Granite-MWCNTs nanocomposite coated with Dialium guineense stem bark extract for enhanced adsorption of chromium(VI)

ABSTRACT The prominent benefit of granite is owned to its physicochemical property and ubiquitous nature. Vast application of granite which also includes its use as an adsorbent in environmental remediation practice, can also be enhanced. To further enhanced the uptake capacity of granite, nanocomposite consisting of multiwall carbon nanotubes (MWCNTs) and granite was fabricated and further modified using Dialiumguineensestem bark extract. The structure and composition of pristine granite (PG) and modified nanocomposite granite (G) based material were examined and confirmed by the FTIR, Raman, TGA, SEM and XRD. Meanwhile, the specific surface areasof PG (1.268 m2/g) and G (16.57 m2/g) were obtained using the BET surface area analyser. The optimization step revealed that the uptake capacities of PG and G were dependent on solution pH, sorbent dose and contact time. Meanwhile, pseudo-second-order and Elovich kinetic models were noticed to best describe the data for the removal of Cr (VI) by PG and G. Equilibrium isotherm study revealed that Freundlich and Langmuir models fitted well to the experimental data obtained for the uptake of Cr(VI) onto PG and G respectively. Furthermore, electrostaticattraction betweentheDialiumguineense stem bark extract on the surface of G and Cr(VI) influenced the uptake of Cr(VI). On the other hand, the interaction between the plant extract and Cr(VI) may result in the attenuation of Cr(VI) via reduction to Cr(III). Finally, the thermodynamically favoured adsorptive process demonstrated high adsorbent reusability with good stability for Cr(VI) uptake.


Introduction
The trace element chromium is a major environmental contaminant, commonly found in its trivalent oxidation state (Cr(III)).In this state,it is an essential micronutrient that is used by plants and animals for some metabolic processes [1][2][3].Similarly, Cr also exists in the hexavalent state (Cr(VI)) in the aquatic environment as oxyanions.The seoxyanions (CrO 4 and Cr 2 O 7 2-) are pH-dependent and are known to form soluble compounds in the aqueous environment [4,5].Meanwhile, Cr(III) ions react with the available hydroxyl ions in the aqueous environment to form stable and insoluble oxyanions.Anthropogenic activities and recent technological advances are the main sources of hazardous metal ions discharged into the environment [6][7][8][9][10].The high volume of metal ions deposited on soils, aquifers and the aquatic ecosystem has been attributed to industrial processes such as chemical manufacturing, electrical and electronic equipment, wood protection, brass, leather tanning, pigments, electroplating, magnetic tapes, metal finishing and catalytic processes among others [11][12][13][14][15][16].
The birth of nanoscience has paved way for the discovery of new materials such as carbon nanotubes.The unique properties of carbon nanotubes include robust mechanical properties [45,46], good electrical property [47], strong chemical stability and large specific surface area [48].The porosity and surface area of these tubes confers on them the capacity to function as an adsorbent in environmental remediation practices.Researchers have extensively used multiwalled carbon nanotubes (MWCNTs) in the adsorption of contaminants from the aqueous phase [49,50].Meanwhile, modified-MWCNTs have demonstrated greater uptake capacity in remediation studies [51][52][53][54].The performance of MWCNTs as adsorbents for various pollutants can be greatly improved by coating MWCNTs on suitable support to enable more adsorbate to assess more active adsorption sites [55].Granite is a relatively cheap and readily accessible support and in the presence of plant extracts, the MWCNTs can be loaded onto the granite to form a nanocomposite with improved adsorption capacity.Plant extracts are generally rich in phytochemicals with functional groups capable of not just binding the MWCNTs to the granite but also serve as adsorption points in the composite.We envisage that the nanocomposite will have improved adsorption efficiency based on the synergic interaction of the combined components.Therefore, this study aim to synthesise a novel nanocomposite composed of granite and MWCNTs coated with Dialiumguineese plant extract with enhanced adsorption potentials for Cr(VI) ion.

Samples and sample preparation
Thestem bark of Dialiumguineense(DGSB) was collected from the premises of the Michael Opkara University of Agriculture Umudike, AbiaState, Nigeria.The DGSB was thoroughly washed using distilled water, dried and pulverised to a fine powder with the aid of an electric grinder.The ground DGSB was preservedfor further use.About 500 g of powered DGSB was suspended in 0.5 dm 3 distilled water for 7 days at room temperature [56].The extract was concentrated and stored for use.

Granite rock preparation
The granite rock was kindly provided by the Geology department, Michael Okpara University of Agriculture Umudike, Nigeria.The granulated rock was further reduced to a fine powder using a ball mill machine.The pulverised granite (10 g) was then treated with 0.1 M HNO 3 (150 cm 3 ) for 3 h under constant stirring at room temperature.Thereafter, the mixture was filtered, washed to neutral pH and oven-dried.

Preparation of nanocomposite
The MWCNTs were purified using HCl and thereafter functionalised in a 3:1 (v/v) nitric acid-sulphuric acid mixture [57,58].About 1 g of the functionalised nanotubes (f-MWCNTs) was weighed into a 100 cm 3 beaker containing 10 g of pulverised granite rock dispersed in 50 cm 3 of deionised water.The mixture was stirred for 6 h, thereafter, 5 cm 3 of glutaraldehyde (crosslinker) was added to the mixture (f-MWCNTs/PG) and stirred to dryness at a temperature of 75°C.The crosslinked f-MWCNTs/PG (nanocomposite) obtained was then dried and preserved for further treatment.About 2 g of the nanocomposite was transferred into 20 cm 3 of Dialiumguineensestem barkaqueous extract.The mixture was stirred to dryness at 50°C.The black product (G) was vacuum oven-dried, ground and stored in an air tight container for further use.

Characterisation
Morphological characteristics of PG and G were performed using field emission scanning electron microscopy (SEM) (ZEISS ultra plus).The XRD patterns of PG and G were achieved by making use of an X-ray diffractometer (XRD Bruker D8 Advance powder x-ray diffraction).Surface functionalities of the adsorbent were obtained using Fourier transform infrared (FTIR) spectroscopy (Thermo Nicolet-870 spectrophotometer).Raman spectra of pristine and Cr-loaded adsorbent were recorded in the region 4000-100 cm −1 on a Bruker RFS 27 FT-Raman spectrometer.TGA measurements were acquired using Mettler Toledo TGA/DSC1 Star System.The specific surface area, pore volume and size of PG and G were estimated using Brunauer-Emmett-teller (BET) nitrogen sorption-desorption method (Micromeritics Instruments Corp., USA) and Barret-Joyer-Halenda (BJH) model.

Batch adsorption experiments
The uptake of Cr(VI) onto PG and G was investigated by making use of the batch adsorption mode.In particular, 50 mg of PG or G was contacted with 25 cm 3 of Cr(VI) solution (adjusted to pH 2) in a 100 cm 3 stoppered glass bottle fixed on a thermostated water bath.The mixtures were withdrawn at the preset contact time and filtered.Thereafter, the remaining concentration of Cr(VI) was measuredvia the colourimetric technique utilising 1,5-diphenyl-carbazide as a complexing agent at 540 nm with the help of the UV-visible spectrophotometer [59].Adsorbent doses, solution pH, contact times, Cr(VI) concentrations and solution temperature were cardinal adsorptive parameters used for the optimisation of the process.The uptake efficiency, adsorption capacity, thermodynamics, kinetics and isotherm analysis of adsorption was evaluated as described in the supplementary information.

Reusability experiments
A reusability study of PG and G was used to assess the limit of the effectiveness of the adsorbents after several cycles.A similar adsorption procedure was used to load Cr(VI) onto the surface of the adsorbents.Meanwhile, the regeneration of PG-Cr and G-Cr was achieved by making use of NaOH.About 0.5 g of PG-Cr or G-Cr was contacted with 25 cm 3 of 0.5 mol dm −3 NaOH for 3 h at 25°C.The regenerated PG and G were washed, dried, and preserved for the next cycle.The percentage removal of PQ and Q for the next cycle was calculated.

Materials characterisation
Figure 1(a) shows a representative SEM micrograph of the purified MWCNTs used in the preparation of the nanocomposite.The nanotubes were observed to have an outer and inner diameter in the range 10-18 nm and 4-5 nm respectively.The SEM micrograph (Figure 1(b)) of the granite showed grains that were irregular in shape with random size distribution.With the DGSB aqueous extract, the helical tubular structures of the MWCNTs were observed to be coated on the surface of the grains forming a network of porous heterogeneous microscopic craters in the corresponding nanocomposite adsorbent developed Figure 1(c).The EDX spectrum of the spent adsorbent showed the presence of Cr among other elements, with a 0.10 wt% representing the area under scan and not the entire adsorbent surface.
The textural properties of the nanocomposite adsorbent (G) in comparison with the pristine granite (PG), showed improved surface area from 1.27 (PG) to 16.57 (G) m 2 g −1 .This represented a 12-fold increment in the surface area, attributable to the presence of the MWCNTs.An increase was also observed in the pore volume, while the diameter was narrowed to 23.56 nm (see Table 1).The increment in both surface area and pore volume could significantly enhance the rate and capacity of adsorption of the nanocomposite.The BET analysis of PG and G also revealed a type III isotherm with similar adsorption and desorption profiles an indication of a possible multilayer adsorption process (see Figure 2).
The composition of the pristine granite (PG) and the developed nanocomposite (G) were revealed by XRD analysis.The diffractograms (Figure 3) showed the presence of quartz, microcline, albite, clinochlore and ferropargasite.Quartz was the most prominent of the identified mineral and it accounted for the silicon content as revealed by the EDX analysis.The diffraction peaks associated with MWCNTs were not observed, which could be attributed to the overwhelming dominant peaks of the granite minerals.The FTIR spectra of the granite (PG), showed characteristic bands of silica at (ν/cm −1 ): 1020 (Si-O-Si asym ), 741 (Si-O-Si sym ), 557 (O-Si-O) and 494 (Si-O-Si bend ).This confirmed the prevalence of quartz in the mineral composition of the granite as revealed by the XRD analysis.Similarly, these bands were present in the spectra (Figure 4) of the derived composite (G) and the corresponding spent adsorbents (PG-Cr and G-Cr).Additionally, the spectra of G and G-Cr showed vibrational bands associated with the organic functional groups present in the plant extract and MWCNTs at (ν/cm −1 ): 3400 (O-H, N-H), 2835 (C-H str ), 1790 (C = O) 1590 (C = C) 1416 and (C-H bend ).However, the intensities of these bands, especially O-H, C = O, significantly diminished in the spectrum of G-Cr, this indicated the most active adsorption points on the nanocomposite.In comparison with PG and G, a shift to lower frequency was observed in the band associated with Si-O-Si asym vibration in PG-Cr and G-Cr, which showed the contributions of the fundamental framework of the granite in the adsorption of Cr ions from water.
The sequestered capture of Cr on the materials was also established with Raman spectroscopic analysis, which revealed the peak at 467 cm −1 characteristics of Cr (III) ion (see Figure 5).Our result obtained is in good agreement with a previous work [60].This peak was not observed in Raman spectra prior to adsorption.The increase in the peak intensity associated with Cr (III) in G-Cr attested to the improved adsorption capacity of the nanocomposite in comparison with the unmodified granite.

pH effect
The stability of Cr(VI) and the surface chemistry of the adsorbents (PG and G) are strongly affected by solution pH.Hence, the uptake capacity of the adsorbents will be affected b ythe variation in solution pH.At solution pH less than or equal to 6, Cr (VI) exists in theiroxyanionic forms (HCrO 4 − and Cr 2 O 7 2-).This pH range is commonly used to ensure that chromium species exist in theirhexavalent state.To examine this phenomenon, the pH PZC (point of zero charge (PZC)) experiment was performed.The pH PZC is the pH when the net charge on the adsorbents (PG and G) surface is zero.The pH PZC of the PG and G was estimatedby making use of the solid addition method [61][62][63][64][65]. Figure 6 revealed that the surface of PG and G will be neutral at a pH of 6.49 and 3.39, respectively.Hence, it indicates that above and below these pH values, the material surface will be negatively and positively charged respectively.Taking into consideration the implication of the pH PZC , it shows that at low solution pH, the surface of the materials (PG and G) will be positively charged, hence, electrostatic interactions between the positively charged sorbents surface and the oxyanions, or the possible reduction of Cr (VI) to Cr (III) in a sufficiently protonated aquatic environment could be responsible for the uptake of Cr(VI) by PG and G. Poor uptake of Cr (VI) at high solution pH could be due to strong electrostatic repulsion between oxyanion and the negatively charged surface of the adsorbent or the higher competition with OH − for the active site on the surface of the adsorbent.The results showed that the low solution pH enhanced the Cr(VI) removal.The capacities of PG and G were optimum at pH 2 and this was selected as the optimum pH (see Figure 7).Meanwhile, the result obtained is in good agreement with other reports [66,67].

Contact time influence
Figure 8 displays the effect of contact time on the adsorption of hexavalent chromium from simulated wastewater.Rapid adsorptive removal of Cr (VI) from 16.52 to 30.25 mg g −1 for PG and 44.78 to 55.34 mg g −1 for G within 5 to 60 min were observed.The aforementioned observation can be attributed to a large volume of vacant uptake sites on PG and G at the early stage of the adsorption process.Meanwhile, a gradual increase within 60 to 180 min was noticed for PG and G.However, within the contact timeframe of 180 to 1440 min, an insignificant increase in the uptake capacity of the adsorbents (PG (34.22 to 34.48 mg g −1 ) and G(60.88 to 61.15 mg g −1 )) were observed as the adsorptive system approaches equilibrium.The slow Cr (VI) adsorption rate with increased contact times can be ascribed to thereduction in the adsorptive driving force resulting from the reduced uptake sites on the materials (PG and G) surfaces and the reduced sorbate concentration with increased contact time.A similar result was documented in the  attenuation of Cr (VI) onto iron-based nanoparticles [68], chitosan-MWCNTs [69] and activated carbon [70].
The experimental data acquired from the contact time experiments were fitted into four kinetic models (see supplementary information) (see Figure 9).The kinetic information obtained from these models are summarized in Table 2.As shown in Table 2, kinetics data obtained for the adsorption of Cr(VI) onto PG and G were better described by the pseudo-second-order model and Elovich kinetic models, respectively.This deduction is based on the goodness of fit measure (the model that has the lowest sum of squared residuals (SSR) and the least residual standard error (RSE) values).Meanwhile, the experimental uptake capacity (q exp ) of PG was very close to the model-calculated uptake capacity (q eq ), confirming the high correlation of Cr(VI) adsorption onto PG to the pseudosecond-order model.The mechanism removal of Cr(VI) by PG, indicates a bimolecular interaction between the oxyanions sorbate species and the positively charged active sites  of PG.The removal of Cr(VI) by G was best described by Elovich, suggesting chemisorption as its mechanism of adsorption, this further supports the fact that the adsorbate may have been reduced from Cr(VI) to Cr(III) followed by the efficient uptake of Cr(III) onto the surface of G.
To sufficiently deduce the mechanism of Cr(VI) uptake on PG and G, Morris-Webber mechanistic model was used to investigate the intrinsic diffusion steps.To achieve this, the experimental data obtained from the contact time experiment were fitted into a linear plot of q eq vs t 0.5 , in which a nonzero intercept was obtained.This suggested that the uptake of Cr(VI) by PG and G were controlled by two or more steps.

Dosage effect
Figure 10 shows the implication of adsorbent dose on the removal of hexavalent chromium from simulated wastewater by PG and G.The experimental result showed that the efficiencies of PG and G were dependent on dosage.The percentage of Cr(VI) uptake increased from 23.95 to 71.17% for PG and from 61.98 to 97.57% for G, as their dosages were increased from 0.01 to 0.4 g.The increasing number of adsorption sites with increased dosage at fixed adsorbate concentration may be responsible for this effect.Meanwhile, the adsorption capacities of the adsorbents were noticed to reduce with increased adsorbents dose.This could be associated with aggregation of the adsorbent which hinders the effective use of the adsorption sites.

Effect of temperature and Cr(VI) concentration
Figure 11 shows the influence of Cr(VI) concentration on the uptake capacity of PG and G at various temperatures.At all temperatures investigated, the adsorption capacity of PG and G was observed to increase with increasing initial Cr(VI) concentration.At 298 K, the amount of Cr(VI) adsorbed by PG and G were 7.34 and 8.56 mgg −1 for the lowest initial Cr(VI) concentration of 11.76 mg dm −3 and increased to 46.0 and 51.24 mgg −1 respectively for the highest initial Cr(VI) concentration of 100 mg dm −3 .Meanwhile, a similar trend was observed at 303, 308 and 318 K.This observation may be due to the decelerated mass transfer resistance on Cr(VI) between the sorbate-sorbent interface as a result of increased driving force induced by the increasing initial Cr(VI) concentration.In addition to this, the uptake of Cr (VI) was enhanced due to an increased number of collisions between the oxyanions of Cr(VI) and Dialiumguineensestem bark extracton the surface of the sorbents as a result of the increasing initial Cr(VI) concentration.On the other hand, the implication of solution temperature on the adsorptive systems were not significant for initial Cr(VI) concentrations less than 70 mg dm −3 , but at a higher concentration.An enhancing impact of solution temperature was observed for G. Hence, the adsorption of Cr(VI) onto G was endothermic.On the contrary, the effect of solution temperature was inconsequential on the adsorption Cr(VI) onto PG.

Adsorption isotherms
The relationship between the residual Cr(VI) concentration in the liquid phase and the amount of Cr(VI) removed per unit mass of PG or G at equilibrium were analyzed by making use of eight isotherm models (see supplementary information).The estimated values of the adsorption isotherm parameters for Cr(VI) adsorption onto PG and G are shown in Table 3. Comparatively, the SSR and RSE of the isotherm models were used as indicators in selecting models that best describe the experimental data.Meanwhile, among the eight isotherms used, Freundlich isotherm models presented satisfactory SSR, hence, was sufficient to describe the experimental data of Cr(VI) uptake onto PG within the studied temperature range.This model assumes a multilayer surface, containing heterogeneous adsorbent sites that are energetically non-equivalent with the tendency to interact with each other [71].The dimensionless Freundlich constant n is associated with the intensity of adsorption and surface heterogeneity, were higher than 1 and less than 10 (1 < n < 10), suggesting auspicious adsorption of Cr(VI) onto PG within the investigated temperature range.On the other hand, the Freundlich constant K F is related to the uptake capacity of PG.However, K F and n were not significantly affected by increased solution temperature.Over the studied temperature range, data acquired from the initial Cr(VI) concentration experiment for the uptake of Cr(VI) onto G was best described by the Langmuirmodel.Hence, it indicates monolayer adsorption of Cr(VI) onto the homogeneous surface of G containing a finite number of binding sites that do not interact [72,73].
The maximumuptake capacity (q max ) of G increased from 60.5 to298.9mgg −1 when the temperature increased from298 to 318 K (see Table 3).Meanwhile, the increase in solution temperature did not affectthe Langmuir constant b.A comparison of the maximum removal capacity(q max ) for Cr(VI) onto PG and G and that obtained for various adsorbents (see Table 4), revealed that G was a more efficient adsorbent for the elimination of Cr(VI) from the aquatic ecosystem.This may be attributed to the enhanced impact of Dialiumguineensestem bark extract on the pristine granite rock.With regards to the mechanism of uptake, electrostatic attraction between the plant extract on the surface of G and Cr(VI) may play a key rôle in the adsorption of Cr(VI) onto the surface of G.In a similar fashion, the interaction between the reductant (plant extract) and Cr(VI) may reduce the toxicoxyanionto Cr(III) which is less harmful.

Thermodynamics and adsorbent reuse
The thermodynamic behaviour of Cr(VI) uptake onto the surfaces of PG and G was investigated and the result suggests the nature of the binding forces responsible for the uptake.As shown in Table 5, the negative values of ∆G° suggest a spontaneous and favorable removal process within the studied range of concentration and temperature.
Positive enthalpy and entropy values were calculated, this also indicates endothermic adsorption process and increased randomness of the sorbate at the sorbent-sorbate interface, respectively.Hence, successful adsorption of Cr(VI) was achieved for the adsorbents.

Reclyclability
Using NaOH as an eluting agent, the reusability of PG and G for the uptake of Cr(VI) was assessed.As shown in Figure 12, The Dialiumguineensestem bark extract modified nanocomposite (G) and the pristine granite (G) was noticed to sustain about 66% and 25% efficiency after the fifth cycle respectively.Meanwhile, a reduced efficiency was observed for both adsorbents with the increase in usage and this could be due to the loss of loosely bonded modifiers on the surface of G and loss of active sites on PG.The result indicated good stability (66% after the fifth cycle (G)) for nanocomposite and hence, G can be recommended for large scale water treatment process.

Conclusion
A novel nanocomposite (G) composed of multiwallcarbon nanotubes (MWCNTs) and granite coated with the stem extract of Dialiumguineensewas successfully synthezied, characterised and used for Cr(VI) adsorption.The nanocomposite (G) showed enhanced thermal stability, good surface morphology, enhanced surface area and undisrupted crystalline phases.The application of the Dialiumguineensestem bark extract as a modifier was noticed to enhance Cr(VI) removal from water by 23.52% at 318 K.The uptake of Cr(VI) onto pristine granite (PG) and G was noticed to be pH, sorbent dose and contact timedependent.Meanwhile, the increase in initial Cr(VI) concentration was observed to enhance the uptake capacity of the adsorbent.The maximum Cr(VI) removal capacity was 29.97 mg g −1 and 60.05 mg g −1 for PG and G respectively.Meanwhile, the kinetics data for the removal of Cr(VI) by PG and G fitted best into the pseudo-second-order and Elovich kinetic models respectively.The nanocomposite displayed a sustainable capacity cafter several cycles, hence, can be recommended for a large scale waste water treatment plants in the decontamination of Cr-loaded wastewater.

Figure 2 .
Figure 2. N 2 adsorption-desorption isotherms profile of PG and G.

39 Figure 6 .Figure 8 .
Figure 6.Effect of initial solution pH on the uptake of Cr(VI) onto PG and G.

Figure 7 .
Figure 7. pH PZC plots of PG and G.

Figure 10 .
Figure 10.Effect of dosage on the (a) efficiency (%) and (b) the uptake capacity (q eq ) of Cr(VI) removal by PG and G.

Table 1 .
Textural properties of PG and G.

Table 2 .
Kinetic parameters obtained for the uptake of Cr(VI) onto PG and G.

Table 3 .
Adsorption isotherm model constants for Cr(VI) uptake onto PG and G.

Table 4 .
Comparison of maximum Cr(VI) adsorption capacity of PG and G adsorbent with other reported works.

Table 5 .
Thermodynamic parameters for the uptake of Cr(VI) onto PG and G.