1. Some metals lack suitable calibration data, which prevents quantitative modeling. One possible solution to this is to correlate model binding parameters with available data on metal ion hydrolysis.
2. Environmental NOM may contain ligands which are not isolated on XAD resins, and thus are not included in fulvic or humic acid calibrations.
3. Environmental processes may alter metal complexation properties of the original NOM; for example, the effects of solar irradiation on Cu(II) binding.
An alternative modeling approach which addresses these problems is a stochastic, agent-based simulation of NOM transformations linked to metal binding by quantitative structure property relationships (QSPRs), which addresses the above concerns: calibration depends exclusively on results from pure compounds, the NOM generated contains not only humic and fulvic molecules but other more (and less) soluble components, and environmental processes like preferential transport and photolysis can be directly modeled. The agent based model gives semi-quantitative agreement with published results without calibrating on metal-NOM data, and can make predictions regarding the nature of complex formation and changes due to environmental processes.