Surface Adsorption Mechanism Research Articles

Estudio de la movilidad de nanofluidos estables basados en hierro para remediación in situ de contaminantes inorgánicos

Las nanopartículas de hierro cerovalente (FeNPs) se han empleado para la remoción de una gran variedad de contaminantes en medio acuoso, transformándolos en especies de menor toxicidad mediante mecanismos de adsorción superficial y reacciones rédox. En tratamientos de remediación in situ de aguas subsuperficiales se inyectan como nanofluidos (NFs), donde las suspensiones de FeNPs deben estabilizarse para evitar su precipitación y favorecer su transporte. En este trabajo se prepararon nanofluidos estables a partir de FeNPs empleando estabilizantes como carboximetilcelulosa (CMC) o goma xántica (GX), y se evaluó su movilidad en columnas rellenas de lecho poroso, a escalas de laboratorio y piloto. Los parámetros del lecho se obtuvieron por modelado de las curvas de ruptura de un trazador de NaCl, empleando un software específico. Los NFs mostraron buena movilidad, mientras que las FeNPs sin estabilizar quedaron retenidas en el medio poroso. Con el objetivo de dilucidar completamente los mecanismos de transporte y los modelos de flujo actuantes se comenzó también con la caracterización reológica de los NFs, encontrándose resultados iniciales interesantes. Mediante una combinación adecuada de los experimentos de movilidad en columna, el modelado y la caracterización, se encontraron resultados alentadores que ayudarán a diseñar un sistema de inyección in situ de los NFs.

Effective removal of radioactive cobalt from aqueous solution by a layered metal sulfide adsorbent: Mechanism, adsorption performance, and practical application

The removal of radioactive Co2+ from aqueous solutions is a critical issue because it is highly toxic and hazardous to humans. Metal sulfides are promising materials for the removal of toxic ions; however, studies on the adsorption of Co2+ by this class of adsorbents as well as their adsorption mechanisms and practical application in complex natural water matrixes are limited. To fill this knowledge gap, in this study, a layered K/Zn/Sn/S metal sulfide nanosheet (KZTS-NS) was employed as an adsorbent for Co2+ removal from aqueous solutions. Besides ion exchange, the mechanism of surface adsorption by forming a Zn0.76Co0.24S composite was found when initial Co2+ concentration was higher than 20 mg/L, which was reported for the first time in layered metal sulfides. The Co2+ adsorption performance of KZTS-NS was systematically studied using batch methods in deionized water, including isotherms, kinetics, influential factors, and selectivity. The results indicated that KZTS-NS exhibited a high adsorption capacity, rapid kinetics, and strong selectivity in a wide pH range. Furthermore, tap water was used as a complex natural water matrix, and the practical application of KZTS-NS in a bench-scale countercurrent two-stage adsorption-flocculation-microfiltration (CTA-F-MF) process for Co2+ removal from tap water matrix was demonstrated. The CTA-F-MF process significantly enhanced the performance of the adsorbent, thereby achieving ultrahigh Co2+ removal (decontamination factor of up to 1176). This study not only provides a new insight into the adsorption of Co2+ by layered metal sulfides, but also forms the basis to enable the practical application of metal sulfides in the field of radioactive wastewater treatment.

New insight into surface adsorption thermodynamic, kinetic properties and adsorption mechanisms of sodium oleate on ilmenite and titanaugite

Titanaugite particles are detrimental to the flotation of fine ilmenite. In this work, the surface adsorption thermodynamic, kinetic properties and the adsorption mechanism of sodium oleate on ilmenite and titanaugite surfaces were investigated by measuring surface wettability, lipophilic hydrophilic ratio, surface free energy, adsorption rate, adsorption density and adsorption thickness. The hydrophilicity of the minerals were determined by the contact angle, lipophilic hydrophilic ratio, γSLW value, and γSAB/γS value. The surface wettability results showed that the hydrophilicity of the minerals decreased following: −20 μm titanaugite >−20 μm ilmenite >−38 + 20 μm ilmenite >− 74 + 38 μm ilmenite. The adsorption process can be represented with Langmuir isotherm and a pseudo second-order kinetic model. Calculation of the adsorption thermodynamic parameters confirmed that the adsorption of sodium oleate on ilmenite surfaces is spontaneous, feasible, and endothermic. And it was alsoconfirmed that sodium oleate was adsorbed onto ilmenite due to both physical and chemical adsorption. In situ AFM imaging visually demonstrated that the adsorption density and thickness of the sodium oleate adsorbed onto titanaugite were lower than those of sodium oleate on ilmenite, which was consistent with the adsorption thermodynamic parameters. It was further confirmed that the electrostatic interaction, chemisorption, hydrogen-bonding adsorption and specific hydrogen-bonding adsorption were the main mechanisms for the sodium oleate adsorbed onto ilmenite, as the zeta potential and XPS spectra analyses.

Multimetal tolerant fungus Aspergillus flavus CR500 with remarkable stress response, simultaneous multiple metal/loid removal ability and bioremediation potential of wastewater

Abstract Microbial bioremediation of multimetal contaminated wastewater can mitigate its environmental toxicity and human health risk. A multimetal tolerating fungus Aspergillus flvaus CR500, able to tolerate As (2000 mg/L), Ni (1600 mg/L), Mn (1600 mg/L), Pb (1200 mg/L), Cr (800 mg/L), Cu (200 mg/L) and Cd (100 mg/L), evident by the tolerance index of 0.22, 0.16, 0.19, 0.09, 0.15, 0.12 and 0.29 respectively A. flavus CR500 adopted multifarious biochemical (enzymatic and non-enzymatic) and morphological strategies (observed by scanning electron microscopic analysis) to deals with As, Pb and Ni toxicity. During the batch study, isolate CR500 could able to remove 99.0, 97.7 and 73.1% of As, Pb and Ni respectively from individual metal contaminated potato dextrose broth (PDB) medium via bioaccumulation and surface adsorption mechanism. A. flavus CR500 also exhibited excellent multimetal (As, Ni, Pb and Cr) removal potential from simulated wastewater (SWW) and Tannery wastewater (TWW) with a high amount of biomass production rate. The removal from 5 mg/L of each metal amended SWW was 97.5, 93.3, 82.2 and 46.6% for As, Pb, Cr and Ni respectively. The toxicity of fungal treated metal solution was reduced, observed from good germination rate and growth of Vigna radiata seeds. Thus A. flavus CR500 has single as well as multimetal removal ability and can be used for ecofriendly treatment of multimetal contaminated wastewater.

Modeling of Mixed Mechanism Adsorption Processes Driven by Surface Adsorption and Internal Ion Exchange

In this work, a number of kinetic models describing adsorption processes that occur via a mixed mechanism of both simultaneous surface adsorption and internal ion exchange were developed. The propo...

Adsorption of Phenol from Wastewater Using Calcined Magnesium-Zinc-Aluminium Layered Double Hydroxide Clay

The presence of priority and emerging aromatic-based pollutants in water sources is of growing concern as they are not bioavailable and are present in reuse plant feed streams. These pollutants have known mutagenic and carcinogenic effects and must therefore be removed. Adsorption has been widely accepted as a suitable remediation technology due to its simplicity. Clay-based adsorbents have attracted significant attention due to their low cost, environmentally benign properties and regeneration potential. The present work focused on the thermal modification of a commercial Layered Double Hydroxide (LDH) clay and its subsequent effectiveness as an adsorbent in the removal of phenol from wastewater. Calcination of the neat clay resulted in the formation of metal oxides with varying phases and crystallinity depending on the treatment temperature. The BET surface area increased by 233% upon calcination at 500 °C. The highest phenol removal (85%) was observed in the clay calcined at 500 °C compared to 10% for the neat clay. Optimization studies revealed a maximum adsorption capacity of 12 mg/g at an adsorbent loading of 10 g/L at pH 7. Phenol adsorption was postulated to occur via a two-stage intercalation and surface adsorption mechanism. The equilibrium data were best fitted on the Freundlich isotherm model which describes heterogeneous adsorption. The adsorption kinetics followed a pseudo-second-order kinetic model with rate constants of 4.4 x 10−3 g/mg.h for the first 12h and 6.1 x 10−3 g/mg.h thereafter.

Enhanced Pb(II) adsorption onto functionalized ordered mesoporous carbon (OMC) from aqueous solutions: the important role of surface property and adsorption mechanism.

Functionalized ordered mesoporous carbon (MOMC-NP) was synthesized by chemical modification using HNO3 and H3PO4 to enhance Pb(II) adsorption. The phosphate functional group represented by P-O-C bonding onto the surface of OMC was verified by FT-IR and XPS. Batch adsorption experiments revealed the improvement of adsorption capacity by 39 times over the virgin OMC. Moreover, the Pb(II) adsorption results provided excellent fits to Langmuir model and pseudo-second-order kinetic model. The adsorption mechanism of Pb(II) onto MOMC-NP revealed the formation of metal complexes with carboxyl, hydroxyl, and phosphate groups through ion exchange reactions and hydrogen bondings. The calculated activation energy was 22.09kJ/mol, suggesting that Pb(II) adsorption was a chemisorption. At pH>pHpzc, the main Pb(II) existing species of Pb(II) and Pb(OH)+ combine with the carboxyl, hydroxyl, and phosphate functional groups via electrostatic interactions and hydrogen bonding. All these findings demonstrated that MOMC-NP could be a useful and potential adsorbent for adsorptive removal of Pb(II). Graphical abstract.

Experimental complemented with microscopic (electronic/atomic)-level modeling explorations of Laurus nobilis extract as green inhibitor for carbon steel in acidic solution

In the present study, comprehensive research efforts were made to experimentally/theoretically examine the corrosion protection capability of Laurus nobilis leaves extract for carbon steel in an acidic medium composed of 1 M HCl. The experimental EIS results evidenced that upon steel exposure to a corrosive electrolyte containing 400ppm L. nobilis leaves extract the maximum protection performance of 92% was obtained after 2.5h. The experiments of potentiodynamic polarization proposed a mixed-type anodic/cathodic anti-corrosion action along with small cathodic prevalence. The electronic features established by the DFT approach highlighted that the surface adsorption mechanism could be described by donor-acceptor interactions.

Thallium(I) sequestration by jarosite and birnessite: Structural incorporation vs surface adsorption

Jarosite and birnessite secondary minerals play a pivotal role in the mobility, transport and fate of trace elements in the environment, although geochemical interactions of these compounds with extremely toxic thallium (Tl) remain poorly known. In this study, we investigated the sorption behavior of Tl(I) onto synthetic jarosite and birnessite, two minerals commonly found in soils and sediments as well as in mining-impacted areas where harsh conditions are involved. To achieve this, sorption and desorption experiments were carried out under two different acidic conditions and various Tl(I) concentrations to mimic natural scenarios. In addition, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and inductively coupled plasma (ICP) analyses were conducted to determine the performance of both minerals for Tl(I) sequestration. Our results indicate that both phases can effectively remove aqueous Tl by different sorption mechanisms. Jarosite preferentially incorporates Tl(I) into the structure to form Tl(I)-jarosite and eventually the mineral dorallcharite (TlFe3(SO4)2(OH)6) as increasing amounts of Tl are employed. Birnessite, however, favorably uptakes Tl(I) through an irreversible surface adsorption mechanism, underlining the affinity of Tl for this mineral in the entire concentration range studied (0.5–5 mmol L−1). Lastly, the presence of Tl(I) in conditions where aqueous molar ratio Tl/Mn is ∼0.25 inhibits the formation of birnessite since oxidation of Tl(I) to Tl(III) followed by precipitation of avicennite (Tl2O3) take place. Thus, the present research may provide useful insights on the role of both jarosite and birnessite minerals in Tl environmental cycles.

Anticorrosive property of hexafunctional epoxy polymer HGTMDAE for E24 carbon steel corrosion in 1.0 M HCl: gravimetric, electrochemical, surface morphology and molecular dynamic simulations

The novel hexafunctional epoxy polymer synthesized in our laboratory, namely hexaglycidyl trimethylene dianiline of ethylene (HGTMDAE), has been evaluated as corrosion inhibitor of E24 carbon steel (CS) in the aggressive environment (1.0 M HCl) by using a gravimetric measurements, electrochemical impedance spectroscopy (EIS) and molecular dynamic simulations (MD). However, to better understand information on the surface adsorption mechanism of the CS/HGTMDAE/1.0 M HCl medium, the corrosion protection was investigated by using the scanning electron microscopy. Furthermore, the gravimetric test results showed that the inhibition performance will increase with increasing in the concentration of HGTMDAE to reach a maximum of 90% at concentration 10−3 M. In addition, the EIS has a better inhibitory efficiency of 95% for the optimal concentration (10−3 M) of the epoxy polymer HGTMDAE. Moreover, the polarization results indicate that this macromolecular binder acts as a mixed inhibitor. The epoxy polymer formed protective layer on surface of the E24 CS by adsorption according to Langmuir model. The inhibitor adsorption phenomena have been tested through the estimation of kinetics thermodynamic parameters. MD simulations were used to describe the electronic and adsorption properties of epoxy polymer HGTMDAE.

Heterogeneous catalyst ozonation of Direct Black 22 from aqueous solution in the presence of metal slags originating from industrial solid wastes

This study developed a low cost catalyst, namely, zinc slag (Zn-S) for the ozonation process of Direct Black 22 (DB22) from aqueous solutions. Among five different kind of low cost metal slags including Fe-S, Cu-S, Cd-S, Pb-S and Zn-S, the Zn-S slag was selected as an efficient catalyst in this study. Zn-S contained mainly zinc (Zn) and calcium (Ca) discharged from zinc slag waste in Vietnam. It was found that Zn-S could effectively decolonize and mineralize DB22 through heterogeneous catalytic ozonation. The degradation kinetic of DB22 followed the pseudo-first order model. The best removal efficiency of DB22 (Zn-S/O3/H2O2 (76%) > Zn-S/O3 (69%) > O3/H2O2 (66%) > O3 (55% for COD) occurred at pH 11 for heterogeneous catalytic ozonation processes with Zn-S as the catalyst as well as ozone alone and perozone processes due to fast decomposition of O3 in alkaline solution to generate powerful and non-selective OH radicals. An increase in decolonization and mineralization rate was observed when increasing the Zn-S dosage from 0.125 g/L to 0.75 g/L for Zn-S/O3 and 0.125 g/L to 1.0 g/L for Zn-S/O3/H2O2. The K values of the pseudo-first order model followed the same sequence as mineralization rates of DB22 in term of COD removal. Ca and Zn constituents in the Zn-S catalyst contributed to the increase in O3 decomposition and improvement of reaction rate with H2O2. Subsequently, the degradation of DB22 by the ozonation process with Zn-S catalyst was enhanced through the enrichment mechanism of hydroxyl radicals (*OH) and surface adsorption. The degradation mechanism of DB22 by hydroxyl radicals was surely affirmed by tests with the decrease in degradation percentage of DB22 in case of the presence t-butanol, Cl− and CO32−.

Suction Stress of Clay Over a Wide Range of Saturation

Quantifying the relationship between water retention and internal stress state of unsaturated soils has long been a challenge. Current effective stress formulations can effectively capture the relationship for coarse-grained soils, where capillary mechanisms dominate internal stress state over a wide range of saturation, but fail for fine-grained soils, particularly at low to intermediate saturation, where surface adsorption mechanisms dominate water retention and corresponding stress state. Internal stress state of compacted kaolinite specimens spanning saturation (S) from 0 to 0.73 is measured through Brazilian tensile strength tests. Changes observed in strength, strain, and stiffness are analyzed to calculate evolution of internal stress with saturation. A model is proposed to quantify the relationship between water retention and internal stress for fine-grained and coarse-grained soils at saturations over the full range (0 ≤ S ≤ 1). Performance of the model is evaluated by comparison with experimental results from the literature.

Sustainable Non-Thermal Plasma-Assisted Nitrogen Fixation-Synergistic Catalysis.

In this Minireview, the multiple chemical synergies present in catalytic non-thermal plasma-assisted nitrogen fixation (NTPNF) are uncovered through a critical exploration of the underlying mechanisms, during which the catalyst, plasma, and reactants play different roles. For the gas-phase NTPNF, the synergies consist of different aspects of the catalytic pathways such as electron-impact dissociation; Zeldovich mechanism in the PNO interactions; and Eley-Rideal, Langmuir-Hinshelwood, surface adsorption, and diffusion mechanisms for the plasma-catalyst interactions. The synergies within the gas-liquid NTPNF involve contributions of plasma and UV excitation to the gas-phase reactions and the UV excitation of molecules at the liquid-surface interface, which improves synthesis of aqueous nitrate, nitrite, and ammonium products. Based on the various synergistic mechanisms during NTPNF, future potential applications are proposed for how NTPNF could benefit the sustainable nitrogen fixation industry.

Computational and experimental studies on the efficient removal of diclofenac from water using ZnFe-layered double hydroxide as an environmentally benign absorbent

Abstract Layered double hydroxide (LDH) has great application in the treatment of organic pollutants. In this study, Zn–Fe LDH was prepared by co-precipitation method and characterized using different analytical instruments including X-ray diffractometer (XRD), zeta potential, Fourier-transform infrared spectrometry (FTIR) spectroscopy, X-ray diffractometer (XPS), Field emission scanning electron microscope (FESEM), and High resolution transmission electron microscope (HRTEM). Also, the efficiency of Zn–Fe LDH as adsorbent for the removal of diclofenac sodium (DCF) from aqueous solution was investigated under different experimental conditions, such as pH, dose of adsorbent, pollutant concentration, and contact time. The Zn–Fe LDH has successfully removed a high percentage of diclofenac from water within 30 min, and its adsorption capacity at equilibrium was 74.50 mg/g. Several isothermal and kinetic models of the adsorption process were investigated. It was found that diclofenac adsorption was best fitted with three and four-parameter isotherms and the pseudo-second order kinetic model. The surface adsorption mechanism was explored computationally by means of Monte Carlo and molecular dynamic simulations in gas and water phases. The simulation results have shown that diclofenac strongly interacts with Zn–Fe LDH through a variety of hydrogen bonds, as well as metal complexation. The in-vivo toxicity study in mice revealed the safety of Zn–Fe LDH on different body organs through estimation the median lethal dose that kill 50% of the animal numbers.

Revealing the effect of preparation parameters on zeolite adsorption performance for low and medium concentrations of ammonium

The effect of preparation parameters on the performance of zeolite for ammonium (20–300 mg N/L) adsorption from simulated wastewater is reported. It was found that the ratios of Na2O/SiO2 and Si/Al had a more important influence than crystallization time on zeolite adsorption properties. Relatively low Na2O/SiO2 ratios were beneficial for fabrication of zeolites with high proportions of micropore area and volume, which led to the surface adsorption mechanism being dominated by surface free energy and pore effects. However, with decreasing Si/Al ratios, the effect of ion-exchange was more prominent due to the high negative surface potential of zeolite. In addition, the concentration of weak acid sites on the zeolites was increased with lower ratios of Na2O/SiO2 and Si/Al, which may promote ammonium removal. Therefore, the most effective zeolite for ammonium removal, which was fabricated at Na2O/SiO2 = 1.375, Si/Al = 4 and crystallization time of 48 hr, exhibited the cooperative effects of adsorption, ion-exchange and a large amount of weak acid sites. The maximum ammonium adsorption capacity (35.06 ± 0.98 mg/g) and the removal efficiency (94.44% ± 4.00%) were obtained at the dosage of 4.0 g/L zeolite NaX at ammonium concentrations of 300 mg N/L and 20 mg N/L, respectively. The Freundlich isotherm and pseudo-first-order kinetics models provided excellent fitting for the ammonium adsorption process. In addition, zeolite NaX showed about 1.23–3.2 times the ammonium adsorption capacity of clinoptilolite. The stable and efficient reusability of zeolite NaX after five regeneration cycles demonstrated that this adsorbent has considerable potential for practical industrial applications.

Surface‐enhanced Raman spectroscopy for successful probing of itraconazole within poly(lactic‐co‐glycolic acid) nanoparticles

AbstractThe present study reports on successful use of surface‐enhanced Raman spectroscopy to investigate itraconazole (ITZ) molecule loaded within poly(lactic‐co‐glycolic acid) (PLGA) nanoparticles (NPs). SER spectra were recorded by depositing samples onto a nanostructured silver film, which was fabricated via electrolyte deposition of silver NPs onto silver substrate. Transmission electron microscopy, dynamic light scattering, zeta potential, and Fourier transform infrared spectroscopy were employed to support our analyses. While compared with normal Raman spectrum of ITZ powder, changes observed in the SER spectra of the samples comprising ITZ‐loaded PLGA‐NPs (sample ITZ@PLGA‐NPs) and ITZ mixed with PLGA‐NPs (sample ITZ+PLGA‐NPs) provided insights regarding the interaction between the ITZ molecule and both the silver film's surface and the PLGA‐NPs. We found spectral evidences that free ITZ molecule (sample ITZ+PLGA‐NPs) interacts with the silver film's surface via the nitrogen heterocyclic closest to the ITZ's phenyl rings. In this condition, it was verified that the ITZ molecules are more likely oriented parallel to the silver film's surface. Additionally, the ITZ molecule in the ITZ@PLGA‐NPs sample is not adsorbed onto the silver film's surface likely due to steric hindrance provided by the PLGA template. Nevertheless, the SER spectrum of the ITZ@PLGA‐NPs sample shows the signature of the ITZ molecule (likely via the electromagnetic mechanism) and the PLGA template (via the surface adsorption mechanism). Moreover, the SER spectrum recorded from the ITZ@PLGA‐NPs sample indicates weak interaction between the loaded ITZ and the PLGA template. We anticipate that the pioneering approach herein introduced can be successfully used to investigate a wide variety of bioactive molecules encapsulated within polymeric templates, thus providing a very promising, sensitive, and robust analytical tool not yet explored in this regard.

Graphene-Based Waist-Enlarged Optical Fibre Sensor for Measurement of Sucrose Concentration

Abstract A novel sucrose sensor based on graphene-coated optical fibre with waist-enlarged bitapers as Mach-Zehnder interferometer is proposed and demonstrated. The sensor is formed by arc fusion splicing a photonic crystal fibre (PCF) sandwiched between two single-mode fibres (SMFs). The intermodal interference is achieved by two waist-enlarged fibre tapers at the coupling points between the PCF and two SMFs. The result shows that the dips of transmission spectra exhibit blue shift with the concentration increase of the sucrose, and the sensor has a high linear response (R2 = 0.98233) to sucrose with an excellent sensitivity of 3.36 pm/ppm in the range of 0–230 ppm. Additionally, the surface adsorption mechanism is also discussed. Such easily fabricated, cost-effective and small volume fibre interferometer could be used for sucrose sensing applications.

A fast and efficient stabilization of firefly luciferase on MIL-53(Al) via surface adsorption mechanism

Stabilization and immobilization on a proper support is essential in processing and production of enzymes. Luciferase is an unstable, thermally sensitive, and light-emitting enzyme that requires stabilization for efficiency and productivity in bio-luminescence reactions. In this work, a known and well-porous framework, MIL-53(Al), was used, for the first time, to support the immobilization of firefly luciferase. Interestingly, all the stability values were improved through the immobilization process, and the content of the loading enzyme on MIL-53(Al) was found to be significant. The results obtained indicated that the stabilization of firefly luciferase on MIL-53(Al) proceeded through the adsorption method. MIL-53(Al) was also characterized by the FT-IR, X-ray diffraction, FE-SEM, TEM, BET, and TGA techniques in order to determine the carboxylic groups and symmetric/asymmetric CO2, percentage of crystallinity and its purity, morphology, size, and shape of the MIL-53(Al) particles, and surface-to-area ratio and thermal gravimetry of support.

Freestanding CNT-modified graphitic carbon foam as a flexible anode for potassium ion batteries

The CNTs/GCF shows long and stable discharge plateaus and the hybrid potassium storage mechanism of surface adsorption and intercalation.

Enhanced dispersion of carbon nanotubes in water by plasma induced graft poly(N,N-dimethylacrylamide) and its application in humic acid capture

Humic acid (HA) is the main culprit of producing the carcinogenic, teratogenic, and mutagenic halogenated alkanes. In present work, multi-walled carbon nanotubes (MCNT) were grafted N,N-dimethyl-acrylamide (NDMA) to enhance its type and concentration of the functional groups by using plasma technique. The MCNT/NDMA composites exhibit better dispersion property and higher HA adsorption efficiency. The addition of amide group could strongly enhance the HA adsorption property of MCNT due to less acidic of MCNT/NDMA (pHpzc = 5.0) than MCNT (pHpzc = 4.5). The results highlight the effect of pH values, ionic strength, Pb(II) concentrations and the Pb(II) addition sequences. HA adsorption by MCNT/NDMA obeyed the Langmuir isotherm model, attributed to an outer-sphere surface adsorption mechanism through π–π interactions and coordination of amide group to the functional groups on HA. The presence of Pb(II) significantly effects the HA adsorption in the three difference addition sequences due to different mechanisms.