Industrial processes often generate wastewater containing impurities from many different sources. Heavy metals are particularly challenging, as they occur in wastewater in many different forms: as dissolved ions, complexes, or solid particles. Of these, dissolved forms – both free metal ions and metal complexes – are most common in acidic industrial wastewater and cannot be removed by mechanical filtration methods. Dissolved metals easily migrate into waterways, where they can accumulate in the food chain and cause long-term environmental and health risks. Free metal ions interact directly with organisms and are therefore particularly harmful, while the behavior of metals in complex forms varies depending on what other substances are present in the water.
The removal of metals depends on their form in water, and one key solution is chemical precipitation, which converts soluble metals into a solid form and separates them from wastewater. Metals that are particularly difficult to treat, such as nickel, chromium, and arsenic, require an optimized precipitation process that is tailored to the characteristics of each wastewater fraction. Successful metal removal is not a simple process, but requires a deep understanding of solubility, reactions, and how different contaminants interact with each other.
How does precipitation work?
Precipitation is a process in which soluble impurities in a solution are converted into a solid form so that they can be removed by sedimentation, filtration, or other separation techniques. The solid material formed in this process is called sludge or precipitate.
Precipitation is commonly used for the removal and recovery of metals, particularly in the treatment of acidic industrial wastewater (Lewis, 2010). Precipitation methods are divided into two main types: chemical precipitation, which uses reagents, and non-chemical precipitation, which is based on physical or biological methods (Mullin, 2003). The choice of method is influenced by factors such as water composition, the substances to be removed, and costs (EPA, 2000). Chemical precipitation is the most commonly used method for removing metals, as it is effective, simple, and cost-efficient (Blais et al., 2008).
The precipitation of metals from water is primarily based on their solubility (S) at different pH values and the types of compounds they can form with ions in solution. Precipitation begins when the amount of ions in the solution exceeds a certain supersaturation limit, which is called the solubility product constant (Ksp) of the compound. Each compound has its own Ksp value, which is not constant but varies depending on factors such as temperature and impurities in the solution. pH does not change the Ksp value, but it does affect whether the supersaturation required for precipitation is achieved – especially in hydroxide precipitation, where pH regulates the concentration of hydroxide ions. All metals exhibit different precipitation behavior depending on their oxidation number and pH conditions (EPA 2000; Benalia, Youcef & Menasra, 2021).
The importance of pH in metal precipitation
A common way to precipitate metals is to raise the pH of water with bases such as calcium or sodium hydroxide. This adds hydroxide ions (OH⁻ ) to the solution, which reduce the amount of hydrogen ions and make the water more alkaline. At high pH, many metals form poorly soluble compounds with hydroxides and precipitate.
However, not all metals behave in the same way; each metal has its own optimal pH value at which it precipitates best. Some metals redissolve as a result of high pH, forming anionic hydroxyl complexes. This is related to their amphoteric nature, i.e., their ability to act as both acids and bases. For example, nickel and aluminum are often found in the same wastewater, but their precipitation requires different pH conditions. Aluminum precipitates at a lower pH than nickel, so achieving the optimal pH for nickel can lead to the redissolution of aluminum (Lewis, 2010; Fu & Wang, 2011). In multi-metal wastewater, this makes precipitation particularly challenging.
The EPSE™ method forms more stable compounds, and excessively high pH levels do not cause redissolution when the pH exceeds the optimal range. This enables the simultaneous removal of several metals in a single treatment step – even when traditional precipitation fails due to redissolution. In addition, there are particularly harmful metals, such as chromium(VI) and arsenic(V), which do not form metal hydroxides, meaning that simply adjusting the pH is not sufficient to precipitate them (Mukhopadhyay et al. 2007; Smedley & Kinniburgh 2002).
What does a successful precipitation process require?
The success of chemical precipitation is influenced by many physicochemical factors, such as temperature, ionic strength, mixing efficiency, and reaction time. Optimization is particularly challenging in multi-metal wastewater, as different metals can react very differently under the same conditions. Therefore, the process must be carefully designed so that the conditions support the formation and stability of the desired compounds (Benalia, Youcef & Menasra, 2021).
The process can also be disrupted by the ionic composition of the solution, impurities, and interactions between metals, which affect the precipitation balance. For example, nickel, copper, and zinc readily form stable complexes with organic compounds such as citrate, ammonia, or EDTA. Such complexing agents often end up in wastewater from certain industrial processes, such as metal surface treatment, electroplating, or industrial washing and rinsing stages, where chelating agents are used to increase solubility or control metals. When these compounds are present in wastewater, they bind metals into complex forms and prevent them from reacting with hydroxide ions, meaning that traditional hydroxide precipitation alone is not sufficient and the complexes often need to be broken down separately (Mullin, 2003; Vardhan, Kumar & Panda 2019; Kowalski n.d.). Successful optimization of the process requires, among other things, control of these phenomena and understanding of the specific characteristics of metal precipitation reactions.
EPSE™ Method for metal precipitation
The EPSE™ method is based on the principles of chemical precipitation, but it has been designed specifically for the treatment of multi-metal and difficult-to-treat wastewater. The method enables the efficient removal of metals even when they occur in complex forms or challenging mixtures – without the risk of redissolution when the pH changes. EPSE treatment produces a stable metal precipitate that withstands pH changes and reduces the risk of redissolution. This makes the process controllable and long-lasting, even in final disposal.
An individual recipe is developed for each wastewater fraction based on research data, which is optimized in stages from the laboratory to piloting – challenges are always solved by utilizing new information and experiments.
Precise optimization of the method reduces chemical consumption and enables resource-efficient, customized solutions for demanding wastewater treatment. Effective treatment is essential, as dissolved metals are at their most harmful when they are freely transported into waterways. Without proper removal, they can cause long-term environmental impacts and health risks.
This article is written by
Anette Anttonen
Laboratory Engineer
anette.anttonen(a)epse.fi
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References in order of appearance:
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Mullin, J.W. 2003. Crystallization and Precipitation. Ullmann’s Encyclopedia of Industrial Chemistry (10). Access restricted. http://dx.doi.org/10.1002/14356007.b02_03 (Accessed: 29.12.2024)
EPA (Environmental Protection Agency). 2000. Wastewater Technology Fact Sheet. Chemical Precipitation. Available at: https://www3.epa.gov/npdes/pubs/chemical_precipitation.pdf (Accessed: 14 July 2025)
Blais, J.-F., Djedidi, Z., Ben Cheikh, R., Tyagi, R. D., & Mercier, G. 2008. Metals Precipitation from Effluents: Review. Practice Periodical of Hazardous, Toxic, and Radioactive Waste Management 12(3). Access restricted. https://doi.org/10.1061/(ASCE)1090-025X(2008)12:3(135) (Accessed: 3.1.2025)
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Mukhopadhyay, B., Sundquist, J. & Klaas, B. 2000. Chromium(VI) removal from groundwater by granular activated carbon. Journal of Hazardous Materials, 129(1–3). Access restricted. http://dx.doi.org/10.2175/106143000X137086
Smedley, P. L. & Kinniburgh, D.G. 2002. A review of the source, behaviour and distribution of arsenic in natural waters. Applied Geochemistry. 17(5). Access restricted. https://doi.org/10.1016/S0883-2927(02)00018-5
Vardhan, K., Kumar, P. & Panda, R. 2019. A review on heavy metal pollution, toxicity and remedial measures: Current trends and future perspectives. Journal of Molecular Liquids. 290. Published online 15 september 2019. Access restricted. https://doi.org/10.1016/j.molliq.2019.111197 (Accessed: 14 July 2025)
Kowalski, A. n.d. Metals removal to low levels using chemical precipitants. Presented at the AESF/EPA Conference for Environmental Considerations in Electroplating and Metal Finishing, Orlando, FL. Available at: https://sterc.org/pdf/awk02/awk02l02.pdf (Accessed: 14 July 2025)
Pictures:
Anna Kivimäki. 2024. Water discharge line into nature.
Alison Emslie Lewis. Review of metal sulphide precipitation. Hydrometallurgy, Volume 104, Issue 2, 2010, Pages 222-234, ISSN 0304-386X, https://doi.org/10.1016/j.hydromet.2010.06.010 (Accessed: 2 September 2025)