Model study of acid rain effect on adsorption of trace elements on soils, 1

Abstract

Acid rain causes a disturbance in the adsorption equilibrium of chemical elements in the soil, which seriously affects the uptake of elements by plants. Using a multitracer technique, the effect of model acid rains on the adsorption of 11 trace elements into kaolin and soil was investigated. Kaolin and Kureha soil were used as the adsorbents. Distilled water and rainwater were acidified with H2S04 and used as simulated acid rains (pH 1.8-5.6). The pH value and gamma-ray spectrum of their supernatants were measured. In two kinds of model acid rains, the Adsorption Percentage (AP) of kaolin was examined. It was found to increase with increasing pH value, except for anionic Se. The AP of Be, Mn, Co, Zn and Sr markedly increased in the pH range 3.0 - 4.0. At pH less than 3.0, their adsorption was low. At pH greater than 5.0, their AP was almost 100 percent, except for Mn. The AP of Y increased with increasing pH value and reached 100 percent at pH 4.0. The AP of Fe, Sc and Zr was high even in the acidic pH range and reached almost 100 percent at pH 4.0. The AP of Se was constant over the pH range studied. The relationship between AP and pH depended greatly on the element and adsorbent. Acid rain reduces the retention of various cationic elements in the soil. This results in a loss of trace elements from the soil and an enrichment of trace elements in the ground water.

酸性雨は土壌中の元素の吸着平衡を乱し、植物の元素取込に重要な影響を与える。マルチトレーサ法を用いて、11の微量元素のカオリンおよび土壌への吸着に与えるモデル酸性雨の影響を解析した。カオリンとクレハ土を吸着物質として用いた。蒸留水と雨水を硫酸で酸性化し、モデル酸性雨(pH1.8?5.6)とした。上澄み液のpH値を測定し、γ線スペクトルの測定を行った。2種の酸性雨について、カオリンへの吸着率を調べた。陰イオンのSeを別にして、吸着率はpH値とともに増加した。Be、Mn、Co、ZnおよびSrの吸着率は、pH3.0?4.0で急激に増加した。pH<3.0では、吸着は少なかった。pH>5.0では、Mnを除くと吸着率はほとんど100%であった。Yの吸着率はpH値と共に増加し、pH4.0で100%に達した。Fe、Sc、およびZrは酸性pH領域でも吸着率が高く、pH4.0でほとんど100%になった。Seの吸着率は調べたpH領域でほぼ一定であった。吸着率対pHの関係は、元素と吸着物質に強く依存した。酸性雨は土壌中での各陽イオン元素の保持を減少させ、微量元素の土壌からの散逸と、地下水への蓄積をもたらす。