Enhanced Cadmium Adsorption Mechanisms Utilizing Biochar Derived from Different Parts of Wetland Emergent Plants Iris sibirica L.

Overview

Li and colleagues pyrolyzed the root, stem, and leaf tissue of Iris sibirica L. (a wetland emergent plant) at 900 degrees C and tested the resulting biochars (BC_R, BC_S, BC_L) as aqueous sorbents for Cd(II). This is primary remediation-method evidence, not food or product occurrence evidence: the measured endpoints are biochar physico-chemical properties, batch adsorption kinetics, isotherm capacities, and mechanism partitioning rather than cadmium concentrations in edible crops, ingredients, consumer products, or drinking water.

Key numbers

Biochar physical properties (Table 1, page 6)

The three biochars differed substantially in surface area and pore structure. Source-reported values:

Raman ID/IG ratios were 0.91 (BC_R), 1.05 (BC_S), and 0.97 (BC_L), with BC_S showing the highest structural defect density (Figure 1d).

Equilibrium adsorption capacity (abstract; Section 3.2.1)

At pH 5.5, initial cadmium (Cd) concentration 20 mg/L, NaNO3 0.01 mol/L, 25 degrees C, biochar dose 1 g/L (0.02 g in 20 mL), 24 h equilibration, the source reports equilibrium adsorption capacities and removal efficiencies:

The abstract orders these BC_S (19.92) > BC_L (19.89) > BC_R (13.22).

Langmuir isotherm fitting (Table 3, page 13)

The Langmuir model fit the equilibrium data at 15, 25, 35, and 45 degrees C with R2 = 0.93-0.98. Maximum monolayer adsorption capacities Qmax (mg/g) and Langmuir constants KL (L/mg):

The authors highlight 2606.11 mg/g (BC_S at 45 degrees C) as the peak adsorption capacity (Section 3.4). Increasing KL with temperature was interpreted as evidence of endothermic adsorption.

Freundlich isotherm fitting (Table 4, page 13)

Adsorption kinetics (Table 2, page 12)

Second-order kinetics fit better than first-order for all three biochars, with BC_S showing the strongest improvement (first-order R2 = 0.663, second-order R2 = 0.964). Intra-particle diffusion fitted in three stages, with R2 > 0.83 for stages 1 and 2 across all biochars. Elovich parameters: alpha (initial sorption rate, mg/g min) = 0.192 (BC_R), 232.320 (BC_S), 1.139 (BC_L); beta (mg/g) = 1.737, 0.595, 0.265. BC_S had the highest initial sorption rate by orders of magnitude.

Mechanism partitioning (Figure 6, page 16)

The source separated Cd adsorption into ion exchange (Qcme), precipitation (Qcmp), and other (surface complexation and Cd-pi interaction; Qother) by sequential acid washing and elution treatments. Reported contributions:

Among exchanged cations (K+, Ca2+, Mg2+), K+ released most readily in exchange for Cd2+, with BC_L releasing the most K+ (up to ~25 mg/L for untreated material, Figure 5d). The authors note BC_L “could be used as a potash fertilizer additive based on high K+ leaching.”

pH and electrolyte sensitivity (Figure 3, pages 9-10)

Maximum Cd uptake occurred at pH 5.5-6 for all three biochars. Below pH 5, H+ competed with Cd2+ for binding sites. Above pH 7, Cd(OH)+ / Cd(OH)2 / Cd(OH)3- complexes reduced free Cd2+ and slightly suppressed uptake.

NaNO3 electrolyte effects (initial Cd 20 mg/L, pH 5.5, 0.01-1.00 mol/L NaNO3): BC_R uptake rose with electrolyte content up to 0.50 mol/L NaNO3 and then plateaued near 18 mg/g, while BC_S and BC_L hovered at 18.17-18.33 mg/g across the range. Zeta potential ranged -10 to -33 mV across pH 2-8, becoming more negative with increasing pH.

Recyclability (Figure 7, page 17)

BC_S retained Cd removal efficiency over five adsorption-desorption cycles, declining from 99.45% (cycle 1) to 87.96% (cycle 5).

Methods (brief)

Iris sibirica L. plants (six months old) were collected from a nursery in the Daxing district of Beijing, washed, dissected into roots (including rhizomes), stems, and leaves, oven-dried at 70 degrees C for 72 h, crushed, and passed through a 100-mesh sieve. Biomass powder (5.00 g) was pyrolyzed in a tube furnace at 900 degrees C for 120 min under argon flow (~200 mL/min), then annealed to room temperature within 200 min, and hand-ground through 100 mesh.

Batch adsorption used 0.02 g biochar in 20.00 mL Cd(II) solution prepared from Cd(NO3)2.4H2O (500.00 mg/L stock), with NaNO3 (0-1.00 mol/L) as background electrolyte, on a horizontal vibrator (HZX-Q100, China) at 200 rpm, 25 degrees C, pH 5.5, filtered through a 0.22 micrometer membrane. Cd, K, Ca, and Mg were quantified by ICP-MS (Agilent 7500cx, USA). Characterization: BET N2 adsorption (Tristar 3020 M), SEM (Hitachi SU8010), XRD (Bruker-AXS), FTIR (Bruker VERTEX 70, 4000-500 cm-1), Raman, Zetasizer Nano (UK), and ash determination per ASTM D1782-84 at 750 degrees C for 6 h. Each biochar sample was tested in triplicate.

The Cd source is a synthetic aqueous solution, not environmental or food samples. The biochars are the engineered adsorbent under test.

Related evidence

• Remediation evidence — drivers and interventions
• Cadmium
• Testing

Update history

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