Agriculture Reference
In-Depth Information
Lighter, mono-aromatics (one ring) compounds include benzene, toluene, ethylbenzene, and
xylenes (NOAA, 1995). Aromatics with two or more rings are referred to as polycyclic
aromatic hydrocarbons (PAHs) (Anderson, et al., 1974). Used lubricating oil contains several
toxic components including up to 30% aromatic hydrocarbons, with as much as 22 ppm
benzo(a)pyrene (a PAH). Upshall et al (1992) reported that motor oil had a density of 0.828
g/ml and contained 14% aromatics and 65.4% aliphatics (by weight). In their study, the sum
of 26 individual PAHs represented 0.17% of the oil, or 1.2% of the aromatic fraction.
The main threats to human health from heavy metals are related with exposure to lead,
cadmium, mercury and arsenic (arsenic is a metalloid but is usually classified as a heavy
metal). Heavy metals have been utilised by humans for thousands of years. Exposure to
heavy metals continues although several adverse health effects of heavy metals have been
known for a long time. For example, mercury is still used in gold mining in many parts of
Latin America. Arsenic is still common in wood preservatives, and tetraethyl lead remains a
common additive to petrol, although this use has decreased dramatically in the developed
countries. Waste-derived fuels are especially prone to contain heavy metals which should be
a central concern in the consideration for their use. Since the mid 19th century, production
of heavy metals increased abruptly for more than 100 years, with associated emissions to the
environment, particularly in less developed countries though emissions have lessened in
most developed countries over the last century.
Some heavy metals are dangerous to health or to the environment (e.g. mercury, cadmium,
lead, chromium), some may cause corrosion (e.g. zinc, lead), some are harmful in other ways
(e.g. arsenic may pollute catalysts). Some of these elements are actually necessary for
humans in minute amounts (cobalt, copper, chromium, manganese, nickel) while others are
carcinogenic or toxic, affecting, among others, the central nervous system (manganese,
mercury, lead, arsenic), the kidneys or liver (mercury, lead, cadmium, copper) or skin,
bones, or teeth (nickel, cadmium, copper, chromium). One of the largest problems
associated with the persistence of heavy metals is the potential for bioaccumulation and
biomagnification causing heavier exposure for some organisms than is present in the
environment alone. Through precipitation of their compounds or by ion exchange into soils
and muds, heavy metal pollutants can localize and lay dormant. Unlike organic pollutants,
heavy metals do not decay and thus pose a different kind of challenge for remediation.
3. Remediation techniques for hydrocarbon and heavy metal contaminated
soil
Internationally, petroleum contamination is widespread, posing serious environmental risks
including surface and groundwater contamination (Balasubramaniam et al. 2007). The
environment can potentially be affected by numerous operations in petroleum exploration,
production and transportation, with common sources of contamination being leaking
underground storage tanks (Nadim et al., 2000). Contamination poses serious
environmental risks, including surface and groundwater contamination, and risks to human
health and safety (Balasubramaniam et al. 2007). Remediation of contaminated soil is an
essential practice. Some of the different techniques used in remediating contaminated soil
are discussed below.
3.1 Physical and chemical remediation techniques
3.1.1 In situ soil vapour extraction
Volatile and some semi-volatile organic compounds (VOCs and Semi-VOCs) can be
removed from unsaturated soils by a process known as soil vapour extraction (SVE). SVE as
