Deng et al. 2021 - Mg-modified biochar for Cu, Pb, and Cd adsorption

Deng and colleagues prepared magnesium-modified corncob biochars and tested their ability to remove Cu(II), Pb(II), and Cd(II) from synthetic aqueous solutions. This is primary laboratory remediation evidence for metal adsorption mechanisms and sorbent performance. It is not occurrence evidence for drinking water, food, ingredients, or consumer products.

Key numbers

Biochar preparation and material properties

Corncob biomass from Ya’an City, Sichuan Province, China was milled to 1 mm, soaked in MgCl2 solutions to target Mg/biomass mass ratios of 5%, 10%, 15%, and 20%, ultrasonicated for 2 h, dried at 80 degrees C, and pyrolyzed under N2 at 450 degrees C for 1 h. The unmodified control was prepared under the same pyrolysis conditions.

Table 1 reports that Mg loading changed both chemistry and pore structure:

The authors selected 15%Mg-BC as the best-performing material because 20%Mg-BC showed poorer pore structure despite higher Mg content.

Single-metal adsorption performance

In initial screening at pH 5.0, 25 degrees C, and 24 h contact time, 15%Mg-BC had sorption capacities of 200.33 mg/g for Cu(II), 164.51 mg/g for Cd(II), and 448.5 mg/g for Pb(II). These were 6.37, 2.36, and 9.34 times higher than unmodified BC, respectively.

The dosage experiment showed rapid capacity increases as 15%Mg-BC dose rose from 0.001 g to 0.01 g in 50 mL solution:

Initial pH strongly affected performance. Sorption increased as initial pH rose from 2.0 to 5.0 and then plateaued at pH 5.0. The authors did not test pH above 6.0 because some metal ions become unstable and precipitate.

Kinetics and isotherms

The kinetic experiment used 0.01 g 15%Mg-BC in 50 mL solution, with 50 mg/L Cd(II), 50 mg/L Cu(II), and 100 mg/L Pb(II), adjusted to pH 5.0, shaken at 180 rpm and 25 degrees C. Cu(II) and Pb(II) reached equilibrium at about 24 h, while Cd(II) increased more slowly and did not fully equilibrate by the end of the test.

Table 2 reports the best-fitting kinetic models:

The isotherm experiment tested Cu(II) and Cd(II) from 10-100 mg/L and Pb(II) from 50-150 mg/L, using 0.01 g 15%Mg-BC, pH 5.0, 25 degrees C, and 24 h. Experimentally observed equilibrium sorption capacities were 296.85 mg/g for Cu(II), 180.93 mg/g for Cd(II), and 527.67 mg/g for Pb(II).

Table 3 model parameters:

The authors summarize affinity as Pb(II) > Cu(II) > Cd(II).

Binary-metal systems

The binary experiments kept one metal fixed while varying the coexisting metal. In Cd-Cu and Cd-Pb systems, Cd(II) capacity fell from 133.23 mg/g to 34.6 mg/g as Cu(II) increased, and to 22.72 mg/g as Pb(II) increased. With Cd(II) varied from 0 to 50 mg/L, Cu(II) capacity declined from 156.4 to 125.02 mg/g and Pb(II) capacity declined from 292.27 to 258.05 mg/g.

When Cu(II) and Pb(II) coexisted, both declined sharply: Cu(II) fell from 156.4 to 80.9 mg/g, and Pb(II) fell from 292.27 to 87.25 mg/g as the competing ion increased. The authors interpret this as competition for sorption sites.

Mechanisms

XRD, FTIR, and XPS results support multiple removal mechanisms: surface precipitation, cation-pi bonding, surface complexation, and ion exchange. The source identifies metal-bearing reaction products including Cu(OH)2, Cu2(OH)2CO3, CdCO3, and Pb3(OH)2(CO3)2.

XPS showed increased Cu, Cd, and Pb atomic percentages and decreased Ca and Mg after adsorption, which the authors use as evidence for ion exchange. The authors also report Pb 4f binding energy at 138.2 +/- 0.1 eV assigned to Pb3(OH)2(CO3)2, Cu at 934.8 +/- 0.1 eV assigned to Cu2(OH)2CO3, and Cd at 405.1 +/- 0.1 eV assigned to CdCO3.

Methods (brief)

All adsorption tests used analytical-grade CuCl2, Pb(NO3)2, and CdCl2 salts in deionized water. Metal concentrations in filtrate were measured by ICP-OES after 0.45 um filtration. Adsorbents were characterized by elemental analysis, ICP-OES for biochar metals, BET surface area and pore structure, FTIR, SEM, XRD, and XPS.

The Mg-content screen, pH screen, dosage experiment, kinetics test, isotherm test, and binary-metal experiments were all batch aqueous experiments. The paper does not test actual wastewater, field water, food-processing water, drinking water, crops, or consumer products.

Implications

Certification: Do not use this source in HMTc occurrence pools. Its mg/g values are adsorbent capacities under controlled lab conditions, not concentrations in market foods, ingredients, products, or water supplies.

App: Useful as remediation-context evidence where a supplier, farm, or processor is evaluating Cu/Pb/Cd removal media. It supports treating real-water validation as a separate evidence requirement because binary systems showed strong competition and lower capacities than single-metal tests.

Courses: Useful for teaching why adsorption capacity, initial challenge concentration, equilibrium concentration, and food/product occurrence concentration are different evidence objects.

Wiki pages this source may touch

• cadmium
• copper
• lead
• index

Verification notes

This page was built from the full PDF, including the abstract, Methods and Materials sections 2.1-2.6, Tables 1-4, Figures 1-9, Results and Discussion sections 3.1-3.6, Conclusions, and data-availability statement. The page keeps products and ingredients empty because the study is a synthetic aqueous adsorption experiment. Copper, lead, and cadmium are reported as Cu(II), Pb(II), and Cd(II) challenge ions; the page does not convert those adsorption capacities into occurrence concentrations or infer performance for real wastewater without field validation.

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