10th Asian Regional Conference of IAEG (2015)

Change in the arsenic leaching from clayey soil by adding slag cement
Yuka ENDO(1), Ryota FUJINAGA(1), Toshifumi IGARASHI(2), Takahiro YAMAMOTO(3)(1) Graduate School of Engineering, Hokkaido University, Japan
E-mail: endo@trans-er.eng.hokudai.ac.jp(2) Faculty of Engineering, Hokkaido University, Japan(3)Pacific Consultants Co., Ltd., Japan

Abstract
Cementitious materials are commonly used to reinforce the structural stability of clayey soils.
However, there are very few data about the changes in arsenic (As) leaching from these soilsdue to the pH increase after the addition of cementitious materials to reinforce foundations. In
this study, batch leaching tests were conducted using a clayey soil with different pH bychanging the amount of addition of slag cement or the concentration of sodium hydroxide(NaOH). The results showed that the leaching concentration of As increased with increasing
concentration of NaOH. However, when slag cement was added, the leaching concentration
only increased at pH less than 10. At pH higher than 10, more slag cement addition translated
to lower leaching concentration of As.
To understand the processes controlling the mobility of As in this system, we used ageochemical modeling software, PHREEQC, and the results suggest that As is adsorbed onto orcoprecipitated with Fe-, Al-, Mg-, and/or Ca-bearing precipitates. By ultrafiltration, we foundthat these precipitates exist as colloidal particles at pH less than 10. Due to their minute sizes,
however, these precipitates were rapidly dissolved at pH greater than 10 releasing As back intosolution. Two Ca/Mgbased materials were also evaluated for their As sequestration capabilities.
When added to the clayey soil-cement mixture, the concentration of As decreased indicatingthat these materials could be used to sequester dissolved As even under hyperalkaline pHconditions.
Keywords: arsenic, slag cement, leaching, pH, coexisting ions1.

Introduction

variable charges depending on the pH. These minerals
have points of zero charge (PZC) around pH 7, so they
exhibit positively charged surfaces at lower pH and
negatively charged at higher pH. This is one of thereasons why As concentration has been observed toincreases with increasing pH. Tabelin and Igarashi(2009) showed that leaching concentration of As fromhydrothermally altered rocks increased at alkaline pH.
Moreover, Yoshimura and Akai (2000) proposed thatAs oxyanion is desorbed from the surface of ironhydroxide colloids with increasing pH, because of
their variable charge.
Therefore, the effects of pH or addition of cement
on As leaching from clayey soils were examined bybatch leaching tests. In addition, the effectiveness of
artificial materials to sequester As and reduce its
leaching from these soils after cement addition wasevaluated.

Cementatious materials are commonly used toreinforce clayey soils in many construction sites. For
example, these materials are used in constructing
embankments to prevent land subsidence. Cementitious
materials are inherently alkaline, so when these areadded to the clayey soils, the pH increases. Thus,arsenic (As) leaching is enhanced when clayey soils
contain significant amount of this hazardous element.
Arsenic could exist in solution in both +3(arsenite) and +5 (arsenate) oxidation states depending
on redox conditions. The former is much more mobilethan the latter because it could exist as an uncharged
oxyanion at certain pH conditions. In contrast,
arsenate is readily adsorbed onto positively charged
surfaces because of it is usually present as anegatively charged arsenic oxyanion (HAsO42-,
H2AsO4-) in surface water and groundwater(Yoshimura and Akai, 2000). The surface ofFe-oxyhydroxides like goethite and hematite have2.

1Materials and methods

10th Asian Regional Conference of IAEG (2015)

2.1 Materials
Clayey soil was obtained from a boring core 14.5
to 16 m deep from a ground surface near Sapporo,Hokkaido, Japan. The soil is structurally weak andleaches As. The collected soil sample was air dried,
lightly crushed using mortar and pestle, and sieved by
a 2 mm aperture screen.
Blast furnace cement was selected as an additive toreinforce the clayey soil sample in this experimentbecause it could improve the structural stability oforganic-rich soils.
Two different types of artificial materials wereused to immobilize As in the leachate: Earth Tight(AT) (Nippon steel & Sumikin cement, Japan), andAdd Rock Magnum (AM) (Okamoto Kogyo, Japan).
The main component of both AT and AM are calcium
and magnesium, respectively.

Fig. 1 NaOH concentration or the amount of cement
added vs. pH.
mixing at 200 rpm for 6 h, pH, EC, ORP andtemperature of the suspensions were measured. They
were then centrifuged and filtered through 0.45 µm
membrane filters. After filtration, the filtrate wasfurther filtered through ultrafilters with cut-off
molecular weight of 200 kDa and 10 kDa.

2.2 Batch leaching tests
Batch leaching tests were conducted under ambientconditions using a solid-liquid ratio of 1:10 andcement to soil mixing ratios ranging from 0.015 to0.3 g cement/15 g soil. The pH, electrical conductivity(EC), oxidation-reduction potential (ORP) andtemperature were measured after mixing at 200 rpm
for 6 h. After centrifugation at 5,000 rpm for 40 to 70
minutes, the leachate was filtered through 0.45 µm
membrane filters. To compare the effects of competing
cations, batch tests were conducted under the same
conditions as above but with the addition of 1 to 20mM NaOH solutions.
Furthermore, ultrafiltration usingultrafilter
syringe units (Advantec, Japan) was used to evaluate
the effects of colloidal particles, which were notremoved by the 0.45 µm membrane filters. Fifteen
grams of clayey soil and 150 mL of deionized water
were mixed with or without 0.15 g of cement. After

2.3 Arsenic sequestration tests
Batch leaching tests to sequester As using AT and
AM were conducted to understand how As leaching
concentration was reduced. Fifteen grams of soil and
150 mL of deionized water were mixed with 0.05 and0.1 g of AM or AT. Furthermore, 15 g of soil and 150
mL deionized water were mixed with 0.15 g of cement
and 0.05 to 0.3 g of AM or AT. The leaching time wasfixed at 6 h and the mixing speed at 200 rpm. After thepH, EC, ORP, and temperature of the suspensions
were measured, the suspensions were centrifuged(5,000 rpm for 60 minutes), and filtered through 0.45
µm membrane filters.

Fig. 2 Leaching concentrations of (a) arsenic, (b) aluminum, (c) iron, (d) calcium and (e) silica vs. pH.

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10th Asian Regional Conference of IAEG (2015)

2.4 Chemical analysisArsenic and other coexisting constituents, such as
Na, Al, Fe, Mg, K, Ca, and Si were analyzed using an
inductively coupled plasma atomic emission
spectrometer (ICP-AES) (Shimadzu Corporation,Japan). Chloride (Cl-) and sulfate (SO42-) ions were
measured by an anion chromatograph. Total organiccarbon (TOC) was also measured by TOC-L(Shimadzu Corporation, Japan).

(a)

2.5 Calculation of saturation indices (SIs) ofminerals
SIs and the speciation of coexisting constituentswere calculated using a geochemical modelingsoftware, PHREEQC (Parkhurst and Appelo, 1999).
3.

(b)

Results and discussion

3.1 Batch tests
Figure 1 shows pH as a function of NaOH
concentration or amount of added cement in batchleaching tests. The pH simply increased with NaOH
concentration and the amount of added cement.
Figure 2 represents As and coexisting constituent
concentration as a function of pH. The red line in Fig. 2(a) represents the environmental standard of Japan for
As. In all cases except one plot, leaching concentrationof As exceeded the Japanese environmental standard.
When NaOH was used for pH adjustment, Asconcentration increased with pH. On the other hand,when cement was used, As concentration increased
with the addition of the cement at pH less than 10, andthen the leaching concentration decreased at pH higher
than 10. These results indicate that As leached fromFig. 4 (a) pH vs. the amount of added Ca/Mg-basedmaterials and (b) As concentration vs. pH.
the soil depends not only on the pH but also on othercoexisting constituents present in the leachate.
Aluminum and Fe concentrations decreased withcement addition. In particular, they decreaseddramatically at pH higher than 10 (Figs. 2 (b) and (c)).
Precipitations of Al- and Fe-bearing minerals wereformed at higher pH because Ca concentration
released from cement increased with pH (Fig. 2 (d)).
Silica concentration also decreased with Caconcentration at higher pH (Fig. 2 (e)).
Concentrations of As and coexisting constituentsusing different filters are presented in Fig. 3. In thebatch tests without cement, As concentration
decreased from 7 µg/L to 2 µg/L by ultrafiltration ataround pH 8.5. Similarly, concentrations of Al, Fe,Mg, and Si decreased to almost zero after 10 kDa
ultrafiltration (Fig. 3 (a)). In contrast,Asconcentration did not decrease at pH 10.3 in the batchleaching tests with cement. However, theconcentrations of Al, Fe, and Mg decreased to almostzero. These suggest that As is adsorbed onto colloidal
particles of Al, Fe, Mg, and/or Si without cement
while As is dissolved with cement.

(a)

(b)

3.2 Arsenic sequestration tests
Changes in pH of the leachate after the addition ofAT and AM are shown in Fig. 4(a). The pH increased
with increasing AT and AM addition and these changes
were similar to those without cement. However, thepH became higher when AT was used compared to AMwith cement. The pH of the leachate with cement wasFig. 3 Chemical properties of leachate (a)withoutcement, (b) with 0.15 g of cement.

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10th Asian Regional Conference of IAEG (2015)higher than that without cement regardless of whether
AT or AM was added.
Figure 4(b) shows the leaching concentration of Aswith and without AT or AM. Although As
concentrations in almost all experiments decreased
with increasing pH by adding these artificial materials,
As concentration increased in some experiments afteradding AT. This could be attributed to the lower pH
and lower concentrations of Ca and/or Mg. Thismeans that the effectiveness the artificial materials tosequester As depended on the pH.

to those of cement as shown in Fig. 5(a). The
differences were the changes in SIs of brucite andgibbsite. These calculated results indicate that As may
be adsorbed onto or coprecipitated with Al-, Mg-,
and/or Ca-bearing minerals.
4.

Conclusion
Arsenic concentration leached from a clayey soil
containing As was increased with pH. When cement
was used, the maximum leaching concentration of As
was observed at pH 10.3. However, As concentration
was lowered below 10 µg/L even under the high pH
condition by adding adequate amount of Ca andMg-containing artificial materials most probably
because of its adsorption and/or coprecipitation withAl-, Mg-, and/or Ca-bearing minerals.

3.3 Calculation of SIs of minerals
Figure 5 shows SI as a function of pH. The SIs ofgibbsite, boehmite, Al(OH)3(amorphous(am)), andFe(OH)3(am) decreased with increasing pH when
cement was added. Only the SI of Al(OH)3(am) waschanged from positive to negative at pH 9.7 whereas
SIs of the other Al- and Fe-bearing minerals werepositive irrespective of pH. It is well known that As isadsorbed onto Fe(OH)3 and Al(OH)3 (Masuda, 2000;Kaneko, 1981). These precipitates were dissolved withincreasing pH.
On the contrary, the SIs of calcite and brucite
increased with pH, and changed from negative topositive. These mean that the precipitates of Mg and/or
Ca are related to As leaching concentration.
The SIs in cases with NaOH solution were similar

References
Kaneko, E. (1981): Effect of silicic acid on thecoprecipitation of arsenic with iron (III) hydroxide,
The Japan Society for Analytical Chemistry, Vol. 30,
No. 2, pp.117-122.
Masuda, H. (2000) : Geochemical cycle of arsenic in
the Earth’s surface and mechanisms of its pollution ofgroundwater, Journal of Groundwater Hydrology, Vol.
42, No. 4, pp. 295-313.
Parkhurst, D.L. and Appelo, C.A.J. (1999): User’sguide to PHREEQC (Version 2) – A Computer
ProgramforSpeciation,
Batch-Reaction,
One-DimensionalTransport,andInverse
Geochemical Calculations. US Department ofInterior and US Geol. Surv. Water-Resour. Invest.
Rep. 99-4259.
Tabelin, C. B. and Igarashi,T. (2009) : Mechanisms ofarsenic and lead release from hydrothermally alteredrock, Journal of Hazardous Materials, Vol. 169, pp.
980-990.
Yoshimura, T. and Akai, J. (2000): Arsenic bahavior insoils and sediments, and groundwater pollution: Areview, Earth Science, Vol. 57, pp. 137-154.

Fig. 5 SIs vs. pH (a) with cement and (b)with NaOH.

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