Book with abstracts from the COST Action 0905 meeting in ... - UMB

Fig 1, 190x showing biochar’s variously
sized pores (A.) and longitudinal, vertically
cut cross-section of those pores at 550x
magnification (B.).
Columns were leached upwards from their
base continuously with de-ionised water (pH
5.5) at 0.1 ml min -1 flow rate, maintained by
a peristaltic pump. Two of the columns
containing biochar were linked in parallel to
the preceding two columns containing soil to
allow biochar to intercept contaminated
eluate directly leached from the soil
columns. Eluate samples (10 ml) from soil
were collected prior to passing through the
biochar columns, by means of a small outlet
at the joint between columns which
remained closed at all times except during
sample collection. The remaining two
biochar filled columns were leached by the
same method, but independently of the soil
columns, to provide a biochar control.
Columns were leached for 8 weeks in total.
For the first 5 weeks (fractions 1-5) eluate
was collected from all of the columns. In
fractions 6, 7 and 8 (weeks 6, 7 and 8)
leaching of soil columns was terminated, but
both the intercepting biochar and biochar
control columns continued to be leached as
previously described. Arsenic, Cd, and Zn
concentrations in the eluate samples were
determined by ICP-MS. For the SEM/EDX
analyses samples of soils and biochar both
before and after leaching were dried and
encased in epoxy resin, stuck to microscope
slide, covered with colloidal graphite to
ensure conductivity and analysed with a Jeol
6400 SEM microscope equipped with an
Oxford detector. Images of EDX dot maps
of As, Zn, Cd and SEM images of biochar
structure were acquired and processed with
INCA software (Oxford Inst.).
Results
Arsenic concentrations were by far the
lowest of all elements measured in the eluate
from the soil (< 10 µg l -1 ), but did increase
slightly when soil eluate was passed through
biochar (Fig 2a & b), suggesting a
cumulative effect given that concentrations
were also very low from the biochar alone.
Unlike arsenic, cadmium and zinc
concentrations in eluate from soil were high,
especially in the first 2 leaching fractions
(200-500 µg l -1 in each fraction) which
resulted in cumulative removals of ~ 4 mg
Cd per kg -1 and ~ 2.5 mg Zn per kg -1 of soil
over the 5 fractions of the column test.
Biochar rapidly and significantly (p < 0.05)
rapidly reduced concentrations of both Cd
and Zn as it intercepted contaminated eluate
from the soil columns (Fig 2e & h). Given
biochar’s large surface area (Fig 1)
SEM/EDX detected greater sorption of As,
Cd and Zn on biochar’s surface following
the column test, corroborating the leaching
test results. Furthermore, this microanalyses
also confirmed de-sorption from the surfaces
of soil, illustrating the relocation of soluble
contaminants from soil to biochar.
Discussion
Biochar alone was free from significant
amounts of As, Cd and Zn in water-soluble
form and would be safe to use as a soil
treatment without itself contributing to any
environmental toxicity effects. Its structure
might explain some of the effects observed
as oxidation could occur most rapidly on
outer surfaces, followed by interior pores,
which might result in differential element
retention between surface and interior sites
as a result of shifting CEC for example.
Nonetheless, biochar has proven to be
effective at reducing high concentrations of
soluble Cd and Zn originating from a
contaminated soil and we can now more
affirmatively say that sorption is one of the
mechanism by which those metals are
retained.