**4. Discussion**

In this study, the phytoextractions of soil Zn, Cu, Fe, Mn, Cd, Cr, Ni, and Pb heavy metals by the four vegetables (brinjal, cauliflower, lettuce, and spinach), grown on six sites equally divided into sandy loam and clay loam soils, and where each soil was irrigated with three types of water—normal, sewage, and normal + sewage—were measured. Since comparing the phytoextraction values of the vegetables grown on different textured soils irrigated with different types of waters would not be satisfying, we additionally compared the vegetable transfer factor (VTF; Cui et al. [20]), which is equal to the heavy metal concentration in the vegetable divided by the heavy metal concentration in the soil. The VTF minimizes biases in comparative phytoextraction values owing to differences in heavy metal concentrations among the soil textures as well as the irrigation waters used in the study.

#### *4.1. Effect of Site/Water on Phytoextraction of Heavy Metals*

In this study, heavy metal phytoextraction of all heavy metals was the highest at the Industrial estate study site, which had a clay loam texture and was being irrigated with the sewage water. The overall phytoextractions of heavy metals by the four vegetables at this site were significantly higher than those at the Khan Village and Sameej abad sites, which had sandy loam and clay loam textures, respectively, and were being irrigated with normal water. Moreover, Shujabad road site (sandy loam) vegetables had also clearly higher phytoextractions than those of the crops at the sites being irrigated with normal water. These results are in line with that of Yargholi [36], who also reported significant increases in heavy metal concentrations in soil (or VTF) in response to irrigation with sewage water of high heavy metal concentrations. The toxic concentration of the heavy metals or other micro/nutrients in the irrigation water has frequently been reported, for example, [37,38] to directly concentrate in the plant roots/shoots/leaves, which food may put the health and life of animals and humans in jeopardy.

The highest phytoextraction by the industrial site vegetables was likely due to the greatest soil concentrations of the respective heavy metals owing to continuous use of sewage water at this heavy soil texture site. It also reveals that the soil particle exchange sites of this heavy texture site tend to be fully occupied and are transferring the surplus to the crops, resulting in high VTF factors. A similar phenomenon is also shown by the Shujabad road site; however, the extent of the transfer of heavy metals to the vegetables was much lower due to the light texture nature of the soil, which may, for example, accumulate or supply lesser heavy metals to the plant parts [39].

Overall phytoextraction of heavy metals by vegetable crops showed that spinach had the highest phytoextraction of all the heavy metals followed by cauliflower and lettuce, while brinjal had the lowest phytoextraction of heavy metals, as was also found by Sharma et al. [17,18]. The phyto uptake capability of the spinach crop we found is comparable to

the findings of Ng et al. [40]. The highest heavy metal phytoextraction of spinach could be attributed to its leafy structure, short stature and growth cycle, and the fastest metabolism, which can uptake toxic compounds or heavy metals in toxic quantities and transform or mineralize them to simpler compounds, which can be assimilated or accumulated for the long term. There was a site \* vegetable interaction for spinach at the Industrial estate site, which has a heavy soil texture irrigated with sewage water, again supporting our findings on the individual effects of site and vegetable on the phytoextraction of the heavy metals, and also support the findings of Zia et al. [34], except they used phytoextraction enhancing chemicals.

#### *4.2. Effect of Site/Water on Vegetable Transfer Factor*

In contrast to the phytoextraction values, the VTF values were found to be the highest at the Sameeja abad site of heavy texture and irrigated with a mix of normal and sewage waters (the exact ratio of the mix is not reported since it kept changing). It was found that the crops had visually much higher coverages and biomasses than those at the other experimental sites. Vegetation may attain maximum coverage and biomass when supplied with exponentially dissolved organic carbon and salts or ions in soils [41]. Therefore, a higher VTF at this site could be attributed to greater uptake of heavy metals (relative to its soil concentrations) along with essential nutrients in contrast to the Industrial estate, which had greater concentrations of heavy metals in both the soil and vegetable crop and the VTF was low due to the much higher soil heavy metal concentrations than those at the Sameeja abad site. The findings of Cui et al. [20] corroborate our explanation of the higher VTFs of vegetables due to lower soil heavy metal concentrations.

Interestingly, spinach had overall the highest VTF values of all the heavy metals, similar to its corresponding phytoextraction values. Additionally, the VTF values of cauliflower, lettuce, and brinjal were also in the order similar to those of the phytoextraction values of these vegetables. These findings complement our results on the phytoextraction capacities of the four vegetable crops used in this study.
