1Graduate School of Science and Engineering, Saitama University, JAPAN (anoozworld@yahoo.com) 2Department of Biotechnology, Chemistry and Environmental Engineering, Aalborg University, DENMARK

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

Anu Sharma1, Chamindu Deepagoda T.K.K.2, Ken Kawamoto1, Per Moldrup2, Toshiko Komatsu1

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 MediaToyoura sandNarita 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)

Saitama University

Porous media

Average diameter, d50 (mm)

Particle density ρs

(g/cm3)

Dry bulk density ρd (g/cm3)

Saturated hydraulic conductivity (Ks) cm/hr

Porosity, ϵ

Toyoura Sand

0.18 2.64 1.58 78.1 0.4

Narita Sand

0.64 2.6 1.56 27.76 0.4

Soil +

ARW

26.3cm, 26.9 cm

Shake 24 hrs,

25oC

Let it stand 20 hrs,

25oC

Filtration 1μm

Soil (125g of VAS) or (10g of RYS)

1L of ARW

Solution pH EC (mS/m)

Turbidity (NTU)

Concentration of CS (mg/L)

ζ potential at natural pH (mV)

ARW 6.5~ 6.8 2.1~2.3 0 -

VAS-WDC 6.5~7.0 2.5~5.8 7.2~10.1 4.8~7.1 -12~-14

RYS-WDC 7.5~8.0 5.5~8.0 126~131 88.7~99.2 -18~-20

ARW3PVs

Colloidal Solution10PVs

ARW7PVs

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 VolumesPore Volumes

(A) RYS-WDC (B) VAS-WDC

LC_HF LC_HF

CS_Natural pH CS_Low pH

Fraction collector

Computer

Data logger

Pressure transducer Nylon

Mesh 105 µm

ARW +CS, Br-

L=10cmdia = 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)

6 0.42 0.30 083 0.44

10 0.40 0.30 0.59 0.42

14 0.39 0.30 0.59 0.30

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 CaCl2

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-SSSA2009 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)

Time (hrs)

RY

S-W

DC

Stability of WDC at different pHζ Potential as function of pH at 0, 48 hours

pH

Time (hrs)

VA

S-W

DC

pH

Particle size distribution at 0, 48 hours

Particle size (µm)

Particle size (µm)

Pore VolumesHC_HF LC_HF

Pore Volumes

HF_HC LF_HCBromide

(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

RYS-WDC VAS- Br-VAS-WDC

VAS-WDC & Toyoura sand

RYS-WDC & Narita sand

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 Conditions Mass balance results

Porous media WDC Condition Flow

RateDarcy Flux (cm/min)

Concentration (mg/L)

Average Flow rate (cm3/min)

Turbidity (NTU) pH Residence

time (min)

Eluted fraction (Me)

Deposited fraction (Ms)

Total fraction (Mt)

LF_HC 0.06 245.63 1.05 396.00 6.2-6.9 70 0.86 0.15 1.01

LF_LC 0.06 33.72 1.16 44.68 5.7-6.6 70 0.51 0.40 0.91

HF_HC 0.53 151.28 9.98 244.00 6.4-6.9 7 0.94 0.28 1.22

HF_LC 0.55 20.00 10.42 32.25 6.2-6.7 7 0.88 0.76 1.64

HF_HC_LpH 0.57 238.58 10.69 384.80 4.6-5.8 7 0.62 0.39 1.01

HF_LC_LpH 0.56 25.78 10.60 41.58 4.9-5.9 7 0.68 0.48 1.16

HF_LC_LpH 0.56 22.24 10.52 4.6 4.6-5.3 7 NM NM NM

HF_HC_LpH 0.56 550 10.54 43.78 4.7-5.8 7 NM NM NM

HF_HC 0.57 567.00 10.79 44.99 7 NM NM NM

HF_LC 0.57 167.00 10.79 15.99 7 NM NM NM

LF_HC 0.16 575.00 3.03 45.57 30 NM NM NM

LF_LC 0.12 159.74 2.27 15.47 30 NM NM NM

NaritaSand

Toyoura Sand

RYS

VAS

Low

High

Low

Natural

Low

Natural