-
Figure 1.
Comparison of aquatic and terrestrial bioavailability models for metals.
-
Figure 2.
Conceptual diagram of chemical controls on metal bioavailability.
-
Figure 3.
Comparative DTPA-extractable metal concentrations across soil types.
-
Figure 4.
Integration of molecular and microbial tools in risk assessment of soil metal bioavailability.
-
Aspect Aquatic systems Terrestrial systems Primary medium Water Soil/sediment Most common bioavailability models Biotic Ligand Model (BLM) Terrestrial BLM, empirical models Exposure route Direct uptake from water Uptake from pore water/soil particles Biological receptor Fish, invertebrates Plants, soil fauna Chemical inputs pH, DOC, Ca2+, Mg2+ pH, CEC, organic matter content, clay content Model type Mechanistic Empirical/semi-mechanistic Regulatory framework EU water framework directive, USEPA Country-specific Standardization Widely used in regulation In development/adaptation stage Table 1.
Key differences between aquatic and terrestrial bioavailability models
-
Chemical factor Impact on bioavailability Mechanism Soil pH ↑ solubility in acidic soils Desorption, dissolution of minerals Redox potential ↑ mobilization under reducing conditions Reductive dissolution of Fe/Mn oxides DOC ↑ or ↓ depending on the complex size Metal-organic complexation Ionic strength ↓ activity of free metal ions Competition with background electrolytes Competing Ions ↓ metal uptake Competition for sorption and biological ligands Table 2.
Key chemical factors influencing heavy metal bioavailability in soil
-
Method category Representative techniques Target fraction Application Chemical extraction Water, DTPA, EDTA, CaCl2 Labile/soluble metals Screening and monitoring Passive sampling DGT, pore water sampling Free ion activity In situ assessments Biological assays Bioaccumulation, avoidance, reproduction Biologically available Ecotoxicity testing Modeling Terrestrial BLMs Effective dose Risk prediction and regulation Table 3.
Categories of bioavailability assessment methods and their typical characteristics
-
Extractant Target fraction Target metals Typical use Remarks Water Labile, mimicking the soil solution All Plant availability, leaching DTPA Labile, organically bound Zn, Cu, Fe, Mn Widely used in agriculture EDTA Potentially mobile Pb, Cd, Ni Pollution assessment Broad-spectrum chelation CaCl2 Soluble, weakly adsorbed Zn, Cd Short-term bioavailability Simple and fast 0.1 M HCl Carbonate-bound Pb, Cd Contaminated soils May overestimate bioavailability BCR sequential Multiple defined fractions Various Detailed fractionation Time-consuming, operational Table 4.
Overview of chemical extraction methods for metal bioavailability assessment
-
Method Analyte Sampling medium Advantages Limitations DGT Free/labile metals Hydrogel + chelex resin Time-integrated, field-validated Requires careful calibration Ion-exchange resins Cationic metals Resin beads Simple and cost-effective Low spatial resolution Microdialysis Soluble ions Semipermeable membrane Dynamic uptake, minimally invasive Limited uptake rate Porewater samplers Total dissolved metals Suction lysimeter Direct measurement of soil solution Disruptive, equilibrium-based Table 5.
Overview of passive sampling techniques for metal bioavailability
-
Assay type Target organism End points Strengths Limitations Plant assay Lolium, Zea, Brassica Root elongation, biomass Direct uptake evidence Sensitive to soil properties Invertebrate assay Eisenia, Folsomia Bioaccumulation, reproduction Ecologically relevant Species variability Microbial indicator Soil bacteria/fungi Respiration, enzyme activity Rapid response High variability Table 6.
Overview of commonly used bioassays for bioavailability assessment
-
Model type Handles non-linearity Interpretability Typical use Example study Linear regression No High Simple soil-metal correlations Hunan Cd rice study Random forest Yes Medium Risk zoning, multi-factor analysis Hunan Cd rice study Support vector machine Yes Low-medium Predictive classification Dutch Zn study Neural networks Yes Low Complex system modeling USDA lead mapping Table 7.
Comparative overview of applications of modeling techniques
-
Tool Target Output type Sensitivity Application stage Metagenomics Community Functional genes High Early detection Transcriptomics Individual Gene expression High Sub-lethal stress Proteomics Individual Protein levels Moderate Mechanism elucidation Microbial biosensors Specific ion Luminescence/fluorescence Very high Field screening Community profiling Microbiome Diversity indexes Moderate Long-term impact Table 8.
Comparative overview of molecular and microbial tools
-
Region or land use Typical metal concerns Data gap description Sub-Saharan Africa Cd, Pb, Zn Limited site-specific data for native soil types Urban gardens As, Pb Lack of long-term monitoring of bioavailable fractions Reclaimed mining sites Ni, Cr, Cu Insufficient post-remediation validation Paddy soils in China Cd, As Poor harmonization of extractant-based tests Table 9.
Key data gaps by region and land use
-
Region/country Extraction method Regulatory status Primary focus EU (ECHA, ISO) BCR, ISO 17402 Standardized, widely adopted Soil and sludge risk assessment USA (USEPA) Limited; Total + BLM (aquatic) Not standardized for soils Human/ecological exposure China (MEE) CaCl2, DTPA Emerging regulatory frameworks Cd and As in agriculture Australia Total + bioassays Guideline-driven Ecological thresholds Canada Total + bioavailability weighting Partial implementation Site-specific risk Table 10.
Overview of regional/national bioavailability protocols
-
Table 11.
International approaches to bioavailability integration in soil policy
Figures
(4)
Tables
(11)