Humic substances (fulvic and humic acids) are key organic substances which can complex with trace metal ions. Humic acids are much preferably complexed with metal ions than fulvic acids. In sediments, there are microbial processes are effectively reacted to decompose the humic substances, but such changes in humic substances increased the functional groups in humic substances. It might result more effectively complextion with humic substances. This is very important for sediments. But in surface water this acts as an opposite behavior. I have enclosed a book chapter that you find in details. 

Khan M. G. Mostofa 

I study humic substances in sediments of a contaminated river (Matanza Riachuelo) and in surface water too just to understand the processes involved in the release or retention of heavy metal from sediment to water column and that is the model i want to develop

I'm also interested in this exact same thing although I use salt marsh sediments. For what I know it might be possible to use WHAM model for your proposes or you can simply try to extract them purify and incubate them with different metal concentrations and make your own langmuir model of binding

Brian Bergamaschi and and his group at USGS Sacramento has published on this.

Here is his profile page, which has links to his papers. Lots of good stuff there.

https://profile.usgs.gov/bbergama

This is a good place to start:

Bergamaschi, B. A., J. A. Fleck, B. D. Downing, E. Boss, B. Pellerin, N. K. Ganju, D. Schoellhamer, W. Heim, M. Stephenson, and R. Fujii. 2011. Methyl mercury dynamics in a tidal wetland quantified using in situ optical measurements. Limnology and Oceanography 56: 1355-1371.

 check out Christian Steinberg  work it is extensive in this area, including I believe a book on it. 

greetings margarete