(80i) Granular Carbon Composite with Thiolated Hydroxyapatite for Lead (Pb2+) Adsorption in Water

Abstract: The present work reports the synthesis of thiol modified hydroxyapatite (HAp-S) employing a one-pot, facile green precipitation method. Furthermore, a novel macro-sized composite, designated as HAp-SC, was developed in-situ using granular charcoal and HAp-S. The materials were characterized thoroughly using XRD, FTIR, FESEM, EDX and zeta potential analyses. The presence of sulfur groups in Hap-S demonstrated enhanced Langmuir uptake capacity for Pb (Q_max: 823.0 mg/g) in aqueous phase compared to the non-functionalized counterpart (Q_max: 491.5 mg/g). HAp-SC also showed a credible Q_max of 95 mg/g (approx.). Additionally, the influence of adsorbent dose, pH, temperature and contact time were discussed to emphasize the promise of the novel carbonaceous composite for adsorptive removal of aqueous lead. The sustainable carbonaceous composite offers credible prospect towards Pb remediation in real applications with its appreciable reusability.

Introduction: Lead (Pb) is one of the top-ten hazardous substances which causes severe damage to the brain, kidneys and the reproductive system. It is identified as a potential carcinogen ^1,2. Strict regulations are set to limit Pb concentration at 1 mg/L in industrial effluent and 10 µg/L in drinking water ^3,4. Adsorption is the most favorable technology due operational simplicity and lower expenses. Adsorbents with nitrogen and sulfur groups (NH₂, SH) in their structure show higher affinity to bind Pb ⁴. The present study showcases the synthesis of thiol-modified hydroxyapatite (HAp-S) using a green precipitation method. The material is initially characterized followed by detailed batch adsorption studies revealing its enhanced Pb uptake performance. Furthermore, HAp-S is encapsulated in granular charcoal producing an efficient composite adsorbent that offers better hydrodynamic control for real field applications compared to the nano-adsorbent.

Synthesis method: 200 mmol of calcium nitrate tetrahydrate (Ca(NO₃)₂).3H₂O) and 30 mmol of mercaptosuccinic acid (MSA) are initially dissolved in 250 mL deionized (DI) water (using a magnetic stirrer) followed by the addition of 125 mmol of sodium dihydrogen phosphate dihydrate (NaH₂PO₄.2H₂O). Lastly, 75 mL aqueous solution of sodium hydroxide (3.5M) was added dropwise to initiate HAp-S crystallization. The material was washed using centrifugation and dried at 75°C overnight and stored finally after grinding to powder form. For HAp synthesis, MSA was not added. To synthesize the HAp-SC, 100 g of granular charcoal was soaked in DI water after which the calcium salt, MSA and phosphate salt was added and soaked for 4 h to ensure uniform wetting of the porous charcoal. The rest of the procedure is similar to HAp-S synthesis. A schematic of synthetic process and images of materials produced are provided in Fig. 1a and 1b, respectively.

Results and Discussion: The XRD analysis presented in Fig. 1c reveal major peaks at 2θ: 25.85Ëš, 31.78Ëš, 40.23Ëš, 46.67Ëš corresponding to (002), (211), (310) and (222) planes of HAp (JCPDS: 09-0432) ⁵. HAp-S shows similar XRD profile indicating that the crystal integrity is intact. The IR spectra in Fig. 1d shows the characteristic HAp peaks near 1021 cm^-1 (PO₄^3-), 873 cm^-1 (HPO₄^2-), 1414 cm^-1 and 1450 cm^-1 (CO₃^2-) and 1650 cm^-1 (OH). The zeta potential analysis (refer Fig. 1e) reveals the negative surface potential of both HAp and HAp-S (marginally more negative) in the pH range of 2 to 12. The Pb adsorption is confirmed using the XPS analysis shown in Fig. 1f. The morphology of HAp particles (refer Fig. 1(g,h)) exhibits well-defined structures with pointed ends and flatter topography which matches well with the reported literature ⁶. In contrast, HAp-S (Fig. 1(i,j)) shows relatively agglomerated structure where crystals are appearing to be fused together. The FESEM image of HAp-SC (refer Fig. 1l) shows HAp particles within the porous structure of charcoal (refer Fig.1k). The EDX analysis confirms the S modification in HAp-SC (refer Fig. 1m and 1n).

The dose of HAp-S (Fig. 1o, bottom) and HAp-SC (Fig. 1o, top) is optimized in corresponding ranges of (0.025-0.15) g/L and (0.5-3.0) g/L, respectively. Pb removal is noted to rise from 22.88% (at 0.025 g/L) to 78.49% at 0.1 g/L dose (HAp-S) after which the increase is limited (< 4%). Hence, 0.1 g/L was selected as the optimum dose. In a similar way, the removal increases from 34% to 78.2% for HAp-SC dose range of 0.1 g/L to 1.5 g/L and thereafter showed marginal change. Thus, the optimum dose for HAp-SC was identified to be 1.5 g/L. HAp-S shows increased Pb adsorption with increasing pH (varied from 2 to 7), although after pH 5, the increase in adsorption efficiency is insignificant (refer Fig. 1p). The presence of excess H₃O⁺ ions in highly acidic medium restricts the interaction between Pb²⁺ moieties and active sites by competing leading to low Pb uptake. Consequently, higher pH facilitates Pb uptake due to the presence of less H⁺ ions and due to higher negative surface potential of HAp-S (shown in Fig. 1e). Since, Pb tends to precipitate as hydroxide over pH 6.5, the optimal pH for subsequent tests is selected as 6 ⁷. Additionally, the kinetic analysis (refer Fig. 1q) reveal pseudo-second order adsorption kinetics (higher R² values), indicating chemisorption as the governing mechanism ⁸. The influence initial Pb concentration (from 5 to 500 mg/L) is shown in Fig. 1r (powder) and Fig. 1s (HAp-SC). It is observed that Langmuir isotherm fits better with the experimental data (higher R² values) compared to Freundlich isotherm suggesting monolayer adsorption ⁷. The maximum Langmuir capacity (Q_max) for HAp, HAp-S and HAp-SC are 491.5 mg/g, 823 mg/g and 95 mg/g, respectively (at 30°C). Furthermore, Q_max increases to 104 mg/g and 115 mg/g at 40°C and 50°C (Fig. 1s). The associated thermodynamics indicate the endothermic nature of adsorption through a positive enthalpy change (ΔH= 39.5 kJ/mol). The composite was effectively regenerated with 0.05M EDTA solution and showed excellent performance (Pb removal > 90%) after 5 cycles of operation with Pb feed of 10 mg/L (refer Fig. 1t).

Conclusively, a novel methodology to produce thiol modified HAp using MSA was demonstrated successfully. This enhanced the capacity of Pb adsorption by nearly 82% due to electronic interaction between S and Pb which is also supported by strong electrostatic forces. Additionally, the appreciable performance of composite sorbent HAp-SC was also evidenced.

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