1. 1 Graduate School of Science and Engineering, Saitama University, JAPAN (anoozworld@yahoo.com) 2 Department of Biotechnology, Chemistry and Environmental Engineering, Aalborg University, DENMARK Mobilization and Deposition of Variably Charged Soil Colloids in Saturated Porous Media Anu Sharma 1, Chamindu Deepagoda T.K.K. 2, Ken Kawamoto 1, Per Moldrup 2, Toshiko Komatsu 1 ABSTRACT 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.  Porous Media  Toyoura sand  Narita sand MATERIALS  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 Properties of porous media Selected properties for eluent and colloidal solution Effect of pH  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). Comparison of porous media COLUMN EXPERIMENTS RESULTS AND DISCUSSIONS  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 Experimental conditions and Mass balance results  Solution Application SUMMARY OF RESULTS  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)  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. Saitama University Porous media Average diameter, d 50 (mm) Particle density ρ s (g/cm 3 ) Dry bulk density ρ d (g/cm 3 ) Saturated hydraulic conductivity (K s ) cm/hr Porosity, Toyoura Sand 0.182.641.5878.10.4 Narita Sand 0.642.61.5627.760.4 Soil + ARW 26.3cm, 26.9 cm Shake 24 hrs, 25 o C Let it stand 20 hrs, 25 o C Filtration 1μm Soil (125g of VAS) or (10g of RYS) 1L of ARW SolutionpHEC (mS/m) Turbidity (NTU) Concentrati on of CS (mg/L) ζ potential at natural pH (mV) ARW 6.5~ 6.82.1~2.30- VAS-WDC 6.5~7.02.5~5.87.2~10.14.8~7.1-12~-14 RYS-WDC 7.5~8.05.5~8.0126~13188.7~99.2-18~-20 ARW 3PVs Colloidal Solution 10PVs ARW 7PVs  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. Red Yellow Soil WDC Fig. 1 Breakthrough and breakdown curves showing the effect of concentration and flow rate for RYS-WDC. 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. Pore Volumes Relative Concentration (C/C 0 ) Pore Volumes Relative Concentration (C/C 0 ) (A) RYS-WDC(B) VAS-WDC LC_HF CS_Natural pH CS_Low pH Deposited colloid (mg, colloid/g, sand) Depth (cm) Fraction collector Computer Data logger Pressure transducer Nylon Mesh 105 µm ARW + CS, Br - L=10cm dia = 4.91cm ARW Extraction procedure for water dispersible colloids  Particle size of RYS-WDC tends to become bigger at low pH resulting in coagulation/aggregation. (Fig. 4 and right table). Pore volume µ (µm) HF_ HC Natural pH σ (µm) µ (µm) HF_HC Low pH σ (µm) 60.420.300830.44 100.400.300.590.42 140.390.300.590.30 Mean particle size of RYS-WDC at high flow rate, high concentration, both at natural pH and low pH condition.  Artificial Rain Water (ARW) 0.085mM NaCl + 0.015 mM CaCl 2 Nishi-Tokyo (Tokyo) Pasture / Agricultural land Volcanic ash soil (Tachikawa loam) Nakijinson (Okinawa) Hilly site Red-yellow soil (Kunigami mahji) Soil sampling sites  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. ASA-CSSA-SSSA 2009 International annual Meeting November 1-5, 2009, Pittsburgh, PA Fig. 3. Particle size distribution for RYS-WDC at high flow, high concentration and low pH condition. Particle size ( µm) Cumulative volume of particle (%) Time (hrs) ζ Potential (mV) Relative Concentration RYS-WDC Stability of WDC at different pH ζ Potential as function of pH at 0, 48 hours ζ Potential (mV) pH Relative Concentration Time (hrs) VAS-WDC pH Particle size distribution at 0, 48 hours Particle size ( µm) Relative volume of particle (%) Particle size ( µm) Relative volume of particle (%) Pore Volumes Relative Concentration (C/C 0 ) HC_HF LC_HF Pore Volumes Relative Concentration (C/C 0 ) HF_HC LF_HC Bromide (A) Effect of concentration (B) Effect of flow rate HF: High flow rate; LF: low flow rate; HC: High concentration; LC: Low concentration; LpH: Low pH; NM: Not measured Effect of pH  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). 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. RYS-Br- Pore Volumes Relative Concentration (C/C 0 ) RYS-WDC VAS- Br- VAS-WDC VAS-WDC & Toyoura sand RYS-WDC & Narita sand Δθ cm 3 /cm 3 Pore diameter, d cm) Toyoura Narita 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). Experimental ConditionsMass balance results Porous media WDCCondition Flow Rate Darcy Flux (cm/min) Concentr ation (mg/L) Average Flow rate (cm3/min ) Turbidity (NTU) pH Residence time (min) Eluted fraction (Me) Deposited fraction (Ms) Total fractio n (Mt) LF_HC0.06245.631.05396.006.2-6.9700.860.151.01 LF_LC0.0633.721.1644.685.7-6.6700.510.400.91 HF_HC0.53151.289.98244.006.4-6.970.940.281.22 HF_LC0.5520.0010.4232.256.2-6.770.880.761.64 HF_HC_ LpH 0.57238.5810.69384.804.6-5.870.620.391.01 HF_LC_ LpH 0.5625.7810.6041.584.9-5.970.680.481.16 HF_LC_ LpH 0.5622.2410.524.64.6-5.37NM HF_HC_ LpH 0.5655010.5443.784.7-5.87NM HF_HC0.57567.0010.7944.99 7NM HF_LC0.57167.0010.7915.99 7NM LF_HC0.16575.003.0345.57 30NM LF_LC0.12159.742.2715.47 30NM Narita Sand Toyoura Sand RYS VAS Low High Low Natural Low Natural