## 3 Results and Discussion
### Heavy Metal Concentration in Sweet Potato Samples  
The concentrations of Pb, Zn, and Cd in the sweet potato samples are presented in Table 5, which shows that the HM concentrations of tubers and leaves differed significantly. The HM concentrations were significantly higher in the sweet potato samples than in the control samples. The highest HM concentrations were found in Field 3, in which the Pb, Zn, and Cd concentrations in tubers were 34.0 \(\pm\) 0.03, 94.2 \(\pm\) 0.07, and 1.19 \(\pm\) 0.03 mg/kg and in leaves were 32.5 \(\pm\) 0.09, 59.4 \(\pm\) 0.01, and 2.75 \(\pm\) 0.01, respectively. Furthermore, in the  
Figure 4: SEM analysis of metals in slag from the lead factory: (**a**) the magnified image is 400 times larger, (**b**) 3000 times larger, (**c**) structural analysis of lead slag. Figure 3: Spatial distribution map of heavy metals: (**a**) Pb, (**b**) Zn, and (**c**) Cd. experimental sites Field 1 and Field 2, the Pb, Zn, and Cd concentrations in tubers varied from 28.7 \(\pm\) 0.07 to 45.1 \(\pm\) 0.09, 70.0 \(\pm\) 0.05 to 94.1 \(\pm\) 0.09, and 0 to 1.80 \(\pm\) 0.02 mg/kg, respectively. At the control site, the tubers showed 0.96 (Pb), 38.5 (Zn), and Cd was not detected. Thus, in the control samples, the Pb and Zn values were 31 and 2.14 times lower. The concentrations of Pb, Zn, and Cd in leaves from Field 3 showed very high values of 32.5 \(\pm\) 0.09, 59.4 \(\pm\) 0.01, and 2.75 \(\pm\) 0.01 mg/kg, respectively. Cd was detected in the sweet potatoes grown at the Field 3 experimental site. Pb concentrations in Field 1 and Field 2 were 2.34 \(\pm\) 0.06 and 10.1 \(\pm\) 0.07 mg/kg, respectively, whereas at the control site, it was 1.09 \(\pm\) 0.02 mg/kg. Additionally, Zn concentrations in Field 1 and Field 2 were 8.24 \(\pm\) 0.06 and 14.1 \(\pm\) 0.01 mg/kg, respectively, whereas the control sample measured 7.6 \(\pm\) 0.01 mg/kg. According to the results in Table 5, Pb concentration was highest in mine tailings and lower in soil and sweet potatoes. However, Zn concentrations were lower in lead slag samples but showed higher concentrations in soil and sweet potatoes than other HMs. Different behaviors were reported for Zn concentration. For example, high Zn concentration accumulated in control and contaminated sites [40]. HM (Pb, Zn, Cd, Cu) in soil samples transferred in grains, but Zn concentration was not predicted by total HM in soil [41]. The results of other researchers highlighted different absorption levels of HM, where sweet potatoes cultivated from contaminated mine tailings soils and sweet potato cultivars influenced accumulation properties [42]. According to data presented by other researchers involving 14 sweet potato varieties, the tendency for Cd and Pb to accumulate in tubers was consistently proved, but a lower index was indicated in shoots and roots [43]. Pb accumulation in roots was found to be characteristic of rapeseed (_Brassica napus_), mustard (_Brassica carinata_) [44], red amaranth (_Amaranthus gangeticus_) [45], and water spinach (_Ipomoea aquatica_ Forsk) [46]. Maize (_Zea mays_ L.) accumulated Cd in its leaves [47], and lettuce was characterized by Cd accumulation in its roots [48]. Other studies considered Zn accumulation. Compared with these results, low concentrations of Zn were found in radish [49]. On the other hand, high concentrations were reported for leek, paddy rice (_Oryza sativa_ L.) [50], and maize [47]. According to our data, there was a clear trend of HM accumulation in tubers (Table 5). Such stability and accumulation of HMs in the plant root system were characteristic of some hyperaccumulator ecotypes [51], in which HMs were predominantly accumulated in the plant roots or root walls [52]. It was proved that in T. _careulescens_, the main part of Cd was stored in the root apoplast [53], and other studies showed that Cd retention after root uptake occurred in the root cell walls [54]. Pb also accumulated in the root walls or in the vacuole [55].