1 Introduction

Water contamination with heavy metals is considered a global issue and caused great concern worldwide [7]. Toxic metal ions can be introduced to the water body from different industrial effluents such as metal plating facilities, mining operations, fertilizer industries, tanneries, batteries, paper industries, and pesticides. Heavy metals are extremely toxic, non-biodegradable, and do not undergo thermal degradation, and once they enter the food chain, they can accumulate at low concentrations in the living organisms [26, 62, 104, 105]. Therefore it has become imperative to develop a treatment process for the removal of such wastes. Different studies have reported that heavy metals such as lead, zinc, cadmium, chromium, and copper are very toxic elements [2, 4, 7, 8]. Drinking water containing copper exceeding the permissible limit for a long period leads to irreversible kidney or liver damage as well as headaches, hair loss, and increased heart rate, and in some cases might cause brain dysfunction, depression, and schizophrenia [19, 61]. According to the World Health Organization (WHO) and Environmental Protection Agency (EPA) guidelines, the maximum allowable concentration of Cu (II) in drinking water is 1.3 and 2.0 mg L−1, respectively [11, 82, 100]. Therefore, the treatment of wastewater containing Cu (II) is crucial in the field of environmental protection [69, 86].

Conventional methods including coagulation, chemical precipitation, ion exchange, adsorption, membrane separation, reverse osmosis, oxidation, evaporation, electro-flotation, and solvent extraction were successfully employed for the removal of heavy metals [73, 96]. However, most of these methods have some disadvantages such as complicated treatment process, high cost, and high energy demand. Among these methods, adsorption (ads) is considered as the most widely used method due to high performance, economical feasibility, and simple application [17, 27, 36, 52, 89, 92]. In general, activated carbon is the most commonly used adsorbent for the removal of heavy metals from industrial wastewater due to its high surface area, microporous structure, and high adsorption capacity. However, it is expensive with relatively high operating costs and there is a need for regeneration after each ads cycle [5]. Hence, there is a growing demand to find efficient, low-cost, and locally available adsorbents to be employed for the removal of copper from wastewater. Natural adsorbents such as sugarbeet pulp [3, 16], peanut hull [117], Cinnamomum camphora leaf powder [17], litter of poplar forests [22], lentil shell [10], wheat shell [9, 10], rice shell [10], almond shell [71], herbaceous peat [28], Tectona grandis L.f. leaf powder [85], base treated rubber leaves [75], pine cone powder [79], spent grain [65], tree fern [32], groundnut shells [94], pretreated Aspergillus niger [72], cedar sawdust [21], crushed brick [21, 102], and sawdust [56] have attracted many researchers and employed for heavy metal removals due to their low cost, high availability, and simple regeneration process.

Different mechanisms were used to explain the removal of heavy metals via natural adsorbents including micro-precipitation, chelation, complexation, ion exchange, and physical adsorption [13, 18]. The ads is related to the attraction force between the functional groups with a negative charge on the surface of the adsorbents with the positively charged metals [88, 91]. Accordingly, a treatment process that can enhance and improve the negative surface charge would significantly increase heavy metal removal performance. Wang et al. [107] enhanced the metal removal of uranium using rice stems by applying alkalinity treatment. Teixeira Tarley et al. [99] showed that applying chemical treatment for rice husk improved significantly the ads capacity of different metal cations. He and Chen [29] showed that the organic functional groups on the surface of bio-sorbents play a leading role in the removal of HM. Luo et al. [67] improved the heavy metal removal by brown algae Laminaria japonica via chemical crosslinking modification using epichlorohydrin potassium permanganate (PC).

Although different research works were published on the use of natural adsorbents for heavy metal removal, factors affecting the improvement of the ads capacity after chemical treatment still need further investigation. In addition, there is a lack of knowledge regarding the effect of surface properties and functional groups on the performance and capacity of natural adsorbents for the removal of metals from wastewater. Moreover, the kinetics and mechanisms of the ads process remain challenges due to the selectivity and complicity of the system.

Given that Pakistan is one of the world’s major producers of rice (5.2 million tons annually), rice husk (RH) which forms 20–23% of the whole rice grain is produced in huge amounts and is considered as unwanted waste material that poses a disposal problem for mill owners [63, 112, 113]. Due to its unique composition including proteins, cellulose, hemicellulose, and lignin that mainly have hydroxyl and carboxyl functional groups, it can be possible natural adsorbents for the removal of heavy metals from wastewater [20, 58,58,60, 78, 90]. It is imperative that the use of RH for heavy metal removal required treatment to rearrange and organize such a functional group on the surface to enhance the removal efficiency [99]. To the best of our knowledge, the ads of Cu (II) onto TRH has not been studied yet. Therefore, the present study reports on enhancing the ads efficiency of the RH by applying chemical treatment using NaOH as a low-cost and easy approach. The changes in surface morphology and the functional group as well as the improvements in the percentage removal of Cu (II) (%RCu) were investigated. The thermodynamics, kinetics, and isotherms of the ads process were also explored. The study also evaluated the effects of different operational conditions including contact time (tc), adsorbent dosage (Dad), initial Cu (II) concentration ([Cu]i), and temperature (T) on the percentage removal of Cu (II) (%RCu). The feasibility of the Cu ads was enhanced by regenerating and reusing the TRH for several times.

2 Experimental

2.1 Adsorbent

The husk of basmati rice (botanical name Oryza sativa) was obtained from a rice mill in Punjab, Pakistan. All experimental works were conducted from the same batch to eliminate any effect of seasonal variation in the rice sample. The collected RH has a bulk density in the range 88–135 kg m−3 and contains 50% cellulose, 23–27% lignin, 15–20% silica, and 10–17% moisture. The stock husk samples were thoroughly washed with water to remove dust particles and were oven-dried at 80 °C until a constant weight was obtained. The dry husk was then stored in a pre-cleaned airtight container and used in the experiments.

The chemical treatment of RH was carried by mixing the dry samples with 1 N NaOH at a ratio of 1 g RH/20 ml NaOH at 35 ± 1 °C. Then, the RH was recovered by filtration through a 0.45-μm filter (Whatman, USA) and washed with distilled water (DW) until the pH of water reached pH 7 ± 0.5 to remove any traces of NaOH remaining in the RH. The RH was then dried as before (80 °C to reach constant weight), denoted as TRH and used in experiments. The chemical measurements of the TRH using neutron activation analysis (NAA, DF-5703(A); DFMC, China) and atomic sorption spectrometry (AAS, WFX220AB; Qualitest, USA) techniques showed the presence of traces elements (Na, K, Pb, and Fe, in mg g−1) and 18.27 ± 0.62% of the TRH as silica.

2.2 Reagents

Chemicals used in the present study are analytical grades purchased from Sigma-Aldrich and used as received. For thermodynamic investigations, the temperature of solutions was controlled at ±0.1°C by immersing the test tubes in a water bath (Gallenkamp thermo stirrer, UK).

2.3 Batch adsorption

Adsorption experiments were carried out using a shaker (Gefellschaft Fur 978, Germany) and temperature-controlled (Gefellschaft Funn 1003, Germany) set up. The pH of the water samples was measured using a pH meter (Oiron 520; Thermo Scientific, USA). The concentrations of Cu (II) was measured using a model AA 100, inductively coupled plasma-atomic emission spectrometry (ICP) (Perkin-Elmer, USA). A four-digit analytical balance with a precision of ±0.0001 mg (Sartorius, CP324-S, USA) was used in weighing samples.

All ads tests were carried out in a batch mode under controlled pH and temperature. A series of 25-cm3 secured cap tubes filled with 4 cm3 of the standard acid solution, a fixed amount of stock radiotracer, and a specific concentration of copper (25 to 200 mg L−1) were mixed with a specific dose of adsorbent (0.05 to 0.6 g). The mixture was agitated in a wrist-action mechanical shaker (Vibromatic, USA) at 500 rpm for 30 min to reach equilibrium. Then, the agitator was stopped, and the solution was centrifuged at 5000 rpm for 30 min to separate solid adsorbent from water. The supernatant was separated from the solid adsorbent, the concentration of Cu (II) in the solution was determined, and percentage removal of Cu (II) (%RCu) was calculated using Eq. (1). Control tests were performed at different pH for samples containing heavy metals alone to evaluate the percentage removal of heavy metal by precipitation. The effect of temperature on the Cu removal was studied by repeating the tests at different temperatures 280.0 ± 1 to 335 ± 1 K.

$$ \%{R}_{Cu}=\frac{{\left[ Cu\right]}_i-{\left[ Cu\right]}_f}{{\left[ Cu\right]}_i}\times 100\% $$
(1)

where [Cu]i and [Cu]f are the initial and final concentrations of Cu (mg L−1), respectively.

2.4 Kinetic study

The kinetic of the ads of Cu (II) onto TRH was tested against Lagergren’s pseudo-first (PFO) [55], Eq. (2), and pseudo-second-order (PSO) [30, 31], Eq. (3), using IGOR Pro 6.1.2, Wave Metrices software.

$$ \frac{\mathrm{d}{Q}_t}{\mathrm{d}t}\kern0.5em =\kern0.5em {k}_1\ \left({Q}_{\mathrm{e}}-{Q}_t\right) $$
(2)
$$ \frac{\mathrm{d}{Q}_t}{\mathrm{d}t}\kern0.5em ={k}_2\ {\left({Q}_{\mathrm{e}}-{Q}_t\right)}^2 $$
(3)

where Qt is the amount of Cu (II) adsorbed on the TRH at any time t (mg g−1), Qe is the equilibrium amount of Cu (II) adsorbed on the TRH (mg g−1), t is the time (min), k1 (min−1) and k2 (g mg−1 min−1) are the PFO and PSO rate constants, respectively. Equations (2) and (3) can be written in a linear form as per Eqs. ((Linear 4) and ((Linear 5) [15, 33, 83].

$$ \log \left({Q}_{\mathrm{e}}-{Q}_t\right)=\log {Q}_{\mathrm{e}}-\left(\frac{k_1}{2.303}\right)\ t $$
(Linear 4)
$$ \frac{t}{Q_t}=\frac{1}{k_2{Q}_{\mathrm{e}}^2}+\frac{t}{Q_{\mathrm{e}}} $$
(Linear 5)

Regression coefficient R2, Eq. (6), and χ2 test, Eq. (7), were used to compare the best fit of the kinetic model,

$$ {R}^2=\frac{\Sigma {\left({Q}_{t, cal}-{\overline{Q}}_{t,\exp}\right)}^2}{\Sigma {\left({Q}_{t, cal}-{\overline{Q}}_{t,\exp}\right)}^2-\Sigma {\left({Q}_{t, cal}-{Q}_{t,\exp}\right)}^2} $$
(6)
$$ {\upchi}^2=\sum \frac{{\left({Q}_{\mathrm{e}}-{Q}_{\mathrm{e},\mathrm{cal}}\right)}^2}{Q_{\mathrm{e},\mathrm{cal}}} $$
(7)

where Qt,exp is experimental ads capacity at any time t (mg g−1), Qt,cal is calculated ads capacity at any time t (mg g−1), ǬQt,exp is average of the experimental ads capacity at any time. (mg g−1), and Qe and Qe,cal are the experimental and calculated equilibrium ads capacity (mg g−1), respectively.

2.5 Adsorption isotherms

For scale-up purposes, the ads process of Cu on TRH was tested against Langmuir (Eq. 8), Freundlich (Eq. 9), and Dubinin–Radushkevich isotherms.

$$ \frac{{\left[ Cu\right]}_e}{Q_{\mathrm{e}}}=\frac{1}{Q_{\mathrm{max}}{K}_{\mathrm{L}}}+\frac{{\left[ Cu\right]}_e}{Q_{\mathrm{max}}} $$
(8)
$$ \log {q}_e=\log {K}_f+\frac{1}{n}\log {\left[ Cu\right]}_e $$
(9)
$$ \ln {q}_e=\ln {C}_{\mathrm{max}}-\beta {\varepsilon}^2 $$
(10)

where [Cu]e is the equilibrium concentration of Cu(II) the solution (mol L−1), Qe is the equilibrium amount adsorbed on TRH per unit mass (mol g−1), Qmax is the maximum ads capacity (mol g−1), and KL (L mol−1) is related to the energy of adsorption, Kf and n are Freundlich constants, Cmax (mol g−1) is the maximum amount of Cu can be adsorbed onto TRH under the optimized experimental conditions, β is a constant related to adsorption energy, and ε (Polyanyi potential) = \( RT\ln \left(1+\frac{1}{C_{\mathrm{e}}}\right) \)where R is the universal gas constant (KJ mol−1 K−1) and T is the absolute temperature (K).

2.6 Adsorption thermodynamic

The thermodynamic of the adsorption process was followed by the changes in Gibb’s free energy (∆Go), enthalpy (∆Ho), and entropy (∆So) following Eqs. (11) and (12) [41, 43,43,45, 49, 50]:

$$ \ln Kc=\frac{\Delta {S}^o}{R}-\frac{\Delta {H}^o}{RT} $$
(11)
$$ \Delta {G}^o=\Delta {H}^o-T\Delta {S}^o $$
(12)

where Kc is the equilibrium constant, R is the general gas constant (8.31 J mol−1 K−1), and T is the absolute temperature (K).

2.7 Characterization

FTIR spectra of virgin and Cu (II) loaded TRH were recorded by employing the attenuated total reflectance (ATR) technique with an FTIR spectrometer (Vector 22; Bruker) having a resolution of 2 cm−1 and a total spectral range of 4000–400 cm−1. Field emission scanning electron microscope (FE-SEM, Sirion200; FEI Company, USA) was employed to study the morphology of virgin and Cu (II) loaded TRH. Similarly, energy-dispersive X-ray (EDX) analysis was used to prove the successful adsorption of Cu (II) onto TRH. The porosity of the TRH was measured using a porosimetry analyzer (Micromeritics Autopore IV 9500 V1.05). Tests were conducted using nitrogen adsorption–desorption isotherm (AD-DE-ISO) at a pressure in the range of 0.1–20,000 psia.

2.8 Statistical analysis

The statistical significance of all the experimental results was evaluated using Student’s paired t test. The test was conducted to investigate the significance of the results using one-way ANOVA (GraphPad Prism statistics software, version 7.04, USA). Only the experimental results that were statistically accepted at a significance level of 5% were used in the results and discussion section.

2.9 Recovery of Cu (II) and recycling the TRH

Attempts were made to recover the adsorbed Cu (II) from the TRH using hydrochloric, sulfuric, and nitric acid as desorbing media. The object is to recover the Cu and reuse the TRH many times for treating wastewater contaminated with Cu.

Desorption of Cu was conducted by mixing the TRH with 10 mL of acids for 30 min and then sonicating for 5 min. After that, the TRH was separated by filtration, washed with DW 10 times, dried, and reused for the consecutive adsorption–desorption process.

3 Results and discussion

3.1 Characterization of treated rice husk (TRH)

The FTIR spectrum of the RH is represented in Fig. 1a. The absorption peaks in the band 3400-3200 cm−1 were assigned to surface O-H stretching, whereas the peaks at 2921–2851 cm−1 correspond to the aliphatic stretching in the C-H bond. Other peaks at 1738.4, 1501.4, and 1365.2 cm−1 were associated with C=O stretching, OH bending of the adsorbed H2O, and aliphatic C-H bending, respectively [38]. Other peaks in the range 1208–1230, 1367-1371, 1740, and 1029 cm−1 were associated with carboxyl group [37, 97].

Fig. 1
figure 1

FTIR spectrum of a virgin TRH and b Cu (II) loaded TRH

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Treating the RH with NaOH has changed the functional group on the surface of the RH as it is evident by the shift in the peak from 2971.6 to 3351.8 cm−1 and the peaks in the range 3173.8 to 1026.8 cm−1. The absence of peaks related to non-conjugated carbonyl functional groups in the range 2750 to 2850 cm−1 indicated the hydrolyses of carbonyl groups during NaOH treatment. Furthermore, the peak at 1074.0 cm−1 corresponds to the anti-symmetric stretching vibration of Si-O, whereas at 476.2 cm−1 indicates the bending vibration of Si-O-Si bond [66, 97]; [111]. Measurements showed that NaOH treatment altered the surface functional groups of the TRH and increased the ionic ligands of carboxyl, phosphate, and amino groups that have high electro-attraction force with the positively charged Cu+2. These conditions increased the TRH capacity to remove Cu+2 from the aqueous solution. Similar trends were observed by He and Chen [29] during the removal of heavy metals by algal biomass. As it was presented in the FTIR results, the chemical treatment showed an increase in the intensity of O-H, C-H, and C=O groups which have high ionic exchange capacity to uptake Cu from the solution [88].

In Fig. 1b, the FTIR spectra of Cu (II) loaded on TRH exhibited changes in the peak positions and relative intensities. The shifting of peaks to 1026.8 and 501.4 and decrease in intensity of 1738.1, 1507.0, 1361.7, and 1216.3 cm−1 were observed for TRH, which suggests that less stretching occurs due to the binding of Cu (II) with carboxyl and silanol groups present in rice husk. It was also observed that a new peak at 467 cm−1 associated with Si–O bending vibration has been observed after the adsorption of Cu (II) onto TRH. Marshall and Johns [70] showed that the treatment of soybean and cottonseed hulls improved sorption capacity for Zn(II) compared with the untreated materials due to the increase in galacturonic acid groups on the surface. Low et al. [64] observed that treatment with NaOH improved the sorption of spent grain. However, Xie et al. [109] indicated that it can dissolve biomass and destroy metal-binding sites. The FTIR analysis before and after the adsorption suggested that there is some sort of interaction among protonated –OH and Si–O groups of rice husk and Cu.

Figure 2 represents the SEM micrographs of TRH before and after the adsorption of Cu (II). The surface of rice husk was irregular and rough with many loops and humps. The pores can be classified according to their sizes. Primary micro-pores are less than 0.8 nm, secondary micro-pores are 0.8–2 nm, meso-pores are 2–50 nm, and macro-pores are 50 nm width in size. The determined pore size present on the surface TRH was 335–1220 nm confirming the macro-porous structure of adsorbents (Fig. 2a, c). The SEM images (Fig. 2b, d) of TRH loaded with Cu (II) showed that the surface of adsorbent becomes smooth and occupied with Cu as a result of adsorption (see red circle in Fig. 2d). The radius of copper is 0.091 nm which was much smaller than the pore sizes of rice husk, so metal ions may be adsorbed through diffusion into pores. The capacity of Cu (II) ads onto TRH was increased from 1.919 × 10−5 to 9.168 × 10−5 mol g−1 because of the tremendous increase in the size and number of pores on TRH. The observed trend can be related to the changes in surface morphology due to disruption of the cross-linking between the metal ions and negatively charged chemical groups as confirmed by Florido et al. [24], Iqbal et al. [34], and Kızılkaya et al. [51] and evidenced by the increase surface ads of Cu.

Fig. 2
figure 2

SEM image of a, c virgin and b, d Cu (II) loaded TRH

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Figure 3 presents the EDX analysis of the energy peak as a result of the ads of Cu (II). Results revealed the presence of Cu (II) onto adsorbent surfaces and copper peaks were present at different energy levels ranging from 0.750 to 8.80 keV confirming the successful ads of Cu (II) onto TRH.

Fig. 3
figure 3

Energy-dispersive X-ray image of Cu (II) loaded TRH

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3.2 Adsorption of Cu on treated rice husk (TRH)

3.2.1 Effect of contact time

Figure 4a shows that the %RCu increased from 35.4 to 95.5% as the tc increased from 0 to 10 min, and then to 97.1% for ct of 30 min. The observed trends suggest that the ads of Cu (II) on TRH was very fast initially for the first few minutes and there was no significant change in %RCu once the equilibrium was established. Therefore, there was no further significant increase in %RCu as the ct increased. The obtained trends can be related to the existence of a large number of empty active sites onto TRH initially, which led to establishing an interaction between Cu (II) and ads sites on TRH. Once the active sites were occupied, the ads slowed down and %RCu decreased due to the movement of metal ions deep into interior pores of TRH. Based on the results from Fig. 4a, it can be concluded that the equilibration time for Cu (II) on TRH was only 15 min, which is a very short time compared with equilibration time for Ce (IV) [48, 113].

Fig. 4
figure 4

a Effect of contact time (tc) on the %RCu by TRH. b Effect of adsorbent dosage (Dad) on the %RCu by TRH. c Effect of initial Cu (II) concentration ([Cu]i) on the %RCu by TRH. d Effect of temperature (T) on the %RCu by TRH

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3.2.2 Effect of the adsorbent dosage (D ad)

Figure 4b presents the effect of the adsorbent dosage (Dad) of TRH (0.05 to 0.6 g) onto the %RCu of Cu (II). The %RCu was enhanced from 18.2 to 97.8% by increasing the Dad of the TRH from 0.05 up to 0.3 g, beyond which no significant enhancement in the %RCu was observed. An amount of 0.20 g of TRH was observed to be the optimum Dad that generates maximum %RCu under the studied conditions.

Fomina and Gadd [25] reported an improvement in heavy metal removal by increasing Dad to a certain limit due to the filling up of active sites via adsorbed metal. Lee and Chang [57] showed that heavy metal removal is indirectly related to the Dad. Thus, the removal rate decreases by increasing the Dad, whereas the work of Bhatti et al. [14] suggested that increasing the Dad decreases the percentage removal of heavy metals. It was also proposed that the interference between the adsorbent sites with one another resulted in intracellular electrostatic interactions and reduced the percentage of heavy metal removals. The attraction between adsorbent and heavy metal was examined by FTIR and revealed that metals are attached to the peaks related to OH, N–H, and S=O. The obtained results suggest that Cu was successfully attached to the TRH surface via electrostatic force after the ion exchange of H+ with water [6, 23].

3.2.3 Effect of initial Cu (II) concentration ([Cu]i)

The effect of [Cu]i on the %RCu is depicted in Fig. 4c. At the low [Cu]i, the number of active sites is higher than the [Cu]i and the %RCu was high, while at high [Cu]i the concentration of active sites is limited and will be filled fast leading to a notable decrease in the %RCu. The %RCu was decreased from 97.5 to 55% as the [Cu]i increased from 25 to 175 mg L−1. The obtained trends were related to the saturation of ads active sites, leaving some of the metal ions unabsorbed at higher [Cu]i. Similar results were reported by various researchers [47, 93, 108, 113, 114].

3.2.4 Effect of temperature (T)

The effect of T on the %RCu from aqueous solution by TRH was investigated using the optimized conditions and presented in Fig. 4d. There was an increase in the %RCu by increasing the T, which indicates higher ads capacity at elevated temperatures. The %RCu increased by increasing the operating temperature from 279 to 338 K. The observed trends can be either due to the acceleration of some originally slow ads steps or to the creation of some new active sites on the surface of the sorbent. Moreover, the obtained behaviors can be related to the fact that at higher T will provide the required energy for the rate-determining of the chemisorption mechanisms allowing Cu to reach new active sites and enhance the removal efficiency [39, 40]. Zafar et al. [113] observed similar results during the removal of cerium ions from aqueous solution using rice husk. Khan et al. [46] used Citrus sinensis leaves and Morus alba leaves for the removal of Congo red dye from aqueous solution and reported comparable results. The capacity of the TRH showed higher capacity compared with other adsorbents from literature as shown in Table 1.

Table 1 Ads capacity of Cu (II) for different adsorbents
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3.3 Adsorption kinetics

The nonlinear plots of PFO and PSO kinetic models for the ads of Cu (II) are represented in Fig. 5. Linear forms of the PFO (Fig. 6a) and PSO (Fig. 6b) following Eqs. (4) and (5) were used to determine the corresponding kinetic and equilibrium constants as shown in Table 2. The calculated values of R2 for linear equations and χ2 values for nonlinear equations are shown in Table 2. Results show that the PSO fits well the ads Cu on TRH with R2 of 0.99 compared with PFO (R2 = 0.75).

Fig. 5
figure 5

Nonlinear plots of pseudo-first-order and pseudo-second-order kinetics models for the ads of Cu (II) onto TRH

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Fig. 6
figure 6

a Linear plot of pseudo-first-order for ads of Cu (II) onto TRH and b linear plot of pseudo-second-order for ads of Cu (II) onto TRH

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Table 2 PFP and PSO kinetic parameters of the ads of Cu on TRH
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Similarly, a small value of χ2 of the PSO kinetic model indicates that PSO model seems to be more suitable to predict the kinetic parameter of Cu(II) ads onto TRH, and the PFO kinetic is not an appropriate model to explain the kinetic data of copper ion ads onto TRH. The data in Table 2 show that the percentage of error for the PFP is greater than PSO conforming that the PSO fits better the ads kinetic of Cu on TRH. The values of the K2 constant show excellent and fast Cu ads kinetics compared with the data published on similar systems as reported by Khan et al. [42].

3.4 Adsorption isotherms

The experimental results were tested against Langmuir, Freundlich, and Dubinin–Radushkevich isotherms. These isotherms were widely used for the analysis and design of the ads process and scale-up to commercial level. Besides, the isotherms provide fundamental data about the applicability of the process as a unit operation. Langmuir isotherm has been used by various workers for the ads study of a variety of systems [54]. Langmuir model supposes homogeneity of the adsorbing surface, no interactions between adsorbed species, and no transmigration of adsorbate species in the plane of the surface. The difference in ads capacities of two adsorbents for the same adsorbate is believed to be largely due to their physicochemical properties or the chemistry of solution containing adsorbing species. In general, Qmax and KL are functions of pH, ionic media, and ionic strength. The value of Qmax for Cu (II) was computed following the Langmuir isotherm using Wavemetrices IGOR Pro 6.1.2 software. A representative plot of the linearized form of the Langmuir isotherm model for ads of Cu (II) onto TRH is represented in Fig. 7a. The corresponding equilibrium constants are given in Table 3. An essential feature of a Langmuir isotherm can be expressed in terms of a dimensionless constant “separation factor” parameter, RL, that is used to predict if an ads system is “favorable” or “unfavorable” and can be expressed as in Eq. (16):

$$ {R}_{\mathrm{L}}=\frac{1}{\left(1+{k}_{\mathrm{L}}{\left[ Cu\right]}_i\right)} $$
(16)
Fig. 7
figure 7

a Linear form of Langmuir isotherm for ads of Cu (II) onto TRH. b Linear form of Freundlich isotherm for ads of Cu (II) onto TRH. c Linear form of Dubinin–Radushkevich (D-R) isotherm for ads of Cu (II) onto TRH. d Plot of 1/T vs. lnKc for ads of Cu (II) onto TRH

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Table 3 Langmuir, Freundlich, and Dubinin–Radushkevivh isotherms parameters
Full size table

where [Cu]i is the initial concentration of Cu (mol L−1) and KL is the Langmuir sorption equilibrium constant (L mol−1). The value of RL indicates if the ads process is either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1), or irreversible (RL = 0). Langmuir ads process deals with the formation of a monolayer of Cu on the surface of the TRH, and afterward, no further ads take place. The values of RL for ads of Cu (II) was calculated from Langmuir constant KL and initial concentrations of metal ion and results are summarized in Table 3, which confirmed that the ads of Cu (II) onto TRH was favorable as indicated by the fractional values of RL between one and zero.

Freundlich isotherm assumes an exponentially decay in the density of the active site concerning the heat of ads. In this sense, Freundlich isotherms can be used as an indicator of the heterogeneity of the adsorbent surface. The constants in Freundlich isotherms (Kf and n) represents the ads capacity and intensity, respectively. Fitting of the experimental data (Fig. 7b) to Freundlich isotherm suggests that this isotherm is not suitable to represent the ads of Cu on TRH, and the corresponding isotherm parameters are given in Table 3. The value of n signifies the heterogeneous surface of the TRH. The values of n in the range 2–10 indicating good ads capacity, 1–2 as moderate ads capacity, and less than 1 as poor ads capacity [98, 115, 116]. The linearized form of D-R isotherm is given in Eq. (10). From β value, the mean ads energy (E) can be computed as in Eq. (17) [35]:

$$ \mathrm{E}=\frac{1}{\sqrt{-2\upbeta}} $$
(17)

which is the mean free energy of transfer of one mole of solute from infinity to the surface of the adsorbent. The plots of straight lines were obtained by using the linear form of D-R isotherm of Eq. (17) and are presented in Fig. 7c. The values of Cmax and β were calculated from intercepts and slopes of the plot of ln{qe} vs. ε2 using a least-square fit program and are presented in Table 3. The D-R constants (Cmax) for ads of Cu (II) onto TRH was evaluated from the intercept of straight lines using the least square fit program to be (1.9514 ± 0.477) × 10−4. Using a β value of (2.105 ± 0.544) × 10−3, the ads free energy (E) was calculated to be 14.43 ± 4.885 kJ mol−1. The high numerical values of E suggest that the chemisorption phenomenon between Cu (II) and TRH is well established with a strong bond.

3.5 Adsorption thermodynamics study

The feasibility and spontaneity of the ads process were investigated by performing thermodynamic tests following Eqs. (11) and (12). The plot of lnKc vs. 1/T for the ads of Cu onto TRH is depicted in Fig. 7d. The values of ∆Ho and So were calculated from the slope and intercept of Fig. 7d and are shown in Table 4. On the other hand, ΔGo was also measured from Eq. (21) and is represented in Table 4. The values of ΔGo decrease by increasing the T representing a decrease in the required T to achieve high %RCu. Furthermore, the negative values of ΔGo for the ads of Cu (II) onto TRH show that the ads process is spontaneous. The positive value of Ho depicts that the ads of Cu (II) onto TRH is an endothermic process. Similarly, the positive value of So represents the enhancement in randomness at the adsorbent–adsorbate interface during the sorption of Cu (II) onto TRH.

Table 4 Thermodynamic parameters for ads of Cu (II) onto TRH
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3.6 Recovery of Cu (II) and recycling the TRH

A series of experiments were performed with varying concentrations of acid solutions for the desorption of Cu (II) from TRH. Among the several acid solutions, HNO3 solution gave maximum recovery of Cu (II) (~99%) with 1.0 mol L−1. On the other hand, the recovery of Cu (II) from TRH was lower for an aqueous solution of HCl (1.0 mol L−1) and H2SO (1.0 mol L−1). The lower concentration of HCl and H2SO4 was used because their higher concentration might damage the active site on the TRH. The obtained result was consistent with the phenomena explained in the effect of acid concentration that the ads process was decreased with an increase in the concentration of acids. As indicated above that Cu is attached to the TRH surface via electrostatic force after ion exchange of H+ [6, 23]. Acid likely plays an important role in the regeneration or recovery of Cu from TRH. Another set of experiments was carried out to determine the number of cycles the TRH can be used for the removal of Cu. Results showed that the %RCu did not significantly change for 10 cycles. The reductions in %RCu after 5, 8, and 10 cycles were no more than 4, 7, and 9%, respectively, suggesting a reasonable service life and good ads capacity.

4 Conclusions

Herein, the batch removal of Cu (II) by TRH from aqueous solution was studied. The percentage removal of Cu (II) was increased by increasing the Ct, DAd, and T, whereas it decreased by increasing the [Cu]i. Chemical treatment improved the surface properties of the RH, increased the negative functional groups and enhanced Cu removals by 1.8 compared with untreated RH. Electrostatic interaction force controls the main mechanism between algae strains and HM. The ads of Cu on TRH is endothermic and spontaneous and well fitted into the pseudo-second-order kinetic and Langmuir isotherm. Furthermore, the recovery of Cu (II) was maximum for HNO3 (1.0 mol L−1) solution suggesting that the TRH could be used as an excellent and cheap adsorbent for removal of Cu (II) from aqueous solution at room temperature.