Acid drainage (AD) refers to the effluent from mining, metallurgy, and various other industrial operations, including tanneries, electroplating, power plants, and battery manufacturing. AD is one of the most obnoxious environmental challenges worldwide, causing contamination to public water supplies, pollution of aquatic ecosystems, and even damage to infrastructure by corrosion.
AD continues to contribute to water pollution significantly. It is characterized by strong acidity, with pH values as low as 2.0, as well as toxicity from a diversity of potential contaminants, particularly heavy metal cations and oxyanions (e.g., cobalt, nickel, cadmium, copper, lead, zinc, chromate, and phosphate). Considering the detrimental effects on aquatic plants, wildlife, and groundwater, the discharge of untreated acid waters into public streams should be strictly prohibited.
Treatment of AD requires removal of cationic and anionic contaminants along with neutralization of the pH. However, currently developed treatment methods such as ion exchange, reverse osmosis, adsorption, and precipitation face significant limitations in satisfying the aforementioned requirements. The methods of ion exchange, reverse osmosis, and adsorption are useful for the removal of toxic contaminants, but generally show little efficacy in pH neutralization.
The approach of precipitation favors the removal of toxic heavy metal cations as well as the consumption of H+, but generally is not efficient in removing anionic contaminants at their relatively low concentrations. Moreover, in order to precipitate metal cations, the liquid pH must be increased to a high value, sometimes upwards of 9.7. Thus, to meet the process demands, a common strategy is to combine two or more methods (e.g. the combination of adsorption and precipitation), leading to higher associated costs.
In the latest work, Minwang Laipan and his co-workers from Guangzhou Institute of Geochemistry (GIG), Chinese Academy of Sciences (CAS) and University of Connecticut (UConn) found that Nature has already provided us effective material to simultaneously remove cationic/anionic contaminants from AD and neutralize its pH. The material belongs to a category of anionic clays called layered double hydroxides (LDHs).
Naturally occurring (i.e., mineralogical) examples of LDH are classified as members of the hydrotalcite supergroup, named after the Mg-Al carbonate hydrotalcite, which is the longest-known example of a natural LDH phase. More than 40 mineral species are known to fall within this supergroup. An LDH is comprised of positively-charged metallic layers with interlayer anions and water. The general formula of LDH is [M2+1-xM3+x(OH)2]x+[An-x/n]x+·mH2O, where M2+and M3+are a metallic bivalent cation and a metallic trivalent cation, respectively, An-is an interlayer anion, and X=M3+/(M2++M3+) is the surface charge, determined by the ratio of the two metal cations. To be noted, M3+can be replaced by M4+in some special cases.
In their research, Laipan et al. took Mg-Al carbonate hydrotalcite as an example and found that its calcined product, MgAl-CLDH, was effective in pH neutralization as well as in the removal of various cationic/anionic contaminants (Co2+, Ni2+, Cd2+, Cu2+, Pb2+, Zn2+, chromate, and phosphate). Simultaneously removing contaminants and neutralizing pH with MgAL-CLDH is made possible by the “memory effect” of LDHs. The memory effect refers to an interesting structure characteristic of LDHs, which are also known as double metal oxides, in that the calcined products can rehydrate and recover to LDH in an aqueous environment.
In this process, double metal oxides capture hydrogen atoms from water, thereby leaving OH– to increase the pH of the liquid. In this way, the pH can be neutralized and heavy metal cations removed. Meanwhile, once the LDH is formed, the surrounding anions are captured into the interlayer space or onto the surface of LDH to compensate charge, resulting in the removal of anionic contaminants. It was determined that the removal efficiencies of various heavy metal cations by adsorption with MgAl-CLDH were much higher than those by precipitation using NaOH, meaning that the adsorptive ability of CLDH/LDH can further contribute to the removal of heavy metal cations.
These findings of Laipan et al. are detailed in the recent publication of Applied Clay Science, in their article titled “Calcined Mg/Al-LDH for acidic wastewater treatment: Simultaneous neutralization and contaminant removal.” The corresponding authors Runliang Zhu, a professor at GIG & CAS, and Luyi Sun, a professor at UConn, are both committed to the study of minerals such as LDHs and their promising applications. The hope is that these abundant and low-cost minerals can offer improvements to current technologies and will soon play an important role in environmental remediation.