This is simply the passive verb form.

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Note added at 5 hrs (2011-01-11 19:08:27 GMT)
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I erred. This is adsorption, not absorption, i.e., it's the binding of a substance onto a surface, not its transportation into something, that is at issue here. (I'm no chemist or chemical engineer, so feel free to flay me on this characterization. ;-) )

See this at http://www.dsr.kvl.dk/~anya/specialerapport.pdf at page 13:

"Fraktionen af lermineraler (partikler < 2μm) kan med deres store specifikke overflader sorbere relativt store mængder af tungmetaller vha. deres permanente og variable (pH-afhængige) ladninger."

See also this at http://www.geus.dk/grundvandsgruppen/smp_42_p_17_20.pdf at page 17 (actually it's the first page displayed at the link):

"Transporten af et pesticid vil være påvirket af i hvor høj grad stoffet bindes – sorberer – til grundvandssedimenterne."

And since it's adsorption at issue here, I believe "til" is entirely correct, as demonstrated in the sentences above.

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Note added at 23 hrs (2011-01-12 13:33:42 GMT)
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Thanks for suggesting I make a second answer, but I think it would violate the spirit, if not the letter, of the KudoZ rules -- see http://www.proz.com/faq/2902#2902:

"Editing an answer would be unfair to other answerers. You might consider adding a comment to your answer. In some cases, a wrong answer can be hidden by the answerer and the correct answer posted in its place."

I'll see if I can get a response on this from a moderator or some other authority.