Prompt Description

The figure is divided into three main parts: (A) Soil Amendment and Heavy Metal Passivation Mechanisms Composition of Composite Amendments: Sludge (provides organic matter, humic acid, and nutrient sources) Attapulgite Clay (possesses numerous layered structures and surface hydroxyl groups, providing adsorption and ion exchange sites) Biochar (carbon source, surface functional groups –COOH, –OH, –C=O, pore structure can adsorb metal ions) Main Action Pathways: Physical Adsorption and Fixation: Biochar pores and attapulgite interlayer pores adsorb heavy metal ions such as Cu²⁺, Pb²⁺, and Cd²⁺. Ion Exchange and Surface Complexation: Si–OH and Mg–OH on the surface of attapulgite form coordination bonds with metal ions; oxygen-containing functional groups on the surface of biochar form stable complexes with heavy metals. Precipitation and Mineralization: Phosphate and carbonate released from sludge form insoluble salts with heavy metals (e.g., Cu₃(PO₄)₂, PbCO₃, CdS, etc.). Changes in pH and CEC: Amendments increase soil pH and cation exchange capacity, reducing the proportion of soluble forms of heavy metals. Enhanced Microbial Activity: Sludge and biochar promote the growth of beneficial microorganisms, which can further passivate heavy metals through biosorption or biotransformation. Results (indicated by arrows in the figure): Decrease in water-soluble and exchangeable heavy metal concentrations Increase in the proportion of residual and carbonate-bound heavy metals Overall performance shows reduced bioavailability (B) Absorption and Barrier Mechanisms in the Maize Rhizosphere Root Surface Layers Exhibit: The root hair zone can adsorb a small amount of heavy metals, but they are bound and passivated by carboxyl and hydroxyl groups. Rhizosphere secretions (organic acids, mucilage, GRPs) synergize with amendments to form metal-organic complexes, reducing the concentration of active ions entering cells. Cell Wall and Membrane Barriers: Metal ions are mostly bound by –COOH and –OH groups on the cell wall. The expression of metal ion transporters (such as ZIP, HMA families) on the plasma membrane is downregulated under low metal availability. Intracellularly released organic acids (such as citric acid, malic acid) form chelates with metals or enter vacuoles for sequestration. In Vivo Transport Pathways: Decrease in the transfer coefficient from root → stem → leaf. Root cells sequester heavy metals in vacuoles. Aboveground parts are mainly transported through symplastic pathways, with a significantly reduced proportion. (C) Quantitative Arrows and Effects After amendment, the arrow points from “active heavy metals → insoluble complexes/mineralized forms” direction. Soil effective metal ↓ (Cu, Cr, Cd, Pb, Zn, Ni) Root uptake ↓ Shoot/Grain translocation ↓ Final Display: Reduced heavy metal mobility and bioavailability → Enhanced maize growth & lower food-chain risk III. Drawing Suggestions Background layering: Maize plants above, root system below, rhizosphere area in the middle, composite amendments and soil layer below. Different colors can be used