*3.2. Fresh Biomass*

After a 14-day exposure, 50 and 250 mg/L C3N4 stimulated the growth of rice as plants grew better than the untreated controls (Figure S1). In the treatments with Cd or As, phenotypic images show overt phytotoxicity to rice in terms of the shoot size; the addition of different concentrations of C3N4 alleviated both As- and Cd-induced toxicity and notably elevated the aboveground biomass (Figure 2A,B). In the C3N4 alone treatment, exposure to 250 mg/L increased the fresh mass of roots and shoots by 17.5% and 25.9%, respectively. Although the presence of 50 mg/L C3N4 also increased the fresh weight of both tissues, large variance across the five biological replicates caused the statistical significance to be

elusive. In the heavy metal alone treatments, both Cd and As resulted in 26–38% and 49–56% decreases in rice root and shoot biomass, respectively, when compared with the corresponding control (Figure 2C,D). It is notable that Cd induced greater phytotoxicity to rice than As. However, the addition of different concentrations of C3N4 alleviated the heavy metal-induced toxicity, leading to significant increases in aboveground biomass in a dosedependent fashion. The addition of 250 mg/L C3N4 increased the Cd-treated root and shoot fresh mass by approximately 47% and 50%, respectively, relative to the Cd alone treatment (Figure 2C,D). Similarly, approximately 51% and 29% increases were evident in As-treated roots and shoots upon exposure to 250 mg/L C3N4 (Figure 2C,D). Similarly, Hao et al. (2021) reported that the addition of 200 mg/L C3N4 increased the shoot height and root length of Cd-treated rice by 14% and 42%, respectively, relative to the Cd alone treatment. Additionally, the authors reported a 20% increase in rice fresh weight upon cotreatment with Cd and C3N4, but a low dose of C3N4 (20 mg/L) had no impact on enhancing the biomass of Cd-treated rice [45]. Nanoscale biochar amendment also increased the dry weight of Cd-treated rice tissues by approximately 20–40% relative to the Cd control; the pigment content in the Cd-treated rice showed a dose-dependent increase with greater nanoscale biochar doses [38]. Conversely, although alkaline fertilizer amendment with or without Mn reduced Cd accumulation in rice, the yield was not significantly altered as compared with the Cd control [56]. The coexposure of nanomaterials and an organic contaminant, TiO2 NPs, also alleviated the tetracycline-induced toxicity to *Arabidopsis* and rice [57,58], suggesting the significant potential of nanoscale materials as amendments for soil remediation, or alleviated heavy metal phytotoxicity.

**Figure 2.** Phenotypic images and fresh weight of rice treated with Cd, As, and C3N4. (**A**,**B**) represent rice images as affected by Cd × C3N4 and As × C3N4 for 14 days, respectively. (**C**,**D**) show the fresh biomass of rice roots and shoots across all the treatments, respectively. Values of fresh weight followed by different letters are significantly different at *p* < 0.05.
