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METALS AND GROUNDWATER PART I - SOURCE AND MOBILITY
An On-Line Version of a Column First Published in the:
National Environmental Journal Vol. 3 No. 6 Nov./Dec. 1993 Pages 24-25

by: David B. Vance

It is estimated that over 60% of CERCLA sites in the United States have contamination due to metals. Similarly, RCRA, general industrial, DOD and DOE facilities also have problems with metals. This, in conjunction with a paucity of innovative approaches to deal with metal problems, means that metal contamination represents a serious environmental concern and a significant market opportunity for environmental service companies.

Metals (or other inorganics) typically become groundwater problems under the following situations:

  • Activities associated with plating shops, where a wide variety of metals are present at high concentrations in forms that are soluble.
  • Sites with releases of radionucleides that, due to unique health risks, can have significant impact at very low concentrations. In addition, the use of chelating and complexing agents, making these contaminants mobile in the environment, is common during processing of these materials.
  • Metals and high levels of inorganic Total Dissolved Solids (TDS) are associated with leaks from sanitary, solid waste and hazardous waste landfills.
  • High TDS impacts are also associated with salt storage areas and petroleum production activities.

 Innovation in the area of hydrocarbon remediation has been significant in the last decade. Metals may only be mobilized or immobilized, unlike hydrocarbons they can not be degraded to less innocuous components (CO2 and H2O for example). This limited reactivity is one of the reasons that innovative remediation technology has not been developed for soils or groundwater contaminated with metals. This will likely change between now and the turn of the century.

Often metal contamination is confined to the upper few feet of soil beneath a contaminated area. However, there are instances where metal contamination has impacted groundwater. The purpose of this column is to discuss the physical/chemical behavior of metals with a particular focus on conditions that make metals mobile and thus able to impact groundwater. The next column will look at remedial alternatives to metal contaminated groundwater. The important issue with regards to metals is mobility. Specifically, under what conditions are metals mobile and what conditions immobile? Metals in the environment can take four fundamental forms: as raw metallic elements, as hydrated ionic salts, in covalently bonded molecules termed inorganic complexes, or associated with a chelating agent.

Elemental metals are not highly soluble under normal groundwater conditions. Although, particles of elemental metal may cause a soil sample to fail a TCLP test.

The mobility of metals as hydrated ionic salts is dependent first, upon which metallic element is participating as the positively charged ion (termed the cation) and secondly, which anion makes up the negatively charged component of the salt.

Following is a brief summary of cationic/anionic solubility relationships:

  • Sodium (Na+), Potassium (K+) and Ammonium (NH4+) are cations that form salts which are all soluble.
  • All metal salts of the Nitrate (NO3-), Nitrite (NO2-), Acetate (C2H3O2-), Permanganate (MnO4-), Perchlorate (ClO4-), and Chlorate (ClO3-) anions are soluble.
  • All Chloride (Cl-), Bromide (Br-), and Iodide (I-) salts are soluble except those of Lead (Pb2+), Silver (Ag+), and Mercury (Hg2+).
  • All Sulfate (SO42-) salts are soluble except those of Barium (Ba2+), Strontium (Sr2+) and Lead (Pb2+).
  • All Oxides (O2-), Sulfides (S2-) and Hydroxides (OH-) are insoluble except those of Calcium (Ca2+), Barium (Ba2+) and Strontium (Sr2+).
  • With the exception of Sodium (Na+), Potassium (K+) and Ammonium (NH4+), all metallic salts of the following anions are insoluble: Carbonate (CO32-); Phosphate (PO43-), Sulfite (SO32-), Borate (BO33-), Fluoride (F-) and Silicate (SiO32-).

 A covalently bonded inorganic molecule that contains several atoms (one of more of which are metal atoms) is termed an inorganic complex. Of particular interest and environmental concern are a class of inorganic complexes termed oxyanions. These are compounds composed of metal and oxygen atoms forming an entire molecule (rather than just an isolated metal ion) that is capable of forming a hydrated complex ion. These oxyanionic complexes are often soluble and more importantly have physical/chemical qualities that make them valuable in various industrial processes. In a modern industrial environment, metallic and metalloid oxyanions not rare substances.

The most troublesome and the most common industrial oxyanion is chromate which contains hexavalent chromium (Cr+6). The chromate molecule in turn forms an extremely soluble anionically charged ion. Contributing to its high mobility in the environment is a property of the chromate ion that allows it to be soluble over the entire range of pH. From acidic to basic conditions, the chromate ion simply changes the overall negative charge it carries, all the while staying soluble and not precipitating. This high degree of mobility, in conjunction with its common use, and the fact that it is a known carcinogen, makes it one of the most common problem metals found to contaminate groundwater.

The ability of chromium to form soluble oxyanionic complexes is not unique to it only. There are other metals and metalloids that form inorganic complexes having more or less the same solubility profiles. This includes: molybdenum, vanadium, tungsten, arsenic, selenium and tellurium. These compounds are not commonly utilized by our industrial society. Although, arsenic and selenium can have significant groundwater impact around mining areas and irrigation complexes in the west.

Industrial chemistry often makes use of complexing agents with more that one point of attachment to a central metal atom in a complex. This type of complexing agent is termed a polydentate ligand or a chelating agent. Chelating agents form strong bonds with metals and are in turn extremely soluble. EDTA (ethylenediaminetetra-acetic acid) is a commonly used chelating agent. Chelating agents are used in industrial chemical systems with transition metals, heavy metals and radionucleides. If released into the groundwater these metal bearing chelates are extremely mobile.

It should also be remembered that under some conditions metals in groundwater can be mobile as colloidal sized particles, even though the metal is in an insoluble form. Although not soluble, colloidal particles are so small they may approach the size of ionized species within an order of magnitude. As such they are transportable in the pore spaces of tight aquifer formations.

Lastly, it is important to understand that metals in groundwater will interact with and adsorb to components of the soil matrix. Clays, other mineral components, and carbonaceous material (especially humic and fulvic substances) can all act in this manner. However in the case of metals, the dominant adsorptive component of the soil matrix is iron. Iron oxides (particularly of Ferric (Fe+3) iron) have a very high adsorptive affinity and total capacity for metal ions and oxyanionic metal complexes. This capacity is so great that ferric hydroxide is often used in waste water treatment systems to aid in the removal of soluble metallic species. Knowing the total iron content of the soil matrix is paramount in understanding the fate and transport of metals in groundwater.

Now the question is, given these properties, how can metal contaminated groundwater be remediated? Biological Electrochemical

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