing colloids with diameter <1µm were applied at water-saturated flow through 10cm-column packed with 0.1-0.5mm Toyoura sand or 0.42-0.85mm (Narita sand) size fraction under different colloid concentrations, flow rates and pH conditions. The colloidal solutions were characterized from the measurement of turbidity, zeta potential, and particle size distribution. 0.001M NaBr was used as a conservative tracer and the pH was adjusted using 0.1M HCl. Mechanisms of colloid transport and retention were studied by measuring colloid effluent concentration, deposition profile, and particle size distribution. The results showed solution concentration of WDC had minimum effects on transport and deposition for RYS-WDC, however low flow rate caused more reversible entrapment of WDC compared to high flow rate condition. The breakthrough and breakdown curves, deposition profile and particle size distribution measurements clearly indicated additional effects of low solution pH in stronger colloid retainment, especially for VAS-WDC. • Effect of pH is evident from breakthrough curves of WDCs at natural pH and low pH (Fig 2), deposition profile and particle size distribution (Fig 3). • Experimental Procedure • The collected effluents were analyzed for • Turbidity • pH • Electrical conductivity • Particle size distribution • Tracer (bromide) concentration • After column experiments • The column was dissected into 1cm sections • Deposited colloid concentration was measured Effect of pH ARW Fig. 2. Breakthrough and breakdown curves for (A) RYS-WDC and (B) VAS-WDC at low concentration and high flow rate with natural and low pH conditions . Experimental conditions and Mass balance results • VAS-WDC is a pH dependent surface charge dominant WDC. With decrease in pH, the VAS-WDC becomes less negatively charged and therefore deposited on the porous media resulting in higher deposition and lower colloid recovery (Fig 2). MATERIALS Properties of porous media Mean particle size of RYS-WDC at high flow rate, high concentration , both at natural pH and low pH condition. Relative volume of particle (%) • Porous Media • Toyoura sand • Narita sand pH Low Natural • Water Dispersible Colloids (WDC) • WDC extracted from two types of soils from Japan • Volcanic ash soil ( VAS-WDC) from Nishi-Tokyo • Red yellow soil (RYS-WDC) from Okinawa Fig. 3. Particle size distribution for RYS-WDC at high flow, high concentration and low pH condition. Particle size (µm) RYS Particle size distribution at 0, 48 hours Relative volume of particle (%) Narita Sand Relative Concentration ζ Potential (mV) High Low Time (hrs) • Particle size of RYS-WDC tends to become bigger at low pH resulting in coagulation/aggregation. (Fig. 4 and right table). Particle size (µm) • Artificial Rain Water (ARW) • 0.085mM NaCl + 0.015 mM CaCl2 VAS Toyoura Sand Natural Soil sampling sites Low Comparison of porous media Selected properties for eluent and colloidal solution • For the same amount of colloid applied (~160mg/L), Toyoura sand irreversibly retained more water dispersible colloids (40%) than Narita sand (10%) (Fig 4). The particle size of Toyoura sand is much smaller than that of Narita sand and therefore, the WDC likely to get deposited in the saturated sand. (Fig. 5) HF: High flow rate; LF: low flow rate; HC: High concentration; LC: Low concentration; LpH: Low pH; NM: Not measured RESULTS AND DISCUSSIONS (B) Effect of flow rate (A) Effect of concentration Δθ cm3/cm3 Extraction procedure for water dispersible colloids Red Yellow Soil WDC RYS-WDC Pore diameter, d cm) Narita Toyoura Fig. 4 Comparison of WDC (RYS and VAS) and porous media (Narita and Toyoura sand) Fig. 5 Pore size distribution of Toyoura and Narita sand. ζPotential as function of pH at 0, 48 hours SUMMARY OF RESULTS Stability of WDC at different pH VAS-WDC • Low flow rate caused more reversible entrapment of WDC than high flow rate for RYS-WDC. • Low solution pH resulted in stronger retainment of colloids, especially for VAS-WDC. • WDC concentration had minimum effect on transport and retention for RYS-WDC • Toyoura sand retained more colloids than Narita sand due to straining caused by small particle diameter. • Numerical analysis will be done to better predict the mechanism of transport and retention of WDC in saturated sand. Fig. 1 Breakthrough and breakdown curves showing the effect of concentration and flow rate for RYS-WDC. • WDC concentration had little effect, while the effect of flow rate was evident in RYS-WDC. • At high flow rate condition, the water dispersible colloids seemed to be deposited irreversibly indicating similar kinetics for both high concentration and low concentration of RYS-WDC (Fig. 1(A)) • Low flow rate (10 times higher residence time) and high concentration of RYS-WDC caused reversible attachment and release of colloids, apart from irreversible attachment (Fig. 1(B)). • Thus, the colloid breakthrough and breakdown curves showed that the overall kinetics of RYS-WDC is flow dependent than concentration dependent. • The colloid characterization results for VAS-WDC showed significant change in colloidal stability and zeta potential with change in pH indicating VAS-WDC as pH dependent surface charge dominant WDC, while a less significant change was observed in case of RYS-WDC suggesting it as permanent surface charge dominant WDC. The particle size distributions also indicate VAS-WDC as less stable WDC resulting in coagulation within short time than RYS-WDC. Acknowledgements: This research was partially supported by a grant from the Research Management Bureau, Saitama University and the grant-in-aid for Young Scientists (A) (No 18686039) from the Japanese Ministry of Education, Science, Sports, and Culture (Monbukagakusho) and grant from Japan Interaction in Science and Technology Foundation (JIST Foundation).