Pyrolysis-derived Prosopis juliflora activated biochar for ciprofloxacin adsorption – comprehensive study notes

Context & Problem Statement

  • Rising global concern: accumulation of antibiotic residues—especially fluoroquinolone drug Ciprofloxacin (CIP)—in freshwater and wastewater.
    • Global consumption grew from 298  t yr−1298\;\text{t yr}^{-1} (1995) to 2318  t yr−12318\;\text{t yr}^{-1} (2015).
    • Concentrations reported:
    • Surface / wastewater effluents: <1  μg L−1<1\;\mu\text{g L}^{-1}
    • Pharmaceutical effluents: up to 30  mg L−130\;\text{mg L}^{-1}.
    • Kazipally Lake (India) exceeded therapeutic blood-plasma levels (WHO alert).
  • Environmental & health risks:
    • Up to 90%90\% of an antibiotic dose excreted unmetabolised (urine), 75%75\% via animal faeces.
    • Fosters antibiotic-resistant bacteria/genes (ARGs) – 18 categories → 271 sub-types (2019).
  • Conventional WWTPs fail to remove CIP completely.

Biomass Choice & Rationale

  • Prosopis juliflora (PJ): an invasive, nitrogen-fixing, lignocellulosic shrub/tree invading millions of hectares in Asia, Africa, Australia, America.
    • High carbon content, robust growth in arid zones, depletes groundwater & absorbs large CO₂ volumes → environmental nuisance → plentiful waste feedstock.
    • Converting PJ into biochar/activated carbon addresses weed control and provides low-cost adsorbent.

Objectives

  • Prepare three PJ-derived sorbents & compare with commercial activated carbon (C-AC):
    1. Raw biomass (PJ)
    2. Biochar (PJB) via slow pyrolysis
    3. ZnCl₂-activated biochar (PJB-AC)
  • Evaluate surface/chemical properties, adsorption capacity, kinetics, isotherms for CIP removal.
  • Benchmark against C-AC and published adsorbents.

Materials & Methods

Chemicals & Reagents
  • CIP (TCI, Japan), C-AC (Fisher Sci), NaCl, ZnCl₂, NaOH, HCl (analytical grade), Milli-Q water.
Biochar & Activated Biochar Synthesis
  • Wood pieces → washed, crushed.
  • Pyrolysis: 550  ∘C,  5  ∘C min−1,  1  h,  N2550\;^{\circ}\text{C},\; 5\;^{\circ}\text{C min}^{-1},\;1\;\text{h},\;N_2 → PJB.
  • Chemical activation:
    • Impregnation: 10 g PJB + 100 mL 3 M ZnCl₂, 12 h, RT, filter.
    • Second carbonisation: 900  ∘C,  3  h,  N2900\;^{\circ}\text{C},\;3\;\text{h},\;N_2.
    • Acid wash: 1 M HCl + DI water to neutral pH → dry → PJB-AC.
Characterisation Techniques & Findings
  • BET-N₂ physisorption (ASAP 2020):
    • Isotherm type: mixed I/II with H₄ hysteresis (micro- & mesopores co-exist).
    • SBETS_{BET} values:
    • PJ 2.04, PJB 6.14, PJB-AC 360.5 m² g⁻¹, C-AC 589 m² g⁻¹.
    • Micropore/mesopore volumes provided (see table).
  • Elemental analysis (%wt): PJB-AC carbon ↑ to 68.7 % vs 45.3 % (PJ).
  • FTIR (4000–400 cm⁻¹) key bands pre-adsorption (PJB-AC):
    • ∼3449\sim3449 (–OH/–NH), 29932993 (–CHₓ), 15731573 (aromatic C=C), 10471047 (alkoxy C–O).
  • Post-adsorption shifts introduce CIP signals:
    • 16251625–1623  cm−11623\;\text{cm}^{-1} (C=O of fluoroquinolone), shift of aromatic C=C to 15891589 cm⁻¹ → indicates π–π Electron Donor–Acceptor (EDA) interactions.
  • SEM:
    • PJB: rough, heterogeneous texture with some pores.
    • PJB-AC: honey-comb network, abundant micro-mesopores; after CIP loading, cavities filled.

Batch Adsorption Experiments

  • Variables tested
    • pH 2–10, initial CIP 20–100 mg L⁻¹, adsorbent 0.2–1.2 g L⁻¹, T 298–318 K, 5 h contact, 150 rpm.
  • Key Equations
    • Percent Removal: %R=C<em>0−C</em>tC0×100\%R=\frac{C<em>0-C</em>t}{C_0}\times100
    • Capacity at time tt: q<em>t=(C</em>0−Ct) Vmq<em>t=\frac{(C</em>0-C_t)\,V}{m}
pH Effect
  • Optima:
    • PJ / PJB: pH ≈ 4
    • PJB-AC & C-AC: pH ≈ 6
  • Explanation: CIP speciation (cationic <pKa1!≈5.9pK_a1!\approx5.9; zwitterionic 5.9<pH<8.95.9<pH<8.9; anionic >8.98.9) + surface charge dictate electrostatic / hydrogen bonding / π–π.
Initial Concentration
  • qeq_e increases 20→100 mg L⁻¹.
    • PJ: 9.7→41.4 mg g⁻¹
    • PJB: 22.8→76.3 mg g⁻¹
    • PJB-AC: 40.2→160.4 mg g⁻¹ (comparable to C-AC 147.5 mg g⁻¹ at 100 mg L⁻¹).
Adsorbent Dosage
  • Higher dosage ↓ unit capacity but ↑ overall % removal due to more active sites.
  • At 1.2 g L⁻¹: PJB-AC 150 mg g⁻¹ > C-AC 140 mg g⁻¹.
Temperature Effect (298–318 K)
  • PJB-AC shows slight capacity rise then decline → exothermic chemisorption; PJ/PJB less temperature-sensitive.

Isotherm Modelling (25 °C)

ModelBest fit adsorbentR²Notes
Langmuir (q<em>e=Q</em>0K<em>LC</em>e1+K<em>LC</em>e)\big(q<em>e=\frac{Q</em>0K<em>LC</em>e}{1+K<em>LC</em>e}\big)C-AC (R² 0.985), PJB-AC (0.973)Monolayer, homogeneous sites. Q0Q_0 PJB-AC ≈223.8  mg g−1≈223.8\;\text{mg g}^{-1}
Freundlich log⁡q<em>e=log⁡K</em>F+1nlog⁡Ce\log q<em>e=\log K</em>F+\frac1n\log C_ePJ (R² 0.999), PJB (0.998)Multilayer on heterogeneous PJ/PJB surfaces. nn 3.36–4.25 (favourable).
Other models (Temkin, D–R, Jovanovic, Hurkins–Jura, Elovich, Halsey) calculated for mechanistic insight—see paper tables.

Kinetic Modelling

  • Models & core equations:
    • Pseudo-1st: log⁡(q<em>e−q</em>t)=log⁡q<em>e−k</em>1t2.303\log(q<em>e-q</em>t)=\log q<em>e-\frac{k</em>1t}{2.303}
    • Pseudo-2nd: tq<em>t=1k</em>2q<em>e2+tq</em>e\frac{t}{q<em>t}=\frac1{k</em>2q<em>e^2}+\frac{t}{q</em>e}
    • Intra-particle diffusion: q<em>t=k</em>it1/2+Cq<em>t=k</em>i t^{1/2}+C
    • Elovich: qt=1βln⁡(αβ)+1βln⁡tq_t=\frac1\beta\ln(\alpha\beta)+\frac1\beta\ln t
    • Brouers–Sotolongo (fractal): q<em>t=q</em>e[1−(1+(n−1)(tτc)α)−1/(n−1)]q<em>t=q</em>e\big[1-(1+(n-1)(\tfrac{t}{\tau_c})^\alpha)^{-1/(n-1)}\big]
  • Best fit: Pseudo-second-order for all adsorbents (R² > 0.98; χ2\chi^2 ≈0) → chemisorption dominant; corroborated by B–S fractal kinetics (heterogeneous active sites, n>1n>1 rapid uptake).
  • Intra-particle diffusion plots show multi-linearity → boundary & pore diffusion both control rate.

Comparative Performance (Literature Benchmark)

  • PJB-AC 158 mg g⁻¹ > many waste-derived carbons (tea-biochar 112, MoS₂-biochar 39, rabbit manure 30 mg g⁻¹) and similar to C-AC.
  • Superior materials exist (grapefruit peel AC 566 mg g⁻¹, bamboo AC 362 mg g⁻¹) but often require harsher activation or costlier feedstocks.

Mechanistic Insights

  • Dominant forces for PJB-AC–CIP:
    • π–π EDA interactions between aromatic rings of biochar & electron-deficient quinolone ring.
    • Hydrogen bonding via –OH/–NH groups.
    • Minor electrostatic attraction at pH ≈6 (zwitterionic CIP, mildly negative surface).
  • ZnCl₂ activation effects:
    • Low melting point (~290 °C) → molten salt penetrates char matrix, promotes dehydration/aromatization, generates micropores; ZnCl₂ recoverable.

Practical / Environmental Implications

  • Converts invasive weed waste into value-added adsorbent, mitigating ecological damage.
  • PJB-AC offers comparable efficacy to commercial AC with lower resource cost.
  • Potential for scale-up in rural/industrial wastewater treatment targeting emerging antibiotics.
  • Regeneration/reusability not detailed—future research needed.

Key Numerical Summary

  • BET Surface (m² g⁻¹): PJ 2.0, PJB 6.1, PJB-AC 360.5, C-AC 589.
  • Langmuir Q0Q_0 (mg g⁻¹): PJB-AC 223.8, C-AC 195.8, PJB 113.2, PJ 41.0.
  • Optimal pH: 6 (PJB-AC), 4 (PJ/PJB).
  • Best kinetic constants (PJB-AC, 100 mg L⁻¹): k<em>2=0.008  g mg−1!min−1,  q</em>ecal=158.4  mg g−1k<em>2=0.008\;\text{g mg}^{-1}!\text{min}^{-1},\;q</em>e^{cal}=158.4\;\text{mg g}^{-1}.
  • Activation yield: Fixed-C rises PJ→PJB-AC from 45.3 %→68.7 %.

Study Limitations & Future Work

  • No regeneration/cycle data; assess adsorbent stability & cost per cycle.
  • Real wastewater matrices may contain ions/DOM influencing performance.
  • Explore column studies, techno-economic & life-cycle analysis.

Take-Home Messages

  • ZnCl₂-activated P. juliflora biochar (PJB-AC) is a promising, low-cost, sustainable adsorbent for CIP elimination.
  • Chemisorption (pseudo-2nd order) + monolayer Langmuir behaviour dominate.
  • Transforming invasive biomass into functional materials aligns with circular bio-economy and pollution control goals.