The National Institute of Advanced Industrial Science and Technology (AIST), jointly with Mitsui Engineering & Shipbuilding Co., Ltd. (MES), has successfully developed a remediation technology to efficiently extract toxic organoarsenics from contaminated soils. This technology is expected to contribute to the solution of an environmental pollution problem which has recently arisen.

AIST, in collaboration with MES, has developed a new soil remediation process to extract almost 100% of organoarsenics from contaminated soils using a washing solution composed of alcohol and a small amount of phosphoric acid. With this technology, (1) contaminated soils can be cleaned up, (2) spent washing solution can be regenerated and recycled, (3) organoarsenics can be recovered in a small volume, which leads to a reduction of remediation cost.

As next steps, suppression of the leaching of the organoarsenics remaining in the soil after the treatment, and process systematization suited for the contamination site are necessary.

Schematic diagram of soil washing system

A new type of environmental pollution problem occurred in March 2003 in Kamisu City, Ibaraki Prefecture, where significant amounts of organoarsenic compounds such as diphenylarsinic acid were detected in groundwater and soils. Such a pollution case had never previously been reported, and thus the development of an effective remediation technology was required. These organoarsenics are (1) highly toxic, (2) subject to leaching from soils, (3) subject to sorption by soils, (4) not biodegradable and (5) have a risk of producing more toxic inorganic arsenics by thermal and chemical decomposition. Conventional measures such as solidification/stabilization, scatter control, and containment seem insufficient for a thorough solution of the problem because the bulk of the pollutant still remains in the soil after remedial treatment. Chemical washing, namely extraction, seems the most promising way by which the bulk of the pollutant can be removed from soils.

AIST has conducted several research projects to develop remediation processes for soils contaminated by arsenic and heavy metals. Based on these research results, AIST carried out the following joint research projects with MES:

1. Development of remediation process for soils contaminated by organoarsenics in FY 2003 under the fund-offering type collaborative research program, and

2. Remediation of soils contaminated by organoarsenics using chemical washing agents in FY 2004 under the patent-application type collaborative research program.

To better understand the behavior of organoarsenics in the soil environment, the adsorption mechanism of organoarsenics onto soils has been elucidated using model soils contaminated by diphenylarsinic acid or phenylarsonic acid, as well as genuine contaminated soils. In addition, various inorganic and organic agents were evaluated in terms of efficiency of organoarsenic extraction from contaminated soils. The results were that alcohols, such as methanol, containing 3 to 5% phosphoric acid exhibited outstanding efficiency, with almost 100% of organoarsenics being removed from a soil containing as much as 3570 mg/kg of arsenic. The mechanism of the effect of this washing solution consists of the function of phosphoric acid in breaking the bonding between the arsenic atoms and the soil constituents and the alcohol’s function in solubilizing organoarsenics into a liquid phase. The spent washing solution can be regenerated to recover alcohol which can be recycled and the extracted organoarsenic can be recovered in a small volume, which leads to reduction of hazardous waste.

Mechanism of extraction of organoarsenics from soil by phosphoric-acid containing alcohol