Pan et al. 2021 - Lignosulfonate/chitosan adsorbent for Pb2+ in water

Pan and colleagues synthesised a sodium lignosulfonate/chitosan (LS/CS) composite by free-radical polymerisation of acrylic acid with lignosulfonate and chitosan, optimised the formulation by a four-factor orthogonal experimental design, and characterised the resulting adsorbent for Pb2+ removal from aqueous solution. This is primary remediation-method evidence, not food or product occurrence evidence: the measured endpoints are adsorbent capacity, kinetics, isotherm parameters, and adsorption thermodynamics in synthetic Pb(NO3)2 solutions, not concentrations in edible commodities or consumer products.

Key numbers

Optimised formulation and headline capacity

Abstract and Section 3.1 report a maximum Pb2+ adsorption capacity of 345 mg/g for the optimised LS/CS adsorbent at pH 7, adsorbent dose 0.01 g, initial Pb2+ concentration 100 mg/L, and 20 C. Table 3 lists the experimental qe at these conditions as 345.356 mg/g.

The orthogonal experiment (Table 1 design, Table 2 results) varied four factors at three levels each. Best-performing levels were: sodium lignosulfonate (C20H24Na2O10S2) 0.3 g, chitosan ((C6H11NO4)n) 0.3 g, potassium persulfate (K2S2O8) 0.07 g, and NMBA crosslinker 0.0115 g. Factor extrema (R values) from Table 2 give the influence order on adsorption capacity: K2S2O8 (R = 91) > chitosan (R = 65) > sodium lignosulfonate (R = 37) > NMBA (R = 29).

Orthogonal-experiment Pb2+ adsorption capacities for the nine runs (Table 2, Qe-Pb2+ in mg/g):

Dose and pH dependence

Figure 4A: as adsorbent dose increased from 0.01 g to 0.07 g, the per-gram adsorption capacity declined steadily from a maximum near 350 mg/g at 0.01 g toward roughly 300 mg/g at 0.07 g (values read from the figure). The Section 3.3 prose narrates the dose sweep as running to 0.15 g, but Section 2.4 specifies the experimental range as 0.01-0.07 g and the Figure 4A x-axis stops at 0.07 g; this is a paper-internal text inconsistency rather than two different experiments. Figure 4B and the inset zeta-potential plot give the zero-point pH (pHpzc) as 4.1; above pH 4 the LS/CS surface carries net negative charge and Pb2+ uptake increases sharply, reaching the reported maximum at pH 7. Below pH 4 the surface is net positive and Pb2+ uptake is suppressed.

Adsorption kinetics

Kinetic experiment conditions: contact time 5-240 min, initial Pb2+ 100 mg/L, LS/CS dose 0.01 g, pH 7.0, 20 C. Table 3 reports pseudo-first-order and pseudo-second-order fits against an experimental equilibrium capacity of 345.356 mg/g:

The pseudo-second-order model is the better descriptor of the kinetic data.

Adsorption isotherms

Isotherm experiment conditions: initial Pb2+ 100-900 mg/L, LS/CS dose 0.01 g, pH 7.0, 20 C. Table 4 reports a Langmuir-better fit with an “actual adsorption” value of 524.95 mg/g listed alongside the model parameters:

The very small RL (0.005) is consistent with strongly favourable adsorption.

Adsorption thermodynamics

Thermodynamic experiment conditions: initial Pb2+ 100 mg/L, LS/CS dose 0.01 g, pH 7.0, temperatures 20-60 C. Table 5:

Negative DG0 across the range identifies the process as spontaneous, the magnitude growing with temperature; positive DH0 identifies it as endothermic.

Adsorption mechanism

Section 3.5 and Figure 8 attribute Pb2+ uptake to single-monolayer chemical adsorption: Pb-pi coordination with aromatic functional groups, Pb-O coordination with hydroxyl groups, and Pb-N coordination with amino groups, together with electrostatic attraction between Pb2+ and the negatively charged LS/CS surface at pH > pHpzc. FT-IR shifts in the O-H, N-H, C-O-C, and C=C regions before and after Pb adsorption (Figure 2 and accompanying text) and SEM-EDS Pb-K maps (Figure 1) support this mechanism.

Methods (brief)

Adsorbent synthesis

Reagents (all AR grade, Shanghai Guoyao Group): acrylic acid (C3H4O2), chitosan ((C6H11NO4)n), sodium lignosulfonate (C20H24Na2O10S2), potassium persulfate (K2S2O8), N,N-methylene bisacrylamide (NMBA), potassium hydroxide (KOH), ethanol (CH3CH2OH), and lead nitrate (Pb(NO3)2). Deionised water was the default solvent.

Adsorbent preparation: 2.744 g KOH was dissolved in 5 mL water and neutralised with 5 mL acrylic acid added drop-wise. LS, CS, K2S2O8, and NMBA were added to the neutralised mixture together with 10 mL water under stirring, sonicated, then reacted at 70 C for 1 h. The product was washed with absolute ethanol, dried, and ground to obtain the LS/CS adsorbent. The orthogonal design and the Orthogonal Experiment Assistant II (V3.1) software were used to optimise factor levels.

Characterisation

• FT-IR: Nicolet nexus 470 spectrometer (Nicolet, USA), 400-4000 cm-1, KBr pellet at 1:100 sample-to-KBr mass ratio.
• SEM: JEOL JSM-7800F field-emission scanning electron microscope (JEOL Co., Ltd., Mitaka, Japan), 10.0 kV accelerating voltage, gold-coated samples; EDS elemental mapping after adsorption.
• TGA: Netzsch 449 C integrated thermal analyser (Netzsch, Germany), 20-800 C at 58 C/min under N2.
• Zeta potential / pHpzc: Malvern Nano ZS90 potential analyser (Malvern, UK), 20 C.

Pb2+ measurement

Pb2+ concentrations before and after adsorption were measured by flame atomic absorption spectrophotometry on a TAS-986 instrument (Beijing General Instrument Co., Ltd., Beijing, China), wavelength 283.3 nm, acetylene combustion gas at 0.09 MPa flowing at 1.5 L/min, combustion height 6 mm. Adsorption capacity Qe was calculated as Qe = (Ce - Co) V / M, where Ce and Co are equilibrium and initial Pb2+ concentrations (mg/L), V is solution volume (L), and M is adsorbent mass (g).

Batch experiment conditions

All solutions used Pb(NO3)2 dissolved in deionised water; pH was adjusted with 0.1 mM HCl and 0.1 mM NaOH.

Implications

Certification: Do not use this source in any food, infant-food, supplement, cosmetic, or ingredient occurrence pool. It does not measure consumer-product concentrations or document a reduction within a food matrix. It is relevant only as remediation context for low-cost lignosulfonate/chitosan composite sorbents applied to Pb-bearing aqueous waste streams.

App: Context for water-treatment and upstream-mitigation notes. The key takeaway is that a lignosulfonate/chitosan composite synthesised with optimised orthogonal-design factor levels achieved 345 mg/g Pb2+ adsorption capacity in batch tests on synthetic Pb(NO3)2 solution at pH 7 and 20 C, with strongly pH-dependent performance (suppressed below pH 4).

Courses: Useful for teaching the distinction between adsorbent capacity and food-occurrence concentration, the role of pHpzc in heavy-metal sorption, and the analytical separation of pseudo-second-order kinetics from Langmuir-type monolayer isotherms in remediation chemistry.

Wiki pages this source may touch

• remediation-evidence
• lead
• index

Verification notes

Page built from the full PDF including the abstract, Sections 2.1-2.4 (materials, synthesis, characterisation, Pb2+ adsorption protocol), Sections 3.1-3.5 (orthogonal results, FT-IR/SEM/TGA/zeta characterisation, dose and pH effects, kinetics, isotherms, thermodynamics, adsorption mechanism), Section 4 (Conclusions), and Tables 1-5 with Figures 1-8. The source measures only synthetic Pb(NO3)2 solution; frontmatter products and ingredients are intentionally empty because no food, ingredient, or consumer-product matrix was sampled. The frontmatter metals value uses the broader Pb slug while this page preserves the source’s Pb2+ ion-state specificity in prose and tables.

The matrices slugs (lignosulfonate-chitosan-composite, aqueous-sorption-test, wastewater-remediation) are remediation-domain proposals outside the project’s current food-matrix controlled vocabulary; a future taxonomy refresh should decide whether to formalise a remediation-matrix vocabulary or to route remediation-method sources via a separate non-food register. The ”## Wiki pages this source may touch” entries [[mitigation/remediation-evidence]] and [[testing/index]] point to wiki directories outside the four-section (ingredients/products/metals/regulations) taxonomy snapshot used for slug-vocabulary auditing; they are speculative routing leads and follow the same out-of-scope-of-snapshot status as the matrices slugs. If the routing audit reports advisory malformed status for this source on the empty products/ingredients fields, that reflects the genuine absence of food/ingredient/product scope rather than a defect.

Audit subagent (2026-06-02) flagged the dose-sweep range as a body/methods conflation: Section 3.3 prose says “0.01 to 0.15 g” but Section 2.4 specifies “0.01 to 0.07 g” and Figure 4A’s x-axis stops at 0.07 g. Verified against the source - confirmed paper-internal text typo in Section 3.3; body revised to the Figure 4A / Section 2.4 range with a one-sentence note. The audit’s other ⚠️ findings (mitigation/remediation-evidence and testing/index outside the four-section taxonomy snapshot) were applied by extending the existing out-of-snapshot disclosure above. No definite-error (❌) findings on numerical fidelity, speciation, methods, brand firewall, or wiki/HMTc firewall; subagent verdict REVISE handled in one pass.

Page history

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