| A previous study revealed that lead, cadmium, mercury, zinc, arsenic and selenium could be volatilized from both spiked and actual contaminated soils when heated in inert (N{dollar}sb2{dollar}) or reducing (H{dollar}sb2{dollar}) atmospheres. Generally, with the exception of lead, all the elements volatilized near to boiling points of the free metals. Oxygen proved detrimental to the remediation process.; While it is easy to understand how metals and metalloids are removed from soil by volatilization at temperatures near their boiling points, this research seeks an explanation for the volatilization of lead from soil at 800-1000{dollar}spcirc{dollar}C below its boiling point. The laboratory results revealed that many reaction pathways led to the same two-step mechanism: (i) reduction (PbSO{dollar}sb4to{dollar}PbS) or decomposition (PbCO{dollar}sb3to{dollar}PbO{dollar}to{dollar}Pb{dollar}spcirc{dollar}) followed by (ii) evaporation of PbS or Pb{dollar}spcirc{dollar}. Together the lead species present, the clay content, and the amount of reducing agent available in the soil, determine the form of lead vaporized and the vaporization rate. All compounds (PbSO{dollar}sb4{dollar}, Pb(NO{dollar}sb3)sb2{dollar}, PbCO{dollar}sb3{dollar} and PbO) can be reduced to metallic Pb{dollar}spcirc{dollar} before volatilization takes place if the soil organic content is abundant or if sufficient clay is present to provide hydrogen during dehydroxylation. When reducing agents are limiting, PbS and PbO are the predominant lead forms left in the soil.; Attempts were made to convert lead compounds into PbCl{dollar}sb2{dollar} using NaCl and HCl. Preliminary experiments showed that Cl-addition led to satisfactory %Pb removal at a lower temperature (700{dollar}spcirc{dollar}C) even in air.; This study confirms the possibility of removing lead from contaminated soil through pyrolysis. It also suggests the presence of organic material and an inert clay matrix as enhancers of the volatilization process. |
