Abstract
	In this study a combination of reduction and cement-based solidification/stabilization (S/S) was 
used in treating hexavalent chromium contaminated soil. Preliminary studies showed that Cr(VI) was 
difficult to immobilize by cement only and leachate exceeded the TCLP limit. Pretreating the soil with 
ferrous chloride (FeCl2) substantially reduced the leaching of Cr(VI) to an efficiency of 95%.  TCLP results 
showed that the leaching of Cr(VI) and total chromium (Total Cr) was reduced with Fe(II) treated soils. 
Chromium oxide (Cr2O3) was identified as a reaction product by X-ray diffractometry (XRD) analyses. 
The Fe(II) pretreatment enhanced the S/S treatment efficiency of Cr(VI). 

1. Introduction
	The widespread use of  chromium has resulted in the contamination of soils and waters. Cr(VI) 
contamination is of great concern  because of its acutely toxic, mutagenic, carcinogenic and leaching 
potential. Solidification/stabilization (S/S) is one of the remediation technologies that have been developed 
and applied for treating inorganic hazardous waste. This study was to evaluate the potential of using 
cement-based S/S for treating K2CrO4 contaminated clayey soil. 
 
2. Testing program
	Contaminated Soil. The contaminated soil was prepared by adding potassium chromate 
(K2CrO4) into the soil to a concentration of 25,000 mg/kg. The control clayey soil was prepared by mixing 
sand (79%), kaolinite clay (20%) and organic matter (1%). 
	Reducing Agent. The reduction of Cr(VI) was achieved by mixing ferrous chloride (FeCl2) up to 
10% (by weight of soil) to Cr(VI) contaminated soil.  
	Stabilization/Solidification (S/S). Portland cement (Type I) was used as the binder for S/S 
treatment. The cement-to-soil ratio and water-to-cement ratio were 0.2 and 0.5 (by weight), respectively. 
Unconfined compressive strength (UCS) and Toxicity Characteristic Leaching Procedure (TCLP) test  were 
conducted on solidified binders after 28 days of curing.  
	Chromium Measurement. The measurement of Cr(VI) and total chromium (Total Cr) were 
followed AWWA Standard Method 312B. 
	XRD Study. A Siemen's D5000 X-ray diffractometer which had a copper tube operated at 50 kV 
and 40 mA was used for the XRD characterization for reflection angle (2 ) in the range 10 to 50o. 

3. Results
	Leaching of Chromium. The leaching of Cr(VI) and Total Cr from untreated contaminated soil 
was 1,000 and 1,020 mg/L, respectively.  When the soil (S) was treated with cement at a C/S ratio of 0.2, 
the maximum amount of Cr(VI) and Total Cr leached out were 675 and 682 mg/L, respectively. All failed 
to meet TCLP limit of 5 mg/L.  
	Reduction of Cr(VI). Addition of FeCl2 reduced the leaching of Cr(VI) according to the 
following reaction: 
		 		(1) 
The reduction of Cr(VI) by Fe(II) salt resulted in reducing the soil's pH from 6.1 to 5.9, 2.0 and 1.6 for 1%, 
4% and 10% Fe(II) addition.	   
	S/S Treatment. The technique of coupling Cr(VI) reduction by FeCl2 and S/S treatment  reduced the 
leaching of Cr(VI) and Total Cr more effectively. The efficiencies in reducing leaching of Cr(VI) and Total 
Cr from Cem/1%FeCl2+CrSoil were 55 and 40%, respectively. The efficiencies increased to 96 and 45% 
for Cem/4%FeCl2+CrSoil system. They were further improved to 99% and 50% for 
Cem/10%FeCl2+CrSoil system. The transformation of Cr2O3 to Cr(OH)3 in cement hydration condition 
(pH>12) explained the higher efficiency of this technique. All samples had UCS greater than the EPA 
requirement of 0.34 MPa (50 psi). 
	XRD Studies. Potassium chromate was identified at 2  of 29.8o, 30.1o, 34.5o  and 35o. When the 
soil was treated with Fe(II), reaction product Cr2O3 was identified. A formation product Cr(OH)3 was 
identified when the sample was further treated with cement. 

4. Conclusions
1) 	Cement-based S/S did not fix Cr(VI) contaminated soil to below TCLP limit 5 mg/L. 	
2) 	Adding FeCl2 reduced Cr(VI) to Cr(III) and caused soil's pH reduction.
3) 	A process coupling Cr(VI) reduction and cement-based S/S treat Cr(VI) contaminated soil efficiently. 

5. References
1. Katz, S. A., Salem, H., The Biological and Environmental Chemistry of Chromium, VCH Press: New 
York, 1994; pp 1 - 10.
2. U. S. EPA, Natural Attenuation of Hexavalent Chromium in Groundwater and Soils, Washington, D.C., 
1994.
3. Vipulanandan, C., Wang, S., Proceedings of In-Situ Remediation of the Geoenvironment, 1997, pp 363 - 
373.