[detaildevil]

I'm trying to understand something about the chemistry of asphalt emulsions used for sealing driveways, etc. It appears that Cr(VI) is present, a dangerous contaminant, and that chromium compounds are used in modern manufacture of these sealants. I can't believe that this is done simply for pH adjustment. Some early sealant patents have very specific addition amounts and a recent patent mentions only that chromium ions "adjust the pH"; from patent 7547356:

"Chemists skilled in the art of making clay emulsions usually add chemicals that add chromium ions to adjust the pH of the clay in water mixtures. These chemical include chromic acid, sodium chromate, sodium dichromate, and potassium dichromate. In recent years though, it has been found that chromium ions can be harmful, toxic, and hazardous. Aluminum reacts in many similar ways to chromium, but does not share the unhealthy and harmful effects of chromium. Chemists making clay emulsions have not caught on to using aluminum chloride in place of chromium chemicals." [Clay is added for some applications, e.g., roofing?.]

I can't believe modern manufacturers would risk use of chromium compounds only for pH adjustment. The patent suggests they "haven't caught on" to alternatives. Is there some other functional role for chromium in these applications? Emulsion stabililization? Why chromium? Does the replacement by aluminum give any hint as to the role of chromium?

I'd appreciate any thoughts anyone has on the topic. We are trying to address chromium sources in urban runoff. Many thanks.