Dissolved vs total metals. Why it matters from the water treatment perspective?
Updated: Nov 3, 2020
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. Other times, elevated concentrations of heavy metals in the water occur 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 from the water treatment perspective? 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?
Dissolved metal concentration is determined by filtering a water sample through 0.45 uM filter. Water that passes through the filter is analyzed for metals and the result is reported as dissolved metal concentration. Total metal concentration is determined by analyzing the unfiltered sample.
Total metal concentration = Dissolved metal concentration + Particulate (insoluble) metal concentration.
The reason why it is important to know both dissolved and total metal concentrations when evaluating a water sample from the water treatment perspective, is because elevated dissolved metals typically represent a bigger undertaking from the treatment point of view. Dissolved metals can NOT be simply removed by physical filtration while particulate (insoluble) metals in theory can be filtered off. Coagulation, flocculation, sedimentation and filtration, steps commonly used in sediment control processes, can take insoluble metals out of the water, but the same steps may not necessarily be able to remove dissolved metals and other technologies may need to be used to achieve the treatment objective.
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. A popular subset of the chemical precipitation is so called co-precipitation method using aluminum or iron salts. Co-precipitation method, while being a relatively inexpensive approach, has a number of advantages over conventional hydroxide precipitation including its ability to lower solubilities of the precipitated metallic compounds and even potential for reducing concentrations of chelated metal complexes, both subject to finding the optimal treatment conditions for a given water sample.
Other commonly used methods to remove dissolved metals are ion exchange, adsorption, and membrane filtration. Each of these methods has its advantages and limitations, and more often than not a combination of different treatment technologies is needed to solve a particular metal treatment challenge in the most efficient, reliable and cost-effective way.
Can metals in the water be monitored in real-time to determine if the treatment system consistently achieves the water quality objectives? Although most metals need to be tested in an analytical laboratory to verify the concentrations, some proxy parameters such as dissolved oxygen, pH, temperature, conductivity, turbidity or a combination of thereof can be used to monitor treatment system performance in real-time using corresponding online water quality sensors and Flowlink water data software.
You can read our blog post about some easily accessible tools that can be used to detect a number of metals in the field or head over to our Resources to compare your analytical results against freshwater or saltwater water quality guidelines most frequently used for surface water discharges in British Columbia, Canada.