[Stabilization of heavy metals in municipal solid waste incineration fly ash with the thiol collectors]

Three kinds of thiol collector, sodium diethyldithiocarbamate (DDTC), potassium ethyl xanthate (EXT) and ammonium dibutyl dithiphosphate (DDTP), were adopted to stabilize heavy metals from municipal solid waste incineration fly ash (MSWI fly ash). The concentration of the three thiol collectors was all 62.5 micromol x g(-1) fly ash. Scanning electron microscopic observation shows that, the thiol collectors evenly cover on the surface of fly ash which makes the angles of mineral crystal ambiguous. Furthermore, the leaching characteristics of heavy metal Cu, Pb, Cd, Cr and Zn in fly ash were analyzed according to the toxicity characteristic leaching procedure (TCLP) and the horizontal vibration method. Comparing with Na2S, thiol collectors present better stabilization effects for Cu and Pb when the extractant is 0.1 mol x L(-1) acetic acid. DDTC stabilizes almost all the acid-extractable Cu, and DDTP stabilizes 69.2% of acid-extractable Pb. When extracted by water, the stabilization ratios of the five heavy metals by DDTC, EXT and DDTP are 72.6%, 73.5% and 76.8%, respectively, significantly higher than that by Na2S (52.4%). The affinity preference of the thiol collectors for the five heavy metals is generally in the order of Cu > Pb > Cr > Cd > Zn. Also, over 60% of the collector participates in the chelating reaction with the acid-extractable heavy metals. Under neutral and alkali condition (pH > 6) the chelators of heavy metal-thiol collector are steady, but partly dissolved under acid condition (pH < 6). Evidently, in order to obtain better heavy metal stabilization effects, it is important to maintain the acid buffer capacity of stabilized fly ash at a higher level.