*3.3. Cd and As Content in Rice Tissues*

In the As and Cd alone treatments, the shoot and root Cd contents were 105 and 1643 mg/kg, which were 3.5- and 2.3-fold of the As contents in shoot and root tissues, respectively (Figure 3). The Cd and As contents in rice tissues were reduced upon exposure to 250 mg/L C3N4, particularly the roots (Figure 3). For example, addition of 250 mg/L C3N4 decreased the Cd contents in roots and shoots by 32% and 35%, respectively, as compared with the control. However, due to large variance of the shoot Cd content, the decrease was statistically insignificant as compared with the Cd alone treatment (Figure 3A). Similarly, 250 mg/L C3N4 resulted in approximately 25% and 36% decrease in As in the roots and shoots, respectively (Figure 3B), clearly demonstrating the potential of C3N4 to alter the heavy metal and metalloid uptake and distribution in rice. It is worth noting that the Cd and As translocation factors were not significantly altered as compared with the respective metal controls (data not shown). Exposure to 50 mg/L C3N4 had no impact on Cd and As contents. Yue et al. (2020) also reported that nanoscale biochar beyond 500 mg/kg reduced the Cd content in rice tissues by more than 50% and significantly outperformed bulk-sized biochar [38]. Metal-based NPs have also been shown to decrease the bioavailability of heavy metals to crop species. Sharifan et al. (2019) demonstrated that ZnO NPs' exposure reduced the Cd contents in lettuce roots and shoots by 49% and 30%, respectively [59]. Similarly, both bulk- and nano-sized TiO2 NPs reduced the Pb accumulation in rice [60]. In the current study, we hypothesize that the adsorption of As and Cd onto the C3N4 surface in the rhizosphere subsequently reduces metal bioavailability [43].

**Figure 3.** The Cd (**A**) and As (**B**) contents in rice roots and shoots upon exposure to As or Cd with or without the addition of C3N4. Values of As or Cd contents in roots followed by different lowercase letters are significantly different at *p* < 0.05; values of As or Cd contents in shoots followed by different uppercase letters are significantly different at *p* < 0.05. Single asterisk "\*" indicates the significant difference between control and each treatment at *p* < 0.05 using a Student's *t*-test.
