Simultaneous control of cadmium release and acidic pH neutralization in excavated sedimentary rock with concurrent oxidation of pyrite using steel slag

Purpose: The aim of this study was to determine whether steel slag potentially could be utilized as an amendment for the reuse of excavated sedimentary rock naturally containing pyrite and the toxic metal cadmium. For this purpose, this study elucidated the mechanisms for simultaneous pH neutralization and cadmium immobilization of excavated sedimentary rock using steel slag under conditions of concurrent pyrite oxidation. Materials and methods: A batch leaching test was conducted using excavated sedimentary rock (mudstone) that was well mixed with steel slag in combinations ranging from 0 to 50 %wt to evaluate the relationship between the dosage of steel slag and both cadmium release and the pH value after 30 days of the treated rock. Another batch leaching test was performed using excavated sedimentary rock combined with steel slag from 0 to 1.5 %wt to monitor cadmium release and the pH as pyrite oxidation proceeded. Cadmium was sorbed onto the steel slag to evaluate cadmium sorption ability at pH values of 6.5 and 9.0. Using the cadmium-sorbed steel slag, the crystalline structure and the level of cadmium release from the steel slag at different pH values were determined. Results and discussion: The pH of excavated sedimentary rock without steel slag decreased, while that with more than 0.5 %wt steel slag remained within a neutral to alkaline pH range after 90 days, when potential pyrite oxidation was complete. The presence of steel slag did not inhibit the process of pyrite oxidation. These findings demonstrate that both pyrite oxidation and pH neutralization independently occurred in the excavated sedimentary rock, even in the presence of steel slag. The addition of more than 0.5 %wt steel slag maintained the level of cadmium released at less than 0.1 mg kg⁻¹ throughout the 90-day period. The cadmium sorption test and X-ray diffraction analysis showed that cadmium immobilization using the steel slag was mainly governed by the (co)precipitation of carbonate minerals. Conclusions: This study indicates that an addition of 0.5 %wt steel slag to the excavated sedimentary rock can achieve pH neutralization and suppress cadmium release simultaneously, even with the occurrence of pyrite oxidation. Thus, this study suggests that steel slag potentially could be utilized as an amendment to allow the reuse of excavated sedimentary rock.