Factbox : Solutions to China's soil contamination crisis

BEIJING (Reuters) - China's campaign to clean up land contaminated by industrial and agricultural waste will not only require billions of dollars of investment, but also a range of different technologies. The pollutants are manifold, and include toxic heavy metals like lead or cadmium, volatile organic compounds (VOCs) such as benzene, and persistent organic pollutants (POPs) derived from pesticides and other chemicals. Waste left by fossil fuel combustion is also a serious problem. Experts say that while there are a number of solutions available, more cost-effective technologies need to be developed to handle the sheer scale of the problem. China also needs to improve detection and monitoring. PHYSICAL SOLUTIONS With developers often racing against the clock to complete expensive real estate projects, the most common method used at contaminated industrial sites is the removal and replacement of polluted soil. In some cases, the soil is treated elsewhere but often it is just sequestered in depots, raising the problem of storage space. Thermal desorption involves the use of heat to break apart contaminants, making them easier to remove from the soil, with some developers claiming it can remove 99.99 percent of pollutants. "Bioslurping" has also proved successful, and involves the creation of a vacuum that allows contaminated liquids to be extracted from soil through tubes. CHEMICAL SOLUTIONS The use of lime is the most frequently deployed chemical solution in China. Applying lime to soil allows heavy metal contaminants to oxidize, making it harder for plant roots to absorb them. But if the soil becomes acidic, the contaminants break down and become soluble again. Activated carbon is also used to bind contaminants and make them easier to remove. BIOLOGICAL SOLUTIONS Experimental projects involving micro-organisms and plants that can absorb toxins from the soil are under way, but the technologies are unlikely to meet China's need for cheap fixes that allow land to be redeveloped as quickly as possible. There is also no "one-size-fits-all" solution. A species of Indian mustard has been shown to be effective in absorbing selenium, and Chinese ferns can accumulate arsenic, but they have little impact on other pollutants. (Reporting by David Stanway; Editing by Dean Yates)