Water Supply and Quality - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 155

micropores, which strongly adsorb hydro-
phobic organic compounds already at low
concentrations (Kleineidam et al ., 2002).
With the suggested application of biochars
in agricultural soils, consideration of carbon-
type specific contaminant sorption becomes
ever more relevant (Bushnaf et al ., 2011;
Khan et al ., 2013). Urban soils, but also soils
in flood plains of industrial countries, also
contain various forms of tars, coke, chars
and soot, for example from sealcoats, coal-
based steel and electric power production,
which are highly loaded with polycyclic
aromatic hydrocarbons (Yang et al ., 2008,
2010). How stable these different forms of
carbon are in soils and whether carbon-
sorbed contaminants would eventually be
released is unclear. 'Normal' soil organic
matter derived from modern plant residues
may be much less stable than the thermally
altered form, and degradation of organic
carbon may lead to water-soluble fractions
of organic carbon.
The water purification capacity of soils
has come under increasing pressure since
the industrial and agricultural revolution.
In industrial soils, organic carbon of natural
or anthropogenic origin was frequently loaded
with legacy contaminants, to such an extent
that soils became a long-lasting secondary
source of pollution of seepage water or air.
Similarly, rural soils have been, and still are,
sinks for atmospheric pollutants or pesti-
cides, and may become sources in the future.
Whether or not soil pollutants finally end up
in groundwater depends on the pollution at-
tenuation by sorption and biodegradation.
Both processes are crucially influenced by
soil organic carbon. This applies especially
for organic compounds, but also for many
inorganic species.

Water Filtration

The role of soil organic carbon in the
sorption and retention of pollutants

Soils are the most important filters for rain-
water on its way down to groundwater.
Almost all drinking waters at some point
have been filtered by soils. The organic pol-
lutant retention capacity of soils typically
increases with organic carbon content, and
organic carbon normalized sorption coeffi-
cients play an important role in predicting
the organic pollutant fate. Soil organic mat-
ter also contains functional groups such as
carboxyl, hydroxyl and amine groups, which,
depending on pH, may contribute substan-
tially to the soil cation exchange capacity,
and thus the retention of cationic inorganic
pollutants. Increased soil organic carbon
contents are thus beneficial for the purifica-
tion of seepage water and the retention not
only of, for example, pesticides in agricul-
ture (Werner et al ., 2013) but also of the
many compounds from urban space or at-
mospheric pollution.
Soil organic matter comes in various
forms (Plate 8) that interact distinctly with
contaminants (Grathwohl, 1990). More ma-
ture, thermally altered forms of organic car-
bon such as coals, or combustion-derived
carbon such as charcoals and soot adsorb
hydrophobic organic chemicals much more
strongly than normal soil organic matter.
On the other hand, more mature, thermally
altered forms of organic carbon tend to have
lower elemental oxygen-to-carbon ratios,
and hence a lower abundance of functional
groups. The extreme heterogeneity in terms
of the nature of particulate organic matter in
soils is largely unknown, although fluores-
cence microscopy techniques exist to iden-
tify and quantify a large variety of organic
matter and black carbon particles (Ligouis
et al ., 2005) (some examples are shown in
Plate 8). These different types of organic mat-
ter particles in soils have different sorption
characteristics that depend on organic mat-
ter facies (Kleineidam et al ., 1999; Karapa-
nagioti et al ., 2000). Thermally altered organic
matter (e.g. kerogens, coals, chars) contains

Combined Effects of Sorption and
Biodegradation on Water Filtration

Contaminant sorption by soil organic car-
bon is beneficial in the short term, as it re-
moves pollutants from seepage water, and
may reduce toxic effects significantly by
reducing the contaminant availability for