When extraction is performed on retted material,the scutching step has a significant impact on the strength and modulus compared to the breaking step.However,as a large part of the fibres weakened by the scutching stage are eliminated during hackling and transformed into hackling tows,this explains the slight rise in modulus observed at the hackling stage.In this case,the modulus and breaking strength after hackling are not significantly different to the ones determined after the breaking step.The tensile property analysis also shows that dew-retting has no significant impact on the strength and modulus of the fibres after hackling.Student’s tests have also shown that there is no significant difference between the fibres obtained at the end of hackling and the reference material,both for retted or un-retted stems.The mechanical potential of the elementary hemp fibres is not affected by the scutching/hackling steps and the level of tensile property and modulus of elasticity.This is globally lower than the potential of flax fibres extracted manually and reported in the literature by about 20 %,but highly sufficient for load bearing composite use.The properties obtained in this work cannot be compared directly as very few studies considered the tensile properties of hemp fibres after scutching and hackling extraction.In most of the studies,the fibres were extracted by hand or with more aggressive devices such as hammer mills,or a mechanical fibre opener.The tensile properties obtained in this work are also larger than the ones obtained by Liu et al.for hemp extracted manually,therefore showing the quality of the fibre extraction.The results obtained at the end of hackling are also compared to those collected at the end of industrial scutching/hackling devices carried out on retted stems from the same batch as the one extracted on the lab-scale device.Fig.8 shows that industrial extraction has a strong and significant impact on both the modulus and the strength with strong decreases compared to extraction on the lab scale device.

At this stage,cannabis grow equipment it has not been possible to identify which process is the most damaging for the fibres in comparison to the lab scale.Both processes are expected to contribute to the fibre property loss in comparison to the lab scale results.In any case,the process parameters used at the industrial scale are expected to be too aggressive and probably damage the fibre by generating defects on their structure.In order to confirm these observations,an analysis of the kink-band defects observed on the fibres at the end of hackling for the two batches extracted with the lab scale and industrial scale equipment was carried out.The results presented in Table 3 show that the number of kink bands varies little from one batch to the other.When the scutching/hackling of the retted material is carried out industrially,there is still no significant difference in the number of defects in the fibres,but the percentage of the fibre surface occupied by the kink bands has been increased ,but still insignificantly,from 12.2 % to more than 17 %.Industrial scutching/hackling therefore probably causes larger defects on the fibres and this may explain the decrease in both modulus and strength for industrially extracted fibres.To improve both fibre yield and tensile properties after industrial extraction,the authors recommend to reduce the processing speed especially with well-retted stems which are more delicate and to introduce the hemp stems as homogeneously as possible.The magnitude of the processing speed will depend of the hemp level of retting,but a reduction by a factor ranging from 1.5 to 2 is recommended and needs to be tested in next trials.Of course,the reduction in scutching speeds will lead to a reduction in the rate of production.However,it should be considered that the decrease in speed should be accompanied by an increase in the long line fibre yield and the preservation of mechanical properties adapted to load-bearing composite materials,which is not currently the case.With such improved process parameters and way of introduction of the stems it is expected to improve both fibre yields and tensile properties of the fibres to values close to the ones of flax.Phytoremediation is a mainly solar energy-driven technology that uses plants and agronomic techniques to remediate and restore contaminated soils.This method has several advantages ,as well as limitations.The first approach of phytoremediation was to use hyperaccumulators for phytoextraction or accumulators for phytostabilization.However,as the research and field application results were released,it became obvious that a crucial bottleneck in the technology was its slowness and economic viability.

A new phytoremediation approach was subsequently developed,the so-called phytomanagement,in which high-biomass and high-value industrial cash crops are cultivated on contaminated lands to produce useful biomass feedstock for bioproducts and bioenergy.The main advantages of this method are they: produce economic revenue for farmers and rural areas,contributing to the wellbeing of the local population; boost local economies by developing innovative entrepreneurship for industrial applications of the biomass produced; increase the availability of domestic raw materials and the ability for them to be used in new emerging markets; exploit land that cannot be used for food production; mitigate the route of exposure for the intake of contaminants by humans,etc.The main bottleneck of this method is that it becomes effective only when the contaminated site is a large area of unused/abandoned arable land,in which agricultural practices and mechanization can be applied.A group of crops that have many attractive characteristics for phytomanagement purposes are fiber crops.Among them,flax ,kenaf and hemp  are very promising candidates since they have been reported to show a tolerance to toxic trace elements in soils,are fast-growing and yield a high biomass,have low input requirements,use well-known agricultural practices,and are multipurpose.Several scientific works showed that flax is relatively tolerant to and in some cases an accumulator of Cd,Cu,Pb,and Zn ; kenaf accumulates Cd,Pb,Zn,and Cr;and hemp accumulates Cd,Ni,Pb,Zn,Cu,Cr,Hg,As,Mn,Se,Rn,Tl,and Sr.These crops have well-known large-scale agronomic and harvest practices and a plethora of industrial uses.Attention most certainly has to be paid to biomass exploitation since a small amount of heavy metals and metalloids may be taken up by the biomass produced.Especially if fibers are used for clothing,they might represent a risk to health and should be carefully monitored.Nevertheless,De Vos et al.demonstrated that hemp fibers concentrated Cd and Pb in levels far below the thresholds for textile product according to STANDARD 100 by OEKO-TEX 2020,while Zn is not considered a toxic element in textile production.The aim of this work was to find crop and variety differences between FKH in their tolerance and potential for Cd,Ni,Pb,Zn,Cu,and Sb accumulation.

The research should generate data for the possible use of each crop/variety for phytomanagement of contaminated soils.Before sowing,water was added to each pot to adjust the soil moisture to 70% of the field capacity,which was measured by the static approach as defined by Reynolds.The seeds of each crop and variety were sown into pots on 4 March 2020.The initial seed germination rate was taken under consideration,and the target seedling number per pot and the actual sowing number for each variety and crop are shown in Table 3.Ten days after sowing,the number of seedlings per pot was recorded to determine the effect of HM&M stress on the germination rate of each variety.During the experimental period,the seedlings were observed for phytotoxicity symptoms every three days.The soil was kept moist by spraying pots every five days with a 100 mL of water.Weeds and pests were controlled in a timely manner.Seven weeks after sowing,the aboveground part of each crop was harvested and the plant height,stem diameter,number of leaves,and the fresh biomass per pot were measured.Then the plants from each pot were dried ,weighed and ground into powder using a cross-hammer beater mill.Plant samples were digested with 5–10 mL nitric acid in vessels,left for 1 h,and then heated in a microwave digestion apparatus.Subsequently,the vessels were placed on a temperature-controlled electric heating plate for 30 min at 100 ◦C to reduce the volume of the solution to approximately 2 mL and the sample volumes were adjusted to 25 mL with deionized water.The resulting solutions were filtered through a Whatman No.42 filter paper and were analysed for HM&M using ICPMS.At this point it should be noted that under Cu and Zn treatments,the seeds of all varieties did not germinate,indicating the severe toxicity of the applied contamination levels of both heavy metals to FKH.Several studies have been published on the phytoremediation potential of FKH; generally,in these studies,low to moderate soil contamination levels were examined.However,in this experiment,extremely high concentrations of the applied HM&M were used in order to evaluate the tolerance of FKH and their uptake potential.The plants were subjected in even higher stressing conditions since this was a pot experiment with artificially contaminated soil,resulting in higher soil availability of the toxic elements.The presented study was quite complex,covering three varieties of each of three crops in three replicates,while for each treatment,eight measurements were taken.The scientific questions to be answered were: which fiber crop and variety is most tolerant to the tested HM&M; which crop/variety takes up the most HM&M for phytoextraction purposes; and which crop/variety does not take up the tested elements,thereby producing a clean biomass for further transformation to bioenergy and/or bioproducts.

These findings will play a key role when designing a phytomanagement plan for areas heavily contaminated due to manufacturing,mineral extraction,abandonment of mines,accidental spills etc.In this work,vertical grow system seed germination per variety and crop was tested since it is the simplest and rapid method for evaluating the phytotoxic effects of inorganic contaminants on plants.The germination of seeds and the early stages of seedlings growth are more sensitive to metal stress as several defense mechanisms have not yet been developed.The heavy metals Zn and Cu are classified as essential micronutrients,involved in many important biochemical and physiological processes of plants.However,their presence in excess can lead to a significant reduction of seed germination and seedling growth.In this study and throughout the experimental period,no seedlings emerged for the pots under Cu and Zn treatments,indicating that the pollution levels of both metals were beyond the endurance of the supplied varieties.Thus,no further analysis was made for these two metals.Compared with the control,the addition of each heavy metal and Sb significantly affected the seedling emergence rates of all the varieties of FKH.Flax: Low Cd contamination increased the rates of emergence of three flax varieties when compared to the control,with the highest rate of 67.1% for Y2I329 and the lowest of 49.5% for Zhongyama No.1.The high Cd concentration reduced the emergence of all flax varieties,with the highest emergence rate of 53.3% for Y2I328 and the lowest one of 35.2% for Zhongyama No.1.Compared with the control,the three flax varieties did not emerge in Ni220; the emergence rate of Y2I329 was reduced for Ni110,but was significantly increased for Y2I328,and was slightly increased for Zhongyama No.1.Compared with the control,Pb1500 showed a tendency to increase the emergence rate of the three flax varieties,among which Zhongyama No.1 obtained the highest rate but Y2I239 the lowest; Pb3000 had no significant effect on the emergence of flax compared with Pb1500.Compared with the control,Sb50 increased the emergence rate of flax varieties; Sb100 reduced the emergence of Y2I328 and Zhongyama No.1 but increased that of Y2I329.Kenaf: Different metals significantly influenced the emergence rate of this crop.Treatment Cd40 notably or slightly increased the emergence rates of the three kenaf varieties,while Cd80 significantly reduced them.Ni110 significantly reduced the emergence of GSS,but did not notably affect the other two varieties,Ni220 significantly reduced the emergence of HP and slightly reduced that of GSS.Pb in this study showed no significant effects on kenaf emergence rate; these results are not in accordance with the findings of Sultana et al.,when two kenaf varieties were subjected to a combined Pb and Cr treatment of 120 and 120 mg/L,respectively.Treatment Sb50 significantly increased and Sb100 significantly reduced the emergence of Hongyou No.2,but did not notably affect that of GSS and HP.Heavy metal stresses also significantly affected the emergence rate of hemp.Compared with the control,Cd40 significantly improved the emergence of Guangxibama and slightly increased that of Zhongdama1,but notably reduced the emergence of Yunma No.1; compared with Cd40,Cd80 significantly reduced the emergence rate of Guangxibama and Yunma No.1 but did not significantly affect that of Zhongdama No.1.An increased germination rate under Cd treatment was also reported by Linger et al.and Seregin and Ivanov.A possible explanation for this is the expression of stress-inducible genes during germination that adjust seedlings’ metabolism to the stressing conditions and better protect them compared with the control seedlings.