The role of different fractions of humic acid in the physiological response of amaranth treated with magnetic carbon nanotubes

Dissolved humic acid (DI-IA) from soil can interact with multi-walled carbon nanotubes (MWCNTs) and magnetic-modified multi-walled carbon nanotubes (MMWCNTs), and subsequently alter the toxicity of MWCNTs and MMWCNTs to amaranth. This is the first study to compare the effects of MWCNTs and MMWCNTs under natural DHAs on their toxicity to amaranth. When DHAs were combined with 0.5 g/L MWCNTs, 1:2:1 MMWCNTs and 4:2:1 MMWCNTs nanomaterials, DHA(1) and DHA(4) both increased the pH of Hoagland's solutions. DHA(1) more severely decreased the soluble protein levels in shoots than DHA(4) in the 1:2:1 MMWCNT and 4:2:1 MMWCNT treatments. DHA(1) and DHA(4) both increased the chlorophyll concentrations of amaranth treated with MWCNTs, decreased the chlorophyll concentrations in the MMWCNT treatments. Co-exposure of DHAs and carbon-based CNTs caused further decreases in the anthocyanin level as compared to the respective CNT alone treatment. In the nanomaterial alone treatment, both 0.25 and 4:2:1 MMWCNTs greatly lowered the anthocyanin level as compared to the other two CNTs with the same exposure dose. Transmission electron microscopy images showed that the interaction between 4:2:1 MMWCNT and DHA(4) had more serious effects on plant cells across all the treatments.