Introduction

Water is a very important constituent of the ecosystem on the Earth and essential component of life. The demand of water is increasing day by day due to an increase in population and in living standard. The quality of our water resources is deteriorating day by day due to the continuous addition of undesirable chemicals in them. Various water pollutants, such as metal ions, the residues of the drugs and pharmaceuticals (new emerging pollutants), dyes, Chiral pollutants and pesticides in water, are hazardous. Since these pollutants are very dangerous and have a direct and indirect impact on the human health life and the surrounding environment, many reviews and studies about have been published [1,2,3,4,5].

The different methods have been developed and used for water treatment. These methods include filtration, screening, oxidation, precipitation, coagulation, centrifugation, flotation, crystallization, sedimentation, distillation, evaporation, reverse osmosis, electrochemical, ion exchange, adsorption, etc. Among them, adsorption phenomenon is considered as one of the appropriate water treatment methodologies because of its ease of operation and the accessibility with a wide range of adsorbents for the removal of different pollutants [6,7,8,9,10,11,12,13,14,15,16].

When oil is explored, transported and stored and its derivatives are used, there is risk for spillage with the potential to cause significant environmental impact [17]. Due to its destructive properties, the entire character of the area is damaged once an area has been contaminated with oil. In addition, when oil encounters something to cling to (e.g., beach, rocks, feathers of a duck or a bathers hair), it is difficult to remove [18]. Therefore, pollution by petroleum oils affects sea life, economy, tourism and leisure activities due to the coating properties of these materials [17].When oil is spilled in water or on land, the physical and chemical properties of oil change progressively. The spilled oil contributes an undesirable taste and odor to drinking water and causes severe environmental damage [19]. Oil pollution from food production, catering industry, petroleum chemical and petroleum mining has drawn extensive attention of many researchers in recent years. Many methods, such as mechanical extraction, in situ combustion and chemical degradation, have been used to the cleanup of oil from polluted areas. Owing to better economical and environmental benefits, the use of sorbent is considered as an effective method to concentrate, transfer and absorb spilled oil [20]. High-efficient oil sorbent is required to possess desirable characteristics, such as excellent hydrophobicity and oleophilicity, high uptake capacity and fast oil sorption rate, low water uptake and insolubility, low cost and high buoyancy. So far, various materials including natural materials such as seaweed, alginate, dead biomass, rice hulls, carrageenan, lignins, cellulose, chitin and chitosan [21], inorganic mineral materials [22] and organic synthetic fibers [23] have been used for oil sorption [24]. Chitosan, discovered by Rouget in 1859 [25], is a technologically important poly-saccharide biopolymer. Chemically, it is a high molecular weight linear poly-cationic hetero polysaccharide consisting of two monosaccharides, N-acetyl-D-glucosamine and D-glucosamine, linked together by β-(1 → 4) glycosidic bonds (Fig. 1). The relative amount of the two monosaccharides in Chitosan may vary, giving samples of different degrees of deacetylation (75–99%), molecular weights (50–2000 kDa), viscosities, pKa values, etc. [26,27,28]. Therefore, the term Chitosan does not refer to a uniquely defined compound; it merely refers to a family of copolymers with various fractions of acetylated units.

Fig. 1
figure 1

FT-IR of Chitosan and Chitosan-g-Octanal Schiff base

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Scheme 1
scheme 1

Chemical structure of cellulose, Chitin and Chitosan

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Chitosan is the deacetylated derivative of Chitin, which is the second most important natural polymers on Earth after cellulose (Scheme 1), [28] distributes widely in nature and is mainly present as the structural component of crustacean shells [29].

Chitosan is also found in nature, such as in cell walls of fungi of the class Zygomycetes, in the green algae Chlorella sp., yeast and insect cuticles. Recent advances in fermentation technology suggest that the cultivation of fungi (Aspergillus niger) can provide an alternative source of Chitosan. However, Chitosan from both sources differs slightly: whereas the acetyl groups in Chitosan produced from crustacean Chitin are uniformly distributed along the polymer chain, a Chitosan of similar degree of deacetylation isolated from fungal cell walls would possess acetyl residues that are grouped into clusters [28, 30].

Indeed, Chitosan is becoming most popular in different applications due to its excellent unique properties such as bio-degradability, non-toxicity, lower density, ease modification and lower cost [31]. Furthermore, Chitosan can undergo physicochemical modification for allowing the formation of functional derivatives with a highly hydrophilic and/or hydrophobic character such as coating, carboxy methylation [32, 33], grafting and amination [34,35,36]. It was reported that Chitosan has various functional groups like hydroxyl and amino groups which induced simple chemical transformation and modifications. Schiff base compounds containing an imine group are usually formed by the condensation of a primary amine with an active carbonyl. The presence of amine groups in the polymeric chain leads to the possibility of a several chemical modifications, including the preparation of Schiff bases (–RC = N–) by reaction with aldehydes and ketones [37, 38]. Its attractiveness as analytical reagent rises from the fact that they enable simple and inexpensive determinations of various organic and inorganic substances [39]. It was reported that Chitosan has been used for the treatment of wastewater from phenolic compounds [39, 40] using immobilization of bacteria on Chitin and Chitosan flaked which degrade hydrocarbons. Additionally, Chitosan showed high adsorption capacity when it applied as oil adsorbent material for removing oil spills [28].

Different materials have been developed and used as sorbents for oil spills [41, 42]. For many reasons and advantages, nature-based sorbents are recommended [43]. However, drawbacks such as low hydrophobic character and limited oil sorption capacity compromised the wide application [44]. Many ideas have been investigated to overcome such drawbacks, such as induced lauric acid to the structure [45], grafting with acrylate polymers [46,47,48] and formation of Schiff bases [49]. Among natural-based materials, Chitosan comes in the lead for treating contaminated waters due to its reach structure with both hydroxyl and amine groups [50,51,52,53,54]. Many modification routes have been investigated to increase Chitosan applications, including the grafting technique [55,56,57,58].

In this study, kinetic, isothermal and thermodynamic studies for the oil spills removal process have been conducted by Chitosan and novel amphiphilic Chitosan-g-Octanal Schiff base adsorbents developed by click chemistry and evaluated successfully in the removal of heavy crude oil spills. The novel amphiphilic Chitosan-g-Octanal Schiff base oil adsorptive materials based on low-cost homemade Chitosan used click grafting technique between the aldehyde groups of Octanal and the Chitosan' amine groups. The developed Chitosan-g-Octanal Schiff base adsorbent acquire a higher hydrophobicity character relative to the Chitosan due to the hydrophobic character of Octanal and consuming the Chitosan' amine groups. Furthermore, other amine groups were engaged in the formation of three dimensions structure via crosslinking with glutaraldehyde. The crosslinking step reduces the hydrophilicity of the Chitosan backbone in addition to induce pores structure. Accordingly, the developed amphiphilic Chitosan-g-Octanal Schiff base can adsorb oils through its hydrophobic Octanal grafted moieties from one hand and their pores on the other hand. The acquired hydrophobicity increases the affinity of the developed amphiphilic Chitosan-g-Octanal Schiff base towards the oils and decreases their affinity towards water. The novel materials were characterized using different characterization tools [28].

Experimental

Materials

Sodium hydroxide pellets were obtained from by EL. Pharanae Co. (Assay 99%, Mwt = 40 g/mol). Acetic acid (Purity 99.8%, Mwt = 60.05 g/mol), glutaraldehyde (GA; 99%) and octanal (Purity ≥ 95%, Mwt = 130.23 g/mol) were obtained from Sigma-Aldrich (Germany).Ethanol (Purity 99%), hydrochloric acid (Assay (37%)) and ethyl alcohol absolute were brought from El-Nasr Company (Alexandria). Shrimp skeletons were provided from commercial resource in Alexandria (Egypt). Sodium chloride and sulfuric acid (Purity 95–97%) were purchased from El-Gomhouria Co., (Egypt). Crude oil sample: Land Belayem (LB) was supplied from Belayem Petroleum Company (Table 1).

Table 1 General characteristics of heavy crude oil
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Methods

Chitosan preparation and purification

First, Chitin was extracted from crushed shells according to the published procedure [59]. The next step was the preparation of Chitosan from previously extracted Chitin via a simple de-acetylating process in an alkaline medium, which resulted in the removal of acetyl groups from the Chitin [60]. The resulted Chitosan was further purified according to the previous method [61]. The degree of de-acetylation was calculated using FT-IR, and potentiometric titration methods show very close results, 94.4% and 93.15%, respectively [62].

Preparation of Chitosan-g-Octanal Schiff base

Chitosan primary amino groups react with active carbonyl groups of Octanal to produce the corresponding Schiff base as presented in Scheme 2. Previously purified Chitosan (0.4 g) was dissolved in 20 ml of 2% acetic acid and stirred at room temperature for 6 h. The resulting viscous solution was filtered through cheese cloth to remove un-dissolved particles and then added 10 ml ethanol to the viscous solution under stirring at room temperature; then, 5 ml of ethanol containing definite amount of Octanal was added to solution under stirring to have homogenous solution. This mixture was stirred for 6 h at 80 °C. The formation of gel refers to formation of the Chitosan-g-Octanal Schiff base. Then 100 µl of glutardehyde was added to the resulting product for 1 h at 80 °C. The resulting product was added to excess of 1 M sodium hydroxide solution. The precipitate was filtered and washed with distilled water and ethanol several times to remove un-reacted Octanal. The product was filtered and dried in a vacuum oven at 60 °C overnight. One mole of Chitosan click reacted with two different Octanal concentrations, namely 0.51 and 5.06 mol, and the prepared samples were coded as CH-Oct0.5 (38.25 G%) and CH-Oct5 (81.75 G%), while the Chitosan was coded as CH-Oct0 (0.0 G%).

Scheme 2
scheme 2

Chitosan and Chitosan-g-Octanal Schiff base (Octanyl Chitosan Schiff base) adsorbents

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The grafting percentage of Octanal was calculated according to the following equation [28]:

$${\text{GP}}\left( \% \right) \, = \, [(W_{{1}} - W_{0} )/W_{0} ] \, \times { 1}00$$
(1)

The W0 and W1 are the weights of Chitosan and the resultant Chitosan-g-Octanal Schiff base.

For the preparation of Chitosan adsorbent, the same procedure mentioned above was applied except for the skip of the Octanal addition step.

Material characterization

Physicochemical characterization

Infrared spectroscopic analysis (FTIR)

The Chitosan and Chitosan-g-Octanal Schiff base structures were investigated by FTIR spectroscopic analyses using Fourier transform infrared spectrophotometer (Shimadzu FTIR—8400 S, Japan). Samples (2–10 mg) were mixed thoroughly with KBr, and the absorbance of samples was scanned from 500 to 4000 cm−1.

Thermal gravimetric analysis (TGA)

Analysis by TGA of samples was carried out using a thermogravimetric analyzer (Shimadzu TGA –50, Japan) under Nitrogen, at a gas flow rate of 20 mL/min, to evidence changes in structure as a result of the modification. In addition, samples were measured their weight loss starting from room temperature to 800 °C at a heating rate of 10 °C/minute.

Scanning electron microscopic analysis (SEM)

Samples were fixed on aluminum stubs and vacuum-coated with gold before being examined by scanning electron microscopy. Morphological changes of the sample's surface were followed using a secondary electron detector of SEM (Joel JSM 6360LA, Japan). Two magnifications factors were used: 1000X and 5000X under 20 keV.

Determination of ion exchange capacity

A known weight of Chitosan or Chitosan-g-Octanal (0.1 g) was added to known volume of 0.1 M H2SO4 solution (20 mL) and the mixture was kept aside for 2 h. The mixture was filtered, and an aliquot was titrated against standard solution of NaOH. Similarly, control titration without the addition of Chitosan was also run. From the difference in the volume of NaOH required for neutralization, ionic capacity of Chitosan samples were calculated using the following equation:

$${\text{Ion}}\,{\text{exchange}}\,{\text{capacity }} = \, \left( {V_{2} - V_{1} } \right) \, a \, / \, w \, \left( {{\text{meq}}/g} \right)$$
(2)

where V2 and V1 are the volumes of NaOH required for complete neutralization of H2SO4 in the absence and presence of Chitosan, respectively. a is the normality of NaOH, and w is the weight of sample taken for analysis.

Water uptake

Water uptake estimation was performed by placing a weighed sample in 10 ml water. After 6 h the sample was then filtered off, carefully bolted with a filter paper and weighed. The water uptake was calculated by applying the following equation:

$${\text{Water}}\,{\text{uptake }} = \, \left[ {\left( {M \, {-} \, M_{o} } \right) \, / \, M_{o} } \right]$$
(3)

where M is the weight of the swelled sample at time t and M0 is the weight of the dry sample [63, 64].

Oil uptake

Oil uptake was calculated by soaking a known weight of the samples (0.05 g) in 10 ml of oil (mineral oil, diesel oil, kerosene oil) and then placed in a closed glass containers for different time intervals, carefully bolted with a filter paper and weighed immediately in a closed balance. Oil uptake can be expressed by the following equation:

$${\text{Oil uptake }} = \, \left[ {\left( {W_{t} {-} \, W_{o} } \right) \, / \, W_{o} } \right]$$
(4)

where Wt is the weight of the soaked sample in the oil for certain time and w0 is the sample initial dry weight [63, 64].

Results and discussion

Physicochemical characterization

The ion exchange capacity and the water uptake of Chitosan and Chitosan-g-Octanal Schiff base adsorbents have been measured. The measured values were found 9.9, 6.0 and 1.47 (meq/g) of CH-Oct0, CH-Oct0.5 and CH-Oct5 adsorbents, respectively. These findings refer to consumption of the Chitosan amine groups in the formation of Schiff base via click reaction with Octanal aldehyde groups. The same trend was observed when Chitosan reacts with aldehydes and ketons [65].

On the other hand, it was clear a gradual decrease of the water sorption capacity of CH-Oct0, from 3.4, to 2.75, and 0.0 (g/g) of the CH-Oct0.5 and CH-Oct5 samples. The click reaction of Octanal with Chitosan free amine groups replaced the hydrophilic amine group with an alternative hydrophobic aromatic one. The same results were observed by Mohy Eldin et al.[28, 38,39,40, 63].

High-efficient oil sorbent is required to possess desirable characteristics, among them, excellent hydrophobicity and low water uptake and solubility [4]. The range of working pH was widened at which Chitosan adsorbents kept low solubility and water uptake increase its potential as oil spills adsorbent. Amine groups of Chitosan have an essential role in Chitosan solubility. The protonation of amine groups of Chitosan in the acid medium enhances Chitosan solubility. Sogias et al. [66] explored the factors affecting the solubility of Chitosan in Water. Due to its semicrystalline nature, derived mainly from inter- and intramolecular hydrogen bonds, Chitosan is water-soluble only at pH < 6. In acidic conditions, its amino groups can be partially protonated resulting in repulsion between positively charged macrochains, thereby allowing diffusion of water molecules and subsequent solvation of macromolecules.

Water solubility of Chitosan and Chitosan-g-Octanal Schiff base derivatives is shown in Table 2. From the obtained results, it can be seen that the water solubility of CH-Oct0 significantly (p < 0.05) decreased after click reaction with Octanal and as the grafted Octanal percent increased from 38.25 G% (CH-Oct0.5) to 81.75 G% (CH-Oct5) where turned to be almost insoluble even at acidic pH medium. The decrease in CH-Oct0 solubility can be attributed to the formation of covalent imide bonds after click reaction of amine groups of CH-Oct0 with the aldehyde ones of Octanal. These findings in Table 2 acquired the developed CH-Oct0.5 and CH-Oct5 Schiff bases two desirable characteristics to be high-efficient oil sorbent, excellent hydrophobicity and low water uptake and solubility [4].

Table 2 Solubility percent of Chitosan and its Schiff base derivative at different pH
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FTIR spectroscopy

The FT-IR spectra of the CH-Oct0, CH-Oct0.5 and CH-Oct5 were represented as shown in Fig. 1. The FT-IR chart illustrates the regular bands of CH-Oct0 function groups. A broadband around 3425 cm−1 corresponding to the stretching vibration of NH2 and OH groups and O–H bending at 1394 cm−1 indicates the presence of hydroxyl groups [67]. The weak absorption peak is at 2895 cm−1 (C–H stretch) for methyl and methylene groups, and the characteristic peak of Chitosan at 1624 cm−1 assigned to the C = O stretching [68]. The band at 1070 cm−1 ascribed to the stretching of the C–O–C bridge [69]. Structural analysis of Octanal by FTIR spectrophotometer showed the identical spectrum with νmax: 1720 (v C = O aldehyde), 2720 (v, C-H aldehyde) and 1380 cm−1 (Ϩ-CH aldehyde) [70]. Chitosan-g-Octanal Schiff base shows significant FT-IR changes (Fig. 1; CH-Oct0.5 and CH-Oct5). The significant differences are:

  1. (a)

    Firm absorption peaks at around 1566 cm − 1 corresponding to the C = N stretching which formed between the aldehyde groups of Octanal and amine groups of CH-Oct0. It seems stronger in case of CH-Oct5 with 81.75 G% relative to CH-Oct0.5 with 38.25 G%.

  2. (b)

    Absorption band at around 2937 cm − 1 for Chitosan-g-Octanal Schiff base of -CH stretching which also seems stronger in case of CH-Oct5 relative to CH-Oct0.5, and.

  3. (c)

    Absorption band at around 1420 cm−1, which also seems stronger in case of CH-Oct5 relative to CH-Oct0.5, for the merger between –NHCO (Amide III) band at 1395 cm−1 of CH-Oct0 and –CH2 band at 1470 cm−1 and -CH band at 1380 cm−1 of the grafted Octanal [70].

TGA

Thermal gravimetric analysis (TGA) of CH-Oct0, CH-Oct0.5 and CH-Oct5 is presented in Fig. 2. The first weight loss that starts from ambient temperature to about 150 °C resulted from evaporation of moisture that attached from the atmosphere or during the reaction process. The increase of moisture content was attributed to the pseudo-hydrophilic character that associated with porous materials [65]. The subsequent degradation that recognized from 220 to 320 °C was a result of oxidative decomposition of the CH-Oct0. In this stage, first depression was produced from destruction of amine groups to form cross-linked fragments [70].

Fig. 2
figure 2

TGA of Chitosan and Chitosan-g-Octanal Schiff base derivatives

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There is a significant difference in thermal stability between CH-Oct0, CH-Oct0.5 and CH-Oct5, where the temperature required to lose their half weights (T50%) is 343 °C, 385 °C and 325 °C as represented in Table 2. The stability of Schiff base derivatives is confirmed with previously published literature [64]. The third depuration that results from the decomposition which appears at high temperature may produce from the thermal degradation of a new cross-linked material formed by thermal crosslinking reactions occurring in the first stage of the degradation process [71].

Interestingly, CH-Oct0.5 and CH-Oct5 have overall higher stability than CH-Oct0 at 600◦C where 36.46% and 25.86% of CH-Oct5 and CH-Oct0.5 residues left, but in case of CH-Oct0, only 8.43% was remained (Table 3) [72].This finding proves the changes caused by grafting of the hydrophobic Octanal chains onto amine groups of Chitosan.

Table 3 Thermal gravimetric peaks of Chitosan and Chitosan-g-Octanal Schiff bases
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SEM

A scanning electron microscope studied the surface morphological analysis of CH-Oct0, CH-Oct0.5 and CH-Oct5 matrices with different grafting percentages (Fig. 3). The SEM graphs show changes in the surface morphology as the CH-Oct0.5 and CH-Oct5 Schiff base formation [73]. The Chitosan-g-Octanal Schiff base with 38.25 G% grafting percentage, (b) CH-Oct0.5, shows a mix of microparticles structures with different sizes and foils structure. Different exciting changes of Chitosan-g-Octanal Schiff base of 81.75 G% structure have been noticed and more surface roughness obtained covered with homogenous particles structure ((c) CH-Oct 5). Different shapes may attribute to the acquired variant of amphiphilic character resulting from the hydrophilic-hydrophobic balance of the CH-Oct0 and the newly formed CH-Oct0.5 and CH-Oct5 Schiff bases. Moreover, the water content plays a fetal role in the arrangement of the polymers chains and, accordingly, the formed polymer structures.

Fig. 3
figure 3

SEM photographs, 1000 × and 5000x, of a CH-Oct0 b CH-Oct0.5 and c CH-Oct5

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Kinetics, isotherms and thermodynamics of oil spills removal

Effect of the adsorption time and kinetics

The effect of variation the adsorption time from 10 to 180 min on oil adsorption percentage of heavy crude oil for the CH-Oct0 and CH-Oct5 Schiff bases adsorbents is studied in Fig. 4A. The conduct experiment was carried out under dynamic condition by the addition of 0.1 g of sorbent material over the oil/artificial sea water surface using heavy crude oil.

Fig. 4
figure 4figure 4

A Effect of contact time on heavy crude oil adsorption percentage (%) using Chitosan and Chitosan-g-Octanal Schiff base adsorbents. B Fitting of heavy crude oil adsorption process with pseudo-first-order model. C Fitting of heavy crude oil adsorption process with pseudo-second-order model. D Fitting of heavy crude oil adsorption process with intra-particle model

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On the other hand, Fig. 4A reveals that the oil adsorption percentage by CH-Oct5 Schiff base was doubled of the CH-Oct0 at equilibrium. This behavior is expected, due to the hydrophobic nature of the CH-Oct5 Schiff base sorbent material which enable them to attach oil molecules easily on the adsorbent surface and inside their porous structures. Therefore, the number of attached molecules increased by increase the contact time between oil and the CH-Oct5 Schiff base adsorbent improving its adsorption capacity. However, after a certain time, the sorbent material became saturated and its affinity towards oil molecules decreased [74]. Moreover, it was observed that the highest heavy crude oil adsorption percentage % values were recorded 92% for CH-Oct5 Schiff base at equilibrium [75].

The knowledge of adsorption kinetics is important information for designing batch adsorption systems. To examine the adsorption kinetics of CH-Oct0 and CH-Oct5 Schiff bases for heavy crude oil, the pseudo-first-order and pseudo-second-order kinetic models and intraparticle diffusion model were examined [24, 67, 76].

  1. (1)

    Pseudo-first-order kinetic model

    $$\log \left( {q_{e} - q_{t} } \right) = \log q_{e} - \frac{{k_{1} }}{2.303}t$$
    (5)
  2. (2)

    Pseudo-second-order kinetic model

    $$\frac{t}{{q_{t} }} = \frac{1}{{k_{2} q_{e}^{2} }} + \frac{t}{{q_{e} }}$$
    (6)

    where qe is the amount of adsorption at equilibrium, g/g; qt is the amount of adsorption at time t, g/g; k1 is the first-order rate constant, min−1; k2 is the second-order rate constants, min−1.

    Constants of pseudo-first-order and pseudo-second-order were determined experimentally by plotting of log (qe − qt) versus t for pseudo-first-order (Fig. 4B) and by plotting a t/qt against t (Fig. 4C). From the figures, it is clear that the pseudo-first- and second-order reaction rate models adequately describe the kinetics of heavy crude oil sorption with a high correlation coefficient: consequently, it was further used to describe all the kinetics of heavy crude oil sorption by CH-Oct0 and CH-Oct5 Schiff bases (Tables 4 and 5).

    The conformity between experimental data and the model predicted values was expressed by correlation coefficient (R2). Comparing Fig. 4B with Fig. 4C, it is found that the pseudo-second-order equation fitted the experimental data better than the pseudo-first-order equation for its higher R2 (all R2 values are closed to 1) [76]. The amount of adsorption equilibrium qe, the rate constants of the equation, k1 and k2, the calculated amount of adsorption equilibrium, qe(cal.), and the coefficient of determination, R2, for heavy oil are shown in Tables 3 and 4.

    It can be seen in Tables 4 and 5 that, with an increase in initial amount of oil, the initial adsorption rate (h0) increases while the rate constant of adsorption (k2) almost decreases. A similar observation was also reported by the earlier researchers [77, 78]. Meanwhile, the calculated amount of adsorption equilibrium (qe(cal.)) from the pseudo-second-order equation is close to the actual amount of adsorption equilibrium (qe(exp.)).

    It can be seen in Table 6 that oil adsorption rate constant k2 in case of CH-Oct0 greater than CH-Oct5 Schiff bases which indicate faster rate diffusion of heavy oils in case of adsorption by CH-Oct0, but adsorption capacity (qe) of CH-Oct5 Schiff bases is higher than of CH-Oct0 in case of heavy crude oil [76].

  3. (3)

    Intra-particle diffusion model:

    $$q_{t} = K_{p} t_{{}}^{0.5} + C$$
    (7)

    where kp is the intraparticle diffusion constant (g/g min1/2). If the intra-particle diffusion is involved in the adsorption process, then a plot of the amount of oil adsorbed per unit mass of adsorbent (qt) against square root of time (t1/2) gave a straight-line and the particle diffusion would be the controlling step if this line passed through the origin [71, 79].

Table 4 Kinetic parameters for heavy crude oil adsorption using Chitosan different initial amounts of oil
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Table 5 Kinetic parameters for heavy crude oil adsorption using Chitosan-g-Octanal Schiff base at different initial amounts of oil
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Table 6 Kinetic parameters for heavy crude oil adsorption with Chitosan and Chitosan-g-Octanal Schiff base at fixed initial oil concentration
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Figure 4D demonstrates the plot of oil sorption capacity qt (g/g), versus t1/2 for heavy crude oil adsorption. The deviation of straight lines from the origin indicates that intra-particle transport is not the rate-limiting step. As seen from this figure, the intra-particle diffusion rate equation fits well to the initial stages of the adsorption process for all the tested CH-Oct0 and CH-Oct5 Schiff bases.

From Table 4 it is obvious that values of Kp for CH-Oct0 increase from 0.292 to 0.806 g/g.min0.5, and values of Kp for CH-Oct5 Schiff bases (Table 5) increase from 0.468 to 2.12 g/g.min0.5 when the initial amount of heavy crude oil was increased from 0.5 to 2.5 g. This is because increasing initial amount of oil facilitates oil diffusion inside the adsorbent particles [80].

Table 6 shows the calculated values of Kp for CH-Oct5 Schiff base are higher than Kp for CH-Oct0 due to oil diffusion inside the adsorbent particle in case of CH-Oct5 Schiff base is higher than in case of CH-Oct0.

Effect of the oil amount and adsorption isotherm

To study the effect of the amount of oil on the adsorption percentage (%) of oil on CH-Oct0 and CH-Oct5 Schiff base, different oil amounts were experimented (Fig. 5A). In brief, 0.1 g of the sorbent material was added to the oil/artificial sea water system using different weights of heavy crude oil (0.5, 1,1.5, 2 and 2.5 g) at constant artificial seawater (20 ml) [6]. The progressive decrease of oil adsorption percentage was observed with the increase in the amount of heavy crude oil from 0.5 to 2.5 g [75].

Fig. 5
figure 5

A Effect of oil amount on heavy oil adsorption percentage (%) at equilibrium using Chitosan and Chitosan-g-Octanal Schiff base. B Langmuir plot of heavy crude oil adsorption by: Chitosan and Chitosan-g-Octanal Schiff base adsorbents at different temperatures. C Dimensionless constant separation factor for Chitosan and Chitosan-g-Octanal Schiff base. D Freundlich plot of heavy oil adsorption by: Chitosan and Chitosan-g-Octanal Schiff base adsorbents at different temperatures

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Generally, increasing the oil amount provides an important driving force to overcome all resistances of the oil between the aqueous and solid phases, thus increasing the uptake. In addition, increasing the initial oil concentration increases the number of collisions between oil and the adsorbent, which enhances the sorption capacity [65, 81]. On the other hand, increasing weight of the initial crude oil is reflected reversely on the oil adsorption percentage because the definition of the oil adsorption percentage of the sorbent is as the ratio of the weight of adsorbed oil to the initial weight of oil. Accordingly, the oil adsorption percentage takes negative trend with the initial amount of oil [65, 75].

The adsorption isotherm indicates how the adsorbate molecules distribute between the liquid phase and the solid phase at equilibrium. The analysis of the isotherm data by fitting them to different isotherm models is an important step to find the suitable model that can be used for design purpose..In this research, adsorption isotherm study was carried out on two well-known isotherms, i.e., Langmuir and Freundlich [65, 82].

Langmuir and Freundlich models

The Langmuir model is obtained below the ideal assumption of a totally homogenous adsorption surface and represented as follows [65, 77, 79]:

$$C_{e} /q_{e} = \frac{1}{{q_{\max } b}} + \frac{{C_{e} }}{{q_{\max } }}$$
(8)

where qe is the oil uptake at equilibrium (g/g); qmax is the maximum Langmuir uptake (g/g); Ce is the final oil concentration at equilibrium (g/l); b is the Langmuir affinity constant (L/g).

Figure 5B shows the Langmuir plots heavy crude oil at different temperatures by (a) CH-Oct0 and (b) CH-Oct5 Schiff base. The Langmuir isotherm constants qm and b were calculated from the particular slope and intercept of the linear plot of Ce/qe vs. Ce, and the values are presented in Table 7. It can be seen that the maximum adsorption capacity (qm) and b constants increased with increasing temperature, which indicates the endothermic nature of the adsorption process [24, 83].

Table 7 Isotherm parameters for heavy crude oil adsorption onto Chitosan and Chitosan-g-Octanal Schiff base at different temperatures
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The important features of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor (RL), which is defined as [67]:

$$R_{L} = \frac{1}{{1 + bC_{o} }}$$
(9)

where b is the Langmuir constant and C0 is the initial oil concentration (g L−1), and the value of RL indicates the type of isotherm to be either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1) or irreversible (RL = 0) [79]. The calculated RL values at different amounts of heavy crude oil for CH-Oct0 and CH-Oct5 Schiff bases are shown in Fig. 5C. It can be seen that RL value drops into the range 0–1 in all experimental systems, which confirms the favorable uptake of the heavy oil process. Lower RL values at higher amount of oil showed that adsorption was more favorable at higher amount of oil.

The empirical Freundlich isotherm, based on sorption on heterogeneous surface, can be derived assuming a logarithmic decrease in the enthalpy of sorption with the increase in the fraction of occupied sites and is given by [65, 76]:

$$q_{e} = k_{f} C_{e}^{1/n}$$
(10)

where KF and 1/n are the Freundlich constants characteristics of the system, indicating the sorption capacity and sorption intensity.

Respectively, (Eq. 8) can be linearized in logarithmic form (Eq. 10) and the Freundlich constants can be determined from their intercept and slope.

$$\log q_{e} = \log K_{F} + \frac{1}{n}\log C_{e}$$
(11)

Figure 5D gives an idea about the Freundlich plots of heavy crude oil at different temperatures by CH-Oct0 and CH-Oct5 Schiff bases, and at room temperature by CH-Oct0 and CH-Oct5 Schiff bases [79]. The Freundlich isotherm constants KF and 1/n in case of heavy oil were calculated from the particular slope and intercept of the linear plot of log qe vs. log Ce, and the values are presented in Table 7. Here, the intensity parameter 1/n indicates the deviation of the adsorption isotherm from linearity. 1/n < 1 shows that the adsorption is favorable, new adsorption sites are available and the adsorption capacity increases. 1/n > 1 indicates that the adsorption bonds are weak, adsorption capacities decrease and un-favorable.

From Table 7 the values of 1/n less than 1 represent a favorable adsorption [59]. The kF values of CH-Oct0 and CH-Oct5 Schiff bases were found to increase with the increase in temperature. This confirms the endothermic nature of sorption [79].

From the correlation coefficient R2 values indicate a good mathematical fit that adsorption process of heavy crude oil more fitted to Langmuir model. So, adsorption of heavy crude oil by CH-Oct0 and CH-Oct5 Schiff bases adsorbents described to occur on a totally homogenous and monolayer adsorption surface [24].

Adsorption thermodynamics

The thermodynamic parameters, such as change in Gibbs free energy (ΔG0), change in enthalpy (ΔH0) and change in entropy (ΔS 0) of light and heavy oil adsorbed by CH-Oct0 and CH-Oct5 Schiff bases, are calculated by Eqs. (12–14) [65, 79]:

The Gibbs free energy change of the adsorption process is related to the equilibrium constant by the equation:

$$\Delta G^{ \circ } = \, - {\text{RT ln }}b$$
(12)

where ΔG◦ is the standard free energy change (KJ mol−1), T the absolute temperature (K) and R (8.3145 J/(mol·K)) is the ideal gas constant, b (mg g−1) an equilibrium constant obtained Langmuir constant.

According to thermodynamics, the Gibbs free energy change is also related to the entropy change and heat of adsorption at constant temperature by the following equation:

$$\Delta G^{ \circ } = \Delta H^{{\mathbf{o}}} - T\Delta S^{o}$$
(13)
$${\text{Ln b}} = \left( {\Delta S/R} \right){-}(\Delta H/{\text{RT}})$$
(14)

where ΔH◦ is enthalpy change (KJ mol − 1), ΔS◦ entropy change (KJ mol−1 K−1), ΔH◦ and ΔS◦ can be calculated from the plot of (ln b) versus (1/T).

The values of ΔS◦ and ΔH◦ are calculated from the slopes and intercepts of the linear plots of lnb versus 1/T (Fig. 6), ΔG◦ is obtained using (Eq. 12). The thermodynamic parameters of heavy oil adsorbed by CH-Oct0 and CH-Oct5 Schiff bases adsorbents are listed in Table 8. The positive values of ΔH◦ suggest that the interaction of heavy oil adsorbed by CH-Oct0 and CH-Oct5 Schiff bases adsorbents is endothermic, which is supported by the increase adsorption of heavy crude oil with a rise in temperature [69]. The negative value of ΔG◦ indicates the spontaneous nature of the adsorption. ΔG◦ value is decreasing with increasing temperature, which suggests that higher temperature makes the adsorption easier.

Fig. 6
figure 6

Chitosan and Chitosan-g-Octanal Schiff base Van t’ Hoff graph of heavy crude oil adsorption

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Table 8 Thermodynamics parameter for the adsorption of heavy oil onto Chitosan and Chitosan-g-Octanal Schiff base adsorbents
Full size table

The positive entropy ΔSº of CH-Oct0 and CH-Oct5 Schiff bases indicates that process was caused by the decrease in degree of freedom of the adsorbed species and this positive value of heavy crude oil suggested that the randomness increased at the solid–liquid interface during the adsorption of heavy oil on CH-Oct0 and CH-Oct5 Schiff bases [76].

Conclusion

The present work investigates the kinetic, isothermal and thermodynamic of the oil spills removal process conducted by adsorption onto Chitosan and novel amphiphilic Chitosan-g-Octanal Schiff base adsorbents, developed by click chemistry. The developed novel Chitosan Schiff base compounds were characterized under advanced analytical tools such as Fourier transform infrared spectroscopy (FTIR), thermal analysis (TGA) and SEM analysis.

The developed Chitosan-g-Octanal Schiff base adsorbent turns to be hydrophobic where no water uptake was detected compared with Chitosan adsorbent; 1.45 g/g. The ion exchange capacity confirmed that behavior where decreased from 9.95 meq/g of Chitosan adsorbent to 1.47 meq/g of Chitosan-g-Octanal Schiff base adsorbent. The obtained results revealed that there was a major increase in the oil adsorption capacity of Chitosan-g-Octanal Schiff base adsorbent compared to the Chitosan due to increasing its hydrophobic characters after Schiff base formation, where the maximum oil adsorption percentage (%) recorded using heavy crude oil was 96.41% for Chitosan-g-Octanal Schiff base adsorbent compared to 64.99% for Chitosan counterpart. In addition, high rate of adsorption was observed where 40% of oil adsorbed within 15 min only using the Chitosan-g-Octanal Schiff base adsorbent compared to 90 min for native Chitosan one. The adsorption process followed the pseudo-second order model and the equilibrium data were sufficiently fitted with the Langmuir model with a maximum adsorption capacity 30.30 g/g at 25 °C. Thermodynamic parameters computed from Van’t Hoff plot confirmed the process to be endothermic, favorable and spontaneous.

The results nominate the developed hydrophobic Chitosan-g-Octanal Schiff base adsorbent as a potential adsorbent for petroleum oil spills removal.