Page 235 - Plant-Based Remediation Processes

11 Flax (Linum usitatissimum L.) and Hemp (Cannabis sativa L.)... 227

The several examples of on site studies mainly show that results of greenhouse
pot experiments (or even hydroponic experiments) very often do not correspond to
field data (very complex environment). Nevertheless, these limited number of on-
site studies with flax and hemp may put the more realistic view on the abilities of
studied crops and help to solve the question, how the phytoextraction technology
may be efficiently connected with practical agricultural production?

11.5.5 Time Needed for Cleaning Heavy Metal-Polluted Soils

The key point for utilisation of flax/linseed and hemp for HMs phytoextraction is total
absorption of metal element from square unit during vegetation period. Probably first
estimation of time needed to clean soil contaminated by Cd in fibre crops, namely in
1
flax was reported by Bo ¨hm et al. (1992). The authors started with value 3 mg Cd kg
soil (after application of sewage sludge) in the upper layer of soil, which represents in
1
total 9 kg Cd ha . If the seed Cd concentration was 3.6 mg and stem Cd concentra-
1
tion 5.6 mg kg 1 DW, and mean yield of seeds 2 tonnes ha 1 and stem 4 tonnes ha ,
1
the total Cd uptake from 1 ha per vegetation season is ca. 30 g ha . Thus, time
1
needed for complete Cd decontamination is 9,000 g /30 g ¼ 300 years. Similar
phytoextraction potential we have estimated on tenfold higher soil Cd concentration
(30–35 mg Cd kg –1 soil; Bjelkova ´ et al. 2011a, b). Kos et al. (2003) estimated flax
–1
phytoextraction potential of above-ground tissues as 49 g Cd ha , 1.99kgPbha –1
–1
and 0.70kgZnha ; phytoextraction potential for hemp was calculated as follows:
1
1
9.57 kg Pb ha ,3.68 kgZnha , and 44 g Cd kg ha 1 (EDTA-stimulated improved
HMs uptake from soil); unfortunately, developmental stage of flax and hemp plant at
the harvest was not provided by authors and calculation was partly done based only
on literature data on above-ground dry matter biomass. It was reported earlier that
maximum concentrations of HMs were determined in vegetative growth and the final
concentrations in the plant maturity are substantially lower (Cieslinski et al. 1996;
Linger et al. 2005). All these facts are necessary to be taken into account during the
estimation of phytoextraction potential—only mature plants have a sense in that
relation. Linger et al. (2002) estimated hemp phytoremediation potential as 126 g
Cd ha 1 per vegetation period. The same authors increased in the their next
1
study this estimation up to 830 g Cd ha per vegetation period (on soil with
17.3 mg Cd) (Linger et al. 2005). Based on these observations/estimations, we can
expect that Cd-phytoextraction potential of flax/linseed on medium-polluted soils
will not overcome 50 g Cd ha –1 per season, in case of hemp this value may be
10 times higher. In such a case, the complete decontamination of soil from Cd may be
hypothetically shortened on some tens of years. Nevertheless, in reality the problem
is not so simple due to need for crop rotation (in order to avoid soil-borne diseases
attack as a result of repeated cultivation on the same field). Examples of HMs
absorption by some fibre crops are provided in Table 11.6.