High‑Efficiency Lead Removal with Silica‑Aerogel‑Supported Hydrozincite and Carbonate‑Hydrotalcite via Precipitation Transformation

Abstract

We report a cost‑effective, scalable synthesis of silica aerogel–supported hydrozincite and Zn/Al carbonate‑hydrotalcite, which exhibit ultra‑thin layers (<5 nm), high surface areas (≈260 m² g⁻¹), and weak crystallinity. Batch adsorption tests against Pb(II) reveal maximum capacities of 684.9 mg g⁻¹ for SA‑Zn‑HZ and 555.6 mg g⁻¹ for SA‑Zn₃Al‑HT, surpassing most inorganic adsorbents. Langmuir and pseudo‑second‑order kinetic fits confirm a surface chemisorption mechanism. XRD, TEM, and EDS mapping show that Pb(II) precipitates as hydrocerussite (Pb₃(CO₃)₂(OH)₂) following a precipitation‑transformation reaction of the support phases, which explains the high efficiency and low pH impact.

Background

Lead contamination in surface and distribution water remains a global environmental and health crisis [1, 2]. Conventional removal methods—chemical precipitation, ion‑exchange, and adsorption—often struggle with low adsorption capacities or harsh operating conditions [3–10]. Adsorption, however, remains the most practical due to its simplicity, high efficiency, and minimal pH disturbance [8, 9, 10]. Existing adsorbents (inorganic oxides [11–15], polymers [16], biomaterials [17, 18], sorption resins [7, 8]) still suffer from limited surface area and sub‑optimal capacities.

Layered double hydroxides (LDHs), or hydrotalcites (HTs), offer a promising platform because of their brucite‑type layers, interlayer anions, and high ion‑exchange potential [19–28]. Carbonate‑intercalated HTs, containing both carbonate and divalent hydroxides, are especially attractive for heavy‑metal adsorption [29]. Yet, conventional HT powders possess low specific surface areas [30]. Supporting HTs on high‑surface‑area carriers—such as silica aerogels (SAs)—can enhance textural properties while remaining inexpensive (

Methods

Materials

Analytical‑grade zinc nitrate, aluminum nitrate, sodium hydrogen carbonate, sodium hydroxide, and lead nitrate were sourced from Aladdin Reagent Co., Ltd. (Shanghai). Hydrochloric acid (36–38 %) was purchased from Sinopharm Chemical Reagent Co., China. Commercial silica aerogel (SA) was supplied by Nano Tech Co., Ltd. (Shaoxing).

Preparation of Adsorbents

SA powder (500 mg) was calcined at 823 K for 2 h to remove surface organics. It was then dispersed in 500 mL deionized water by ultrasound (30 min). A mixed solution of Zn(NO₃)₂ and Al(NO₃)₃ (total 0.0075 mol, ratios 3:0, 3:1, 2:1, or 0:1) was added, followed by alternating drops of 0.5 M NaOH/NaHCO₃. The pH was adjusted to 8.8 or 9.5, and the mixture was hydrothermally treated at 80 °C for 24 h. Products were collected by centrifugation, washed, and vacuum‑dried. The resulting composites were labeled SA‑Zn‑HZ (hydrozincite), SA‑ZnₓAl‑HT (Zn/Al‑carbonate‑hydrotalcite, x = 3 or 2), and SA‑Al‑H (control aluminum hydroxide).

Characterization

Transmission electron microscopy (JEM‑1011), scanning electron microscopy with EDS (JSM‑6360LV), N₂ adsorption–desorption (Micromeritics ASAP TriStar II), X‑ray diffraction (Empyrean XRD), and ICP‑AES (Leeman Prodigy XP) were employed to determine morphology, elemental composition, surface area, pore structure, and crystalline phases.

Adsorption Experiments

Pb(NO₃)₂ was dissolved to prepare 1000 ppm stock solutions, then diluted to 100–1000 ppm and adjusted to pH 6.0 ± 0.1. In a 100 mL Erlenmeyer flask, 50 mg of adsorbent was shaken (150 rpm, 25 °C) for 24 h to reach equilibrium. Remaining Pb(II) concentrations were measured by atomic absorption spectroscopy (Shimadzu AA‑6300). Adsorption capacity was calculated as: \[ q_e = \frac{(C_0 - C_e)V}{m} \] Kinetic studies used 500 ppm Pb(II) with sampling times up to 1440 min.

Results and Discussion

Optimization of Synthesis Parameters

Adsorption capacities increased with higher Zn content and pH 9.5: SA‑Zn‑HZ achieved 680.8 mg g⁻¹, SA‑Zn₃Al‑HT 537.8 mg g⁻¹, SA‑Zn₂Al‑HT 429.5 mg g⁻¹, while SA‑Al‑H was limited to 176.4 mg g⁻¹. These findings underscore the importance of divalent metal abundance and alkaline synthesis conditions.

Texture and Morphology

TEM images (Fig. 2) reveal that both SA‑Zn‑HZ and SA‑Zn₃Al‑HT form ultra‑thin lamellae (<5 nm). BET analysis shows surface areas of 264.1 m² g⁻¹ (SA‑Zn‑HZ) and 233.9 m² g⁻¹ (SA‑Zn₃Al‑HT). XRD patterns confirm hydrozincite (PDF#19‑1458) and carbonate‑hydrotalcite (PDF#51‑1525) phases, with weak diffraction peaks indicating low crystallinity.

Adsorption Isotherms

Pb(II) uptake increased with initial concentration, plateauing at 400–1000 ppm. Langmuir fitting yielded the highest correlation (R² ≥ 0.99) with maximum capacities of 684.9 mg g⁻¹ (SA‑Zn‑HZ) and 555.6 mg g⁻¹ (SA‑Zn₃Al‑HT). The Freundlich, Sips, and Redlich–Peterson models showed lower R² values, confirming monolayer chemisorption dominates.

Adsorption Kinetics

Rapid uptake in the first 50 min followed by a slower approach to equilibrium. Pseudo‑second‑order kinetics (R² > 0.99) best described the data, indicating chemisorption governs the rate. The calculated capacities matched experimental values closely.

Mechanism and Performance Evaluation

Post‑adsorption TEM, EDS, and XRD (Fig. 7) reveal Pb(II) precipitated as hydrocerussite (Pb₃(CO₃)₂(OH)₂), a stable phase with a lower solubility product (K_sp = 3.16 × 10⁻⁴⁶) than Pb(OH)₂ or PbCO₃. This precipitation‑transformation reaction occurs on the surface of hydrozincite or carbonate‑hydrotalcite, facilitated by the release of OH⁻ and CO₃²⁻. pH monitoring (Fig. 8) shows only a minor rise (6.0–6.39) during adsorption, demonstrating environmental compatibility. The adsorbents can be reused for up to four cycles with >93.5 % removal efficiency; beyond saturation, regeneration by calcination is ineffective due to irreversible conversion to hydrocerussite.

Comparison with Existing Adsorbents

The Langmuir capacities (684.9/555.6 mg g⁻¹) surpass most hydrotalcite‑based, graphene oxide, carbon nanotube, and activated carbon adsorbents (Table 4). Combined with low cost and simple synthesis, SA‑Zn‑HZ and SA‑Zn₃Al‑HT represent competitive solutions for Pb(II) remediation.

Conclusions

Silica aerogel–supported hydrozincite and carbonate‑hydrotalcite exhibit ultra‑thin layers, high surface areas, and exceptional Pb(II) adsorption capacities (684.9/555.6 mg g⁻¹). Adsorption follows Langmuir isotherms and pseudo‑second‑order kinetics, confirming surface chemisorption via precipitation transformation to hydrocerussite. The process is efficient, low‑cost, and minimally pH‑disturbing, offering a promising route for developing high‑performance heavy‑metal adsorbents.