Page 207 - Soil Degradation, Conservation and Remediation

196 6 Soil Pollution

The Itai-Itai Episode in Japan
Cadmium concentration in food chain crops grown in non-contaminated soils
−1
is generally low (0.01–1.0 mg kg ). Leafy vegetables (lettuce, spinach)
have the highest; grains (wheat, oats, barley) show the lowest, and root vege-
tables (carrot, radish, onion, potato) have intermediate concentrations of cad-
mium in edible parts. Concentrations of Cd in crops grown on Cd-contaminated
−1
soils may accumulate substantially greater than 1.0 mg kg Cd. Consumption
of cadmium-contaminated food grains may cause severe Cd poisoning.
The well-known episode of Cd poisoning of humans, the itai-itai disease,
was reported in Japan in the mid-1950s (Tsuchiya 1978 ). The name of the dis-
ease came from the screams due to the severe pains in the bones caused by cad-
mium poisoning. It first impairs kidney function and progressively causes

osteomalacia. The outbreak of itai-itai started around 1912 and continued until
1945. The disease was thought to be nutritional and pathological at fi rst. After
many years of investigation, the Ministry of Health and Welfare offi cially
announced in 1968 that the disease was due to chronic Cd poisoning, and the
source of cadmium was the mines in the mountainous areas of Toyama Prefecture.
Mining operations were being done for centuries in the Toyama Prefecture
of Japan. In early 1900s, the Kamioka Mining Co. Ltd. was extracting zinc ores
in these mines for a long time. Wastewater of these operations was released in
the Jinzū River, the water of which became seriously contaminated with cad-
mium. This water was used by local community people for their domestic need

and irrigation in paddy fields. Soils and rice grains, grown in these paddies,
were also contaminated with cadmium. Since the per capita dietary consump-
tion of rice among the Japanese population is high, the impact of Cd enrich-
ment in the rice on the indigenous population was proportionally magnifi ed.

As with all cationic metals, the chemistry of cadmium in the soil environment is
to a great extent controlled by pH. Under acidic conditions, Cd solubility increases
and very little adsorption of Cd by soil colloids, hydrous oxides, and organic matter
takes place. At pH values greater than 6 units, Cd is adsorbed by the soil solid
phase or is precipitated, and the concentrations of dissolved Cd are greatly reduced.
Cadmium forms soluble complexes with inorganic and organic ligands, in particular
with chloride ions. The formation of these complexes will increase Cd mobility in
soils. Chloride can be expected to form a soluble complex with Cd as CdCl ,
+
2+
thereby decreasing the adsorption of Cd to soil particles. In contrast to inorganic
2+
2+
ligand ions, Cd adsorption by kaolinite could be enhanced by the presence of organic
matter via the formation of an adsorbed organic layer on the clay surface (Adriano
et al. 2005 ). Cadmium mobility and bioavailability are higher in noncalcareous
than in calcareous soils. Liming of soil raises the pH, increasing cadmium adsorption
to the soil and reducing bioavailability. A general trend emerges that toxicity
increases in soil when mobility of cadmium is higher; that is, soil toxicity increases
as soil pH, or soil organic matter, decreases.