Chromium compounds are used in many chemical processes as industrial catalysts and pigments for glass, porcelain glazes (bright green, yellow, red, and orange). Approximately 90% of all leather is tanned with chrome, and toxic waste tannery effluents are generated.
1. Introduction
The issue of minimizing waste generation and reducing its harmful potential for the environment has become a permanent feature of new industrial solutions. Research focused on this aspect shows a wide range of possibilities, including the choice of techniques that reduce waste generation (e.g., process optimization) or techniques that enable waste sources to be treated, with a consequent reduction in emissions. Although it is, of course, far better to implement the ‘prevention is better than cure’ procedure, this is not always an option. Therefore, the development of waste source treatment techniques should be prioritized.
One serious environmental problem is the release of heavy metals, including chromium, into the environment. Chromium occurs in nature in the form of minerals such as crocoite (PbCrO
4), chromite (FeCr
2O
4), or lopesite (Na
2Cr
2O
7). This metal is applied to harden steel and protect it from corrosion, which is why it is a component of stainless steels and different alloys, from which special and everyday products are made, such as airplanes, tanks, machinery, industrial installations, kitchen utensils, surgical tools, etc.
[1]. As chromium forms a passivation layer, a fine and solid chromium oxide film, it is used as an external coating to protect the internal metal (steel) from corrosion. Chrome plating is widely used in the leather tanning industry and in the manufacturing of steel products, which has been accelerating the consumption of chromium worldwide for the last 10 years (
Figure 1)
[2]. In 2021, the global chrome plating market was estimated at $16.6 billion, and is expected to grow to $22.2 billion by 2028
[3].
Figure 1. Chromium mine production worldwide from 2010 to 2021
[2].
Chromium compounds are used in many chemical processes as industrial catalysts and pigments for glass, porcelain glazes (bright green, yellow, red, and orange). Approximately 90% of all leather is tanned with chrome
[4][5], and toxic waste tannery effluents are generated and must be efficiently purified before release.
In the aquatic environment, chromium is found naturally in rainwater (0.2–1 µg/dm
3), seawater (0.04–0.5 µg/dm
3), surface waters (0.5–2 µg/dm
3) and groundwater (<1 µg/dm
3)
[6]. Natural water can be contaminated by anthropogenic chromium, especially from tanneries. Chromium is present in aqueous solutions in different forms, trivalent and hexavalent being the prevalent ones. Cr(III) has low solubility and is less dangerous to the aquatic environment than Cr(VI), which shows much better solubility and can easily migrate through the groundwater, mix with it, and contaminate it
[7]. The harmfulness of chromium is well known
[8][9][10]: toxicity limits are 28–80 mg/dm
3 for fish, 0.05 mg/dm
3 for drinking water. However, mutations induced by the chromium(III) complex with 2,2′-bipyridyl were found, among others, in oxidation-sensitive
Salmonella strains TA2638 and TA102
[11].
Although Cr(III) is less toxic to living organisms (negative results in most mutagenicity tests) than Cr(VI) and trace quantities of Cr(III) are even essential for the proper functioning of the human organism, discharge of Cr(III) present in large amounts in spent tanning liquors is burdensome for the environment. For example, groundwater contamination with chromium near tanneries around the world must be carefully monitored
[7][12][13]. An additional risk is the fact that Cr(III) can be oxidized to hexavalent chromium in natural water and soil
[13].
There are no unified discharge limits for Cr(III) or Cr(VI) to the aquatic environment not only in different parts of the world but also within the EU or the World Health Organization. Each country establishes its own standards for the chromium discharge limits to various aquatic systems (marine water, lake, river, and sewer system). The maximum discharge limit to the aquatic environment in the EU is 0.05–2 and 5 mg/dm
3 for Cr(VI) and Cr
total, respectively
[14][15].
It should be noted that, depending on the industry in which wastewater is generated, the presence of associated components, for example, surfactants, other metal ions, acids, and bases must be taken into account. Therefore, it is not possible to unequivocally identify one single best method for chromium removal. Moreover, as science progresses, new materials, techniques, and solutions can be expected to solve the problem of chromium separation from aqueous systems.
2. Chromium in Spent Industrial Effluents
Chromium-containing waste effluents can originate not only from steel plating and tannery processes but also can be generated as a result of the leaching of various types of steel
[16][17][18][19]. The removal of chromium from industrial effluents is important not only for the removal of hazardous metal ions but also for the purification of the effluents before further steps of treatment/recovery of valuable metals.
In order to provide an initial indication of the complexity of the chromium effluent problem, the exemplary compositions of Cr(III)-containing industrial effluents are summarized in Table 1.
Table 1. Origin and composition of Cr(III)-containing spent industrial effluents.
Industrial effluents contain mostly Cr(VI); thus, the research efforts are focused on removal of toxic hexavalent species. However, Cr(III) is present in passivation baths or leach solutions. As can be seen in the summary presented, it is important to keep in mind that the parameters such as concentration and pH will influence the choice of the treatment method
[5]. In general, three methods are possible to reduce Cr(III) concentrations from industrial effluents. These are the following: (i) oxidation to Cr(VI) compounds, (ii) reduction to the metallic element, or (iii) co-precipitation with ions of other low-toxic metals without changing the oxidation state. The former option, due to high toxicity of Cr(VI) relative to Cr(III), is not very promising, since nowadays many technologies are abandoning the use of Cr(VI) compounds.
This entry is adapted from the peer-reviewed paper 10.3390/ma16010378