Effects of cement addition on arsenic leaching from soils excavated from projects employing shield-tunneling method

Abstract Tunnel construction in soft alluvial deposits is generally done using the shield-tunneling method, a technique that requires the mixing of cementitious materials with excavated soils to reinforce and stabilize the excavated tunnel walls. Although the mechanical properties of soft soils and sediments are improved after introduction of cementitious materials, this process could increase the pH of these geological materials and mobilize naturally occurring arsenic (As). In the present study, the effects of cement addition on As leaching from tunnel-excavated soils naturally contaminated with As were investigated. Sequential extraction experiments were also carried out to determine the chemical forms, solid-phase partitioning and leachability of As in the soils. The results of attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) showed slight changes in the chemical properties of soils due to cement addition but the chemical phases of As remained unchanged. In the absence of cement, sequential extraction showed higher As leaching from the soils when the exchangeable As fraction and total As content increased. In contrast, As leaching increased up to pH 10.3 and then decreased at higher pH values when cement was added. This trend was observed irrespective of the soil samples, which indicates that pH adjustment is an important countermeasure in restricting As leaching from excavated soils.