Dissolved & total metals: what’s the difference and why it matters?

Metals, and heavy metals in particular, are common contaminants of concern in groundwater and stormwater run-off. Sometimes, presence of heavy metals in our water is caused by anthropogenic activities, such as industrial production, agriculture, landfilling, mining, and transportation. Sometimes, elevated concentrations of heavy metals in the water are due to naturally occurring minerals, rocks and soils.

Metals in the water can be present in either dissolved (soluble) or particulate (insoluble) state. Why is it important to differentiate them? Or, in other words, why is it important to know both total and dissolved metal concentrations, two analyses frequently performed on water samples by analytical laboratories?

Solids coagulation, flocculation and filtration will take care of particulate metals, but not necessary of dissolved metals.
Solids coagulation & flocculation

The answer is simple.

It is dissolved heavy metals that are most problematic as they can NOT be simply removed by physical filtration while particulate metals in theory can be filtered off. Coagulation, flocculation, sedimentation and filtration, commonly used steps in sediment control processes, can take care of particulate metals, but not necessarily will be able to remove dissolved metals.

To remove dissolved metals, one would need to use an appropriate treatment technology designed for removal of soluble metals or a combination of different treatment steps.

One of the most commonly used methods to lower the dissolved metal concentrations is chemical precipitation. Chemical precipitation converts dissolved metals ions into corresponding insoluble metallic compounds such as a hydroxide, sulfide, or a carbonate which are then filtered out of the solution to yield a clear effluent containing lower metal concentrations. This method has its pros and cons which will be discussed in a separate blog post.

Among other commonly used metals to remove dissolved metals are ion exchange, adsorption, and membrane filtration. Each method has its own advantages and limitations, and often a combination of different treatment techniques is needed to solve a particular metal treatment problem in the most efficient and cost-effective way.