Sedimenrary rocks of marine origin excavated during tunnel construction for railways and roads in Japan contain a variety of toxic elements, such as arsenic (As). When the excavated rocks corne in contact with rain water, these toxic heavy metals could leach out and contanrinate the surrounding environment. It is therefore necessary to understand the leaching behavior of the toxic elements and to dispose the excavated rocks appropriately in order to minimize their negative impacts to the environment. Brucite, one of the hydroxide minerals of magnesium in the serpentinite, has the selectivity for oxyanion adsorption with the affinity sequence being NO3< SO42−< PO43−< AsO43−, making it a potentially effective adsorbent against anions. This paper describes the effect of mixing mudstone containing As with serpentinite for reducing As concentration leached from mudstone excavated during tunnel construction. Column experiments consisting of the mixture of crushed mudstone and serpentinite were conducted in situ to evaluate the leaching properties of As from the mixture and the adsorption characteristics of As to serpentinite.

The results showed that the mixed layer effectively reduced As concentration leached from the colurun. This indicates that mixing may be one of promising methods for disposing waste rocks containing As. Moreover, the comparison of cumulative As adsorption obtained in the column experiments under in situ conditions with the cumulative As adsorption obtained in the batch leaching experiments after slaking revealed that the cumulative As adsorption in the colurun experiments under in situ conditions was lower than the cumulative As adsorption obtained proir to slaking in the batch leaching experiments, which implies that the As adsorption in the colurun experiments under in situ conditions did not reach a saturated adsorption amount.

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