In order to elucidate the fundamental information on the removal of heavy metals from polluted water by the use of bark, the characteristics of adsorption of heavy metals on bark was investigated.
The bark powders(10～20 mesh) of Akamatsu(Pinus densiflora SIEB. et ZUCC), Kuromatsu(Pinus thunbergii PARL.), Oshuakamatsu(Pinus sylvestris LINN.), Benimatsu(Pinus koraiensis SIEB. et ZUCC.), Karamatsu(Larix leptolepis G0RD0N.), Dafurika karamatsu(Larix dahurica TURCZ.), Ezomatsu(Picea jezoensis CARR.), Keyaki(Zelkova setrrata MAKIN0.), Shioji(Fraxinus spaethiana LINGELSH.) and Sawagurumi(Pterocarya rhoifolia SIEB．et ZUCC.) and copper(II), cadmium(II) and chromium(III) as heavy metal were used in this experiment.
The results obtained are as follows :
Heavy metals were generally more adsorbed on the barks of softwood than that of hardwood, and especially the amount of adsorbed heavy metal on the bark of Ezomatsu and inner barks of Benimatsu and Dafurika karamatsu was larger than that of the others. When 1g of bark was shaken with 100ml of heavy metal aqueous solution ranging in concentration from 50 to 1000ppm at 30℃ for 24 hours,the amount of adsorption in equilibrium on each heavy metal was as follows :
Benimatsu (Inner bark) ; 19.3mg <Cd>^^^<2+>/g
Dafurika karamatsu (Inner bark) ; 20.6mg <Cu>^^^<2+>/g
Ezomatsu ; 23.0mg <Cr>^^^<3+>/g
It was found that a part of adsorption capacity is dependent on the content of tannin.
The saturated amount of adsorption and equilibrium concentration obeyed Freundlich-type adsorption isotherm.
Since the amount of adsorbed heavy metal on the bark was directly proportional to the square root of soaking time, the rate-determining step was the rate of diffusion of heavy metal ion in intramicropores of bark. This shows that the adsorption is affected by the pore size distribution and specific surface area of bark.
The amount of adsorbed chromium (III) increased with the rise of temperature and that of cadmium (II) is little affected by temperature. However, the amount of adsorbed copper (II) showed a tendency to decrease with the rise in temperature.
In the adsorption of heavy metals on barks, the optimum pH of the initial aqueous solution was the pH range 5.0-6.0.