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Heavy Metal Removal From Wastewater by Precipitation and Microfiltration*

Removal of soluble heavy metal ions from wastewater is a common industrial treatment requirement. Examples of the industrial processes that generate waste streams containing heavy metals are:

  • Electroplating
  • Electroless Nickel plating
  • Printed circuit board manufacturing
  • Metal forming operations
  • Battery recycling
  • Mining operations

The concentration and chemical form of the soluble heavy metals in the wastewater stream varies with the industry and the mix of operations at a processing site.

Typical removal strategies involve precipitating the metals in an insoluble form such as hydroxides, sulfides, carbonates or some combination, then removing the precipitate with tubular microfiltration for very high quality filtrate, or conventional clarification. The resultant sludge is collected, thickened and dewatered for landfill disposal.

Hydroxide precipitation is a common method as it is relatively simple to operate. Sulfide precipitation has some advantages, but pH and oxidation-reduction potential (ORP) must be carefully controlled to minimize the risk of producing toxic levels of hydrogen sulfide gas. Using hydroxide and sulfide precipitation, in two sequential steps, is also an option, particularly where complexes or chelates are present. Carbonate co-precipitation, using sodium or calcium carbonate, can also be helpful, for instance, for soluble lead reduction; lead carbonate is essentially insoluble (0.00011 g/100 mL @20°C) and will precipitate out. Phosphate precipitation is also an option although this is not such a common process.

Hydroxide Precipitation

Hydroxide precipitation is initiated by adding an appropriate hydroxide to the waste water in stirred reaction tanks to form the insoluble heavy metal hydroxide precipitates:

M+n +nOH- M(OH)n

Many of the heavy metals show marked amphoteric behavior; their hydroxides reaching minimum solubility at a specific pH for each metal.

Theoretical Solubility of Metal Hydroxides vs. pH

Theoretical Solubility of Metal Hydroxides vs. pH

The pH can simply be adjusted to the minimum solubility for the target metal(s). Typical hydroxides that are used include:

  • Sodium hydroxide in solid or liquid form (NaOH)
  • Calcium hydroxide from Lime (CaO)
  • Magnesium hydroxide (MgO)

Lime is generally cheaper than sodium hydroxide but needs onsite slaking and continuous stirring to prevent cake formation. It also acts as a coagulant during the precipitation process and helps improve metals reduction via adsorption mechanisms.

In practice, the presence of several different metals and other substances in industrial wastewater can make large differences to the theoretical solubility. More effective removal of heavy metals can often be achieved by co-precipitation with iron hydroxide. Adding ferric chloride (FeCl3 ) increases the removal of metals and forms an inorganic flock of iron hydroxide. Typical improvements are order of magnitude lower dissolved heavy metals in the permeate than with simple pH adjustment. The actual removal rate depends on many factors, so pilot testing is usually required to establish actual performance characteristics.

Pretreatment

Hexavalent chromium compounds are particularly toxic and should be chemically reduced to the trivalent form in a pretreatment process prior to precipitation. Sulfur dioxide (SO2), sodium bisulfite (NaHSO3) or sodium meta-bisulfite (Na2S2 O5) are generally used for this purpose. Ferrous sulfate FeSO4 is also an option.

Cyanide complexes, often used in plating and mineral extraction are normally removed by alkaline chlorinationoxidation pretreatment. This uses caustic soda (NaOH), and sodium hypochlorite (NaOCl) or chlorine gas (Cl2), in a two step process. Initially, at pH10.5, to oxidize CN- to CNO- , then to complete the oxidation at a pH (target value of 7) to form N2 and CO2.

Other complex and chelated metal salts are often found in wastewater and will decrease the effectiveness of precipitation treatment. Some of these can be broken down relatively easily with pH adjustment, and/or oxidation, while others, like EDTA (Ethylenediaminetetraacetic acid), are very stable. For example, a common industry practice is to use a combination waste treatment method such as acid and high pH lime or caustic soda. 

This process first adjusts the pH of the organo-metallic containing waste to approximately 2 with dilute mineral acid. After the chelate/complex breaking step, the pH is then raised to 9.5–11 to form insoluble metal hydroxides. Chemical oxidation using potassium permanganate (KMnO4 ), ozone, chlorine dioxide, or hydrogen peroxide (H2O2) has also been used to “break” metal complexes and metal chelates allowing subsequent precipitation of the metal ions. Another strategy is to replace target heavy metals in chelates with low toxicity iron using the ‘Permutit Sulfex’ process. Some chelating agents have a stronger affinity for ‘non toxic’ metals, such as calcium, than the targeted toxic heavy metals. Adding calcium chloride, for instance, may help displace some of the heavy metal for subsequent hydroxide precipitation.

The ‘Sulfex’ process uses freshly precipitated ferrous sulfide (FeS) as the source for the S2- ion with the pH typically adjusted to between 8 and 9. The FeS is precipitated by reacting solutions of an iron salt such as ferric chloride (FeCl3) or ferrous sulfate (FeSO4) with sodium sulfide (Na2S) or sodium hydrogen sulfide (NaHS), with the addition of an alkaline such as sodium hydroxide (NaOH) to raise the pH above 7 to prevent evolution of hydrogen sulfide (H2S) gas. It can also be used to reduce hexavalent chromium to the trivalent form for one-step operations. Combined or sequential hydroxide and sulfide precipitation can be used to efficiently remove certain metal ion combinations where complexes and chelates are present.

Long chain polymers are used as wetting agents in some industrial processes. They can form agglomerates that will foul microfiltration membranes. Although performance can be restored by oxidizing pretreatment (NaOCl, H2O2) and alkaline oxidation (NaOH and NaOCl) or simple oxidative (H2O2) chemical cleaning procedures, it is better to select process additives that have short chain length polymers where practical.

Solids Removal

Unlike conventional clarifier techniques, Tubular Microfiltration (TMF) is a very effective method of removing virtually all the precipitated metal hydroxide/sulfides/carbonates from the treated wastewater stream. The resulting high quality permeate can be fed directly to reverse osmosis equipment for high-quality reuse, reclaimed ‘as is’ or discharged. Solids concentrations of up to 18% (w/w) can be processed but most efficient operation is normally at between at 3 and 5% solids. 

The sludge is normally drawn off and passed through conventional thickening, filter press processes and or similar processes to produce dewatered solids, typically for land fill disposal. Supernatants from the sludge handling processes are recycled through the TMF system so that the only outputs are the filtered water and the compact solids cake.

* Porex is committed to a continuing program of product care and improvement. Please be advised that this information is intended to be used as a general guideline and nota complete review which covers all possible specifications, performance, parameters, appearance, and dimensions of all water filtration products or systems. It is necessarily the responsibility of the system builder and operator to obtain appropriate advice concerning specific equipment or particular circumstances. Porex Corporation is not liable for direct, indirect, special, incidental or consequential damages arising out of the interpretation, the use or the results of use of this information.

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