Elution of Sulfuric Compounds from Na-sulfide Waste for Stable Management at a Disposal Site: an Experimental Investigation

We investigated the dissolution and oxidation behaviors of sulfuric waste from secondary batteries by reacting the waste with synthetic rainwater to examine the optimal management of this waste at a disposal site. Morphological observation and chemical analysis of bulk sulfur (S) wastes revealed heterogeneous distributions of sodium sulfide (NaS) and thiosulfate (Na₂S₂O₃). Leaching experiments using crushed S-waste samples with oxygenated synthetic rainwater for 144 h showed that sulfide ion was the dominant eluent from the less-oxidized sample, whereas sulfate (SO₄) and S₂O₃ ions were released from the well-oxidized sample (left for 6 months). An atmospheric oxidation experiment of S-waste was carried out to elucidate the relationship between sulfide oxidation on the S-waste surface and eluted species. X-ray photoelectron spectroscopy showed rapid oxidation of NaS to Na₂S₂O₃ within the first 72 h, and time-of-flight secondary ion mass spectrometry revealed that the sulfide-oligomers species (S₃ and S₄) formed after reaction for 336 h. The kinetic analysis indicates that NaS was easily oxidized by atmospheric oxygen. The oxidation rate was 10–100 times higher than that of pyrite. The NaS oxidation and the formation of sulfide oligomer species inhibited the rapid dissolution of sulfuric components from a well-oxidized sample. Most S-waste could be exposed to atmospheric air before disposal; therefore, the main concern for this waste disposal could be SO₄ and S₂O₃ ion dissolution and interaction (redissolution) with toxic compounds such as enhancement of leaching rate of heavy metals due to complex ion formulation.