## 4 Discussion
### Effect of Plant on the Interactions between Pollutants and Carbonaceous Materials  
The former environmental availability assay reflects the ability of BC and AC to increase pollutant sorption on these materials, but it did not consider plant physiological processes that may influence this interaction. Among plants, it is well known that the Cucurbita family accumulate considerable amounts of POPs in their shoots [28]. For this reason, _C. pepo_ was chosen in this study because it highly extracts and accumulates POPs including PCB from contaminated soils [29; 30; 31; 32; 2, 33]. Plant is a major driver of pollutants availability through a variety of root functions such as uptake, exudation that can alter biogeochemical parameters of the soil (e.g., pH and redox potential, concentrations of pollutants, complexing or chelating compounds) in the vicinity of the roots [34]. Roots release protons, responsible for a substantial decrease of rhizosphere pH, that could in turn change the sorption of PCB congeners on soil substrate that is maximum between pH 6.5 and 7.5 [35]. Root exudates like organic acids also play a great role on the sorption/desorption of pollutants from soil components and organo-mineral complexes and subsequently on their availability for plant uptake (Figure 5). The amendment of soil with citric acid that is strongly exuded in _Cucurbita pepo_ ssp. _pepo_ allowed a significant increase of shoot PCB concentration [36] probably due to PCB desorption [32] and/or to root absorption of PCB via complexation [7]. POPs are transported from roots to shoots via the xylem sap [5; 37; 38] and other transfer pathways to shoots are negligible such as direct soil contamination, atmospheric deposition and volatilization from soil and subsequent redeposition on shoots [39]. Our results showed that only the addition of activated carbons to the soil decreases significantly POPs transfer to shoot and thus their extraction from soil by _C. pepo_ roots. Previous studies also showed a reduction of PCB concentration in _C. pepo_ shoot tissue: 63% of PCB (Arocol 1254) with 12.5% AC amendment [18] and 54% of PCB with 11.1% BC amendment [16]. Furthermore, the addition of AC (0.1 and 0.23%; \(w\)/\(w\)) reduced from 73% to 98% of heptachlor epoxide in the shoot of winter squash (_Cucurbit maxima_) [40]. AC amendment (0.02 to 0.08%) also reduced up to 66% of the dieldrin concentration in cucumbers (_Cucumis sativus_ L.) [17].  
Figure 5: In vivo processes leading to PCB contamination of _C. pepo_ aerial parts. Respective tendencies of each category of PCBs (high weight NDL-PCBs–H-PCBs as PCB 180, 153 and 138 or Low weight NDL-PCBs–L-PCBs as PCB 101) to follow each step were mentioned.  
It may be hypothesized that reduction of PCB concentration in shoot tissue is due to the sorption of PCB into the surface of activated carbon, which prevent these molecules from being available in the soil pore water and absorbed into the root, and thus translocated from the root to shoot tissue. This assay involves the use of a sorbent phase acting as an infinite sink for pollutants. Concerning the in vivo assays, the present study used _C. pepo_ as a model plant which extracts pollutants from the water solution and accumulates them in aerial parts.