Consider this situation (yes, it has really happened, near Rotterdam, The Nether
ID: 208019 • Letter: C
Question
Consider this situation (yes, it has really happened, near Rotterdam, The Netherlands): Houses were built on a site where landfill with dredge sludge from a harbor was used to elevate the ground. The new owners started growing foods in their gardens, only to find that their lettuce had unacceptably high levels of cadmium in it. Further study revealed that the soils were highly contaminated, not just with cadmium, but with other metals and arsenic, and organic compounds, such as polycyclic aromatic hydrocarbons (PAHs). Using the terms chemical speciation, extractability, fractionation and bioavailability, explain how you would go about investigating the site, and what considerations you might have for the design of a remediation and management strategy. (NOTE: I am not asking you to design a remediation and management strategy, but to write about what considerations you might take into account).
Explanation / Answer
The landfills that are dumped with toxic dredge sludge, contain several toxic metals or aromatic hydrocarbons, that impose health hazards to all living creatures, including humans. To analyse the toxicity of these metals, it is important to know the chemical form in which the metal is present in the specific sample to be studied. For application of a suitable technique for bioremediation or extraction of such toxic metal, it is essential to understand their chemical speciation or in which chemical form they exist I that environment. Thus, priority should be given to understand chemical speciation of the toxic metals, rather than determining the total content of the metal in the landfill. Chemical speciation of the metal will evaluate the availability of the metal in a chemical form, its association with other elements in the soil (geochemical form), its reactivity with water, acids or other substance, (that may be utilized for its extraction), its degradability status with microorganism, conditions to be applied for degradation (aerobic or anaerobic).
Assessment may be conducted by calculating the geochemical index of the metal, in a sample as:
Igeo= log2 Ci/1.5Bi
Where, Igeo= geochemical index.
Ci= = measured concentration of the metal
Bi= geochemical background of the metal
Value of, Igeo, determines the index of pollution of a metal in the sample. Example, if Igeo=0, the sample is unpolluted for that metal, Igeo greater than 5, the soil is extremely polluted, while values in between, indicate moderate pollution.
Further, extractability of the polluting element is analysed, by multiple extraction procedure (MEP), using variety of solvents, acid digestion (AD), toxicity characteristic leaching procedure (TCLP), to determine the effective technique that can be applied.
Fractioning of the pollutants are conducted to determine the mobility of metal into different extractables like, residual, oxidizable, reducible, acid extractable, etc.
The chemical speciation of the toxic metal in the sample also indicate its bioavailability. The bioavailability of the metal is the measurement of the amount of chemical forms of the metal that can potentially harm humans or animals.
The toxic exposure to humans or animals may occur via, dermal contact, oral in take or inhalation. Bioavailability measurement, is conducted by evaluation the chronic daily intake (CDI) of the toxic substances, as CDIinh (inhalation), CDIing (ingestion) or CDIdermal(dermal contact).
Thus chemical speciation, extractibility, fractionation and bioavalibilty of the toxic component needs to be analyzed during investigating a site, before specific remediation processes are applied.