## 4 Discussion
Trends of decreasing HM accumulation with urban to rural gradients were also noted by [PERSON] and [PERSON] [64] in a study of HM accumulation in forest soil. In the present work, similar trends were observed in the Cu, Cd, Pb, and Zn concentrations, suggesting that these pollutants accumulated highly in roadside areas compared to rural areas. In rural areas, a high concentration of metals is attributed to the fact that these areas become a wasteland where _C. procera_ grows abundantly. The HMs have been recorded in the order: roadside areas > residential areas > rural areas. In the present study, Zn contents in _C. procera_ samples rangedfrom 103.70 mg kg\({}^{-1}\) in roadside areas to 26.15 mg kg\({}^{-1}\) in rural areas. [PERSON] [73] found Zn concentrations in vascular plants between 15 and 100 mg kg\({}^{-1}\), with 230 mg kg\({}^{-1}\) being toxic. In contrast, [PERSON] and [PERSON] [74] revealed that Zn concentrations in vascular plants ranged from 10 to 200 mg kg\({}^{-1}\), with Zn essential plants [75]. In the unpolluted region, the zinc level in the upper horizon was 9 mg kg\({}^{-1}\), whereas in the contaminated area, the zinc content averaged 71 mg kg\({}^{-1}\) as reported by [PERSON] and [PERSON] [76]. Zn concentration in leaves of _C. procera_ was shown to be safe in both rural and urban areas. Cu is an essential element for photosynthesis and the metabolism of protein and carbohydrates [77]. The soil Cu concentrations ranged from 15.24 \(\pm\) 0.96-3.82 \(\pm\) 0.37 mg kg\({}^{-1}\) at various locations and plant components, whereas the maximum permissible level for Cu concentration is 10 mg kg\({}^{-1}\) in plants [78]. Consequently, the plants collected from the roadside were regarded as Cu-toxic. For photosynthesis and protein and carbohydrate metabolism, Cu is needed [77]. There may be harmful implications if copper levels in shoots or leaves exceed 20 mg kg\({}^{-1}\)[75]. In biochemical methods, _C. procera_ roots were used as a biosorbent to extract Cu from solutions with known amounts of the metal [79]. Pb has no benefit in plant nutrition and is found in plant tissues at concentrations of 1-10 mg kg\({}^{-1}\)[74]. Toy manufacturing, printing and the oil industry produce contaminated water and exhaust gases, which contain Pb. Pb concentrations greater than 5 mg kg\({}^{-1}\) are considered toxic to plants [8]. Pb levels in _C. procera_ plants from roadside areas (19.45 \(\pm\) 0.68) and urban areas (10.36 \(\pm\) 0.89) were toxic, while rural areas (3.29 \(\pm\) 0.50) were normal. It might be because Pb is mainly stationary in soil, lowering its bioavailability and subsequent plant absorption [80]. The concentration of Cd in the soil of different sites and plant parts was found in the range of 3.69 \(\pm\) 0.05 to 0.90 \(\pm\) 0.02 mg kg\({}^{-1}\) for road and rural areas and 2.275 \(\pm\) 0.15 -1.46 \(\pm\) 0.08 mg kg\({}^{-1}\) for roadside and rural areas in plants, respectively. Furthermore, Cd is a hazardous element that occurs naturally, accompanying Zn. Cd is less readily absorbed by soil and organic matter, making it more readily available to plants [81]. Metal manufacturing, plastics, household wastes, automobile fossil fuels, and sewage are the most significant sources of Cd contamination. Plants in an unpolluted environment contain 0.01-0.30 mg kg\({}^{-1}\) Cd [73]; hence, Cd concentrations in all tested environments were in the phytotoxic range. Tables 4 and 5 show the phytoremediation potential of Pb, Zn, Cu, and Cd using the BAF from soil to plant and the TF in plant parts. The BAF can assess the plants ability to take HMs from soil; however, it is not a precise estimate of HMs concentrations [82; 83; 84]. According to [PERSON] et al. [85], if the BAF is <1, plants cannot be used for metal phytorextraction. Plants with a BAF value of <1 are not appropriate for metal phytoextraction [86]. Cd > Zn > Cu > Pb was the order of BAF for HMs by _C.