Introduction

The quest for potable water has been a serious challenge to the global ecological balance, especially in developed countries. The recent interest in water shortage and water pollution has driven researchers very hard, so as to look for a lasting, effective and efficient solution to this menace combating the survival of human lives and the ecology. Today, 800 million people still lack access to potable water. For instance, only 46% of the Oceania population and 39% of the Sub-Sahara Africa population have access to potable water [1–4]. The sporadic rise in human activities over the decades has led to the consistent release of various recalcitrant anthropogenic pollutants into water bodies, thereby causing a sharp drop in the quality of the ecosystem. Eradication of these deleterious pollutants is difficult using classical water purification systems. Report from the World Health Organisation (WHO) showed that 1.8 million people die from polluted water-related diseases [2]. In Nigeria, this problem aggravates the sustainability of its ecology, leading to the precarious survival of flora and fauna. Hence, this has heralded an urgent need for researchers to proffer a sustainable solution to this life quagmire [5–8]. Toxic metals are known to be the most problematic pollutants, due to their non-biodegradable nature. Irrespective of the huge effort that has been put together by researchers to minimise their environmental impact, toxic metals in the environment still have adverse effects on human, fauna and flora. The toxicological effects of Cd(II) and Pb(II) in humans include bone lesions, cancers (kidney and lung), hypertension, inhibition of haemoglobin formation, sterility, infants brain impairment, learning disabilities, abortion, and kidney damage. To date, various remediation technologies have been used to treat polluted water containing toxic synthetic dyes and metals. These are coagulation, chemical oxidation, photodegradation, aerobic or anaerobic oxidation, precipitation, membrane filtration, dialysis, ion exchange, solvent extraction, reverse osmosis, among others [9, 10]. These technologies have some drawbacks, such as low selectivity, incomplete removal, cost ineffectiveness and production of large amount of secondary sludge. Adsorption is known as a green, sustainable and cost-effective technology, which is an alternative to other remediation technologies mentioned above. In recent times, natural or modified biomaterials (biosorbents and agricultural byproducts), which comprise lignin, cellulose, hemicelluloses and other organic compounds that have various functional moieties, have been found to be ubiquitous, easily sourced, cheap, green, sustainable and good adsorbents for clean recovery of toxic synthetic dyes and metals [9]. Some researchers in their previous treatises have used eucalyptus seeds [9], peanut hulls [11], corn cob [12], almond shell, hazelnut shell [13], olive cake [14], mungbean husk [15], mango peel waste [16], orange waste [17], Scolymus hispanicus L. [18], chemically modified orange peel [19], Nauclea diderrichii seed biomass [20, 21], Zea mays seed chaff [22], Mesoporous SiO2/graphene oxide nanoparticles-modified Nauclea diderrichii seed biomass [23, 24], Parkia biglobosa biomass [25], Papaya-clay combo [26], MnO2 nanoparticles-modified Nauclea diderrichii biomass waste [27], TiO2 nanoparticles-modified Nauclea diderrichii seed biomass [28], Pentaclethra macrophylla and Malacantha alnifolia barks [29]. Other researchers in the literature used other adsorbent and techniques to treat wastewaters in addition to adsorption [30–53].

Terminalia ivorensis, a timber tree, is the sole member of the genus that occurs naturally in West Africa. It is an indigenous plant (family Combretaceae) which is recognised in the South-western rain forest of Nigeria and Ghana. The bark is used as a lotion for the treatment of wounds, sores and cuts. Terminalia ivorensis is a large deciduous forest tree. On plantations, weeding up to the second year and line cleaning or creeper cutting from the third to sixth year may be necessary. With the diverse native uses of this species, its utility can be further enhanced for agroforestry development by the present research effort on it [54]. Terminalia ivorensis is found in the rainforest but is predominantly a tree of seasonal forest zones. Terminalia ivorensis has numerous uses in the building, construction, carpentry industries and household equipment. Also, this plant can also use as ethnomedicine, a lotion for the treatment of wounds, sores and cuts because it healed without scar [54].

The seed epicarp of Terminalia ivorensis is initially removed before the seed is planted. This seed epicarp constitutes a colossal amount of obnoxious waste. This has informed our choice of exploring this waste as a biosorbent for this research. Conventional remediation technologies like coagulation, membrane filtration, dialysis, reverse osmosis, etc., utilized to treat water polluted with toxic metal ions and dyes are very expensive and result in secondary pollution. The use of Terminalia ivorensis seed waste will avail researchers and industries the opportunity to substitute the aforementioned cocktails of conventional technologies above with very cheap, convenient, suitable, eco-friendly and secondary pollution-free technology to treat polluted water.

This work explores the remediation potential of green, sustainable, cost-effective, ubiquitous, and locally sourced biomass of Terminalia ivorensis seed waste (TISW) for some toxic pollutants studied. To the best of the authors’ knowledge, TISW is a new biological adsorbent which is reported to have good uptake capacities for MB, CR, Cd(II) and Pb(II).

Experimental

Preparation of Terminalia ivorensis seed waste (TISW)

Terminalia ivorensis seed waste (TISW) was obtained from the Forest Research Institute of Nigeria (FRIN), in Ibadan (7°23′16″ North, 3°53′47″ East), Nigeria. After collection, it was air dried for four weeks and later oven dried for 22 h at 70 °C. The dried bark was cut into small pieces and pulverised. Further drying was carried out for 6 h. The pulverised TISW was sieved to 250 µm, put into plastic containers and used for various adsorption experiments.

Surface microstructures of TISW

Proximate analysis, bulk density, pH of point of zero charge (PZC), specific surface area (SSA), Fourier transform infra red spectroscopy (FTIR), thermogravimetric/differential thermal analysis (TG/DTA), scanning electron microscopy (SEM) and energy dispersive analysis of X-ray (EDAX) of TISW

Proximate analysis and bulk density of TISW were determined according to the published protocol of Ofomaja and Naidoo [55]. The pH of point zero charge of TISW was determined according to the published protocol of Stumm and Morgan [56]. The specific surface area of TISW was determined according to the published protocol of Sears [57]. The Fourier Transform Infra Red spectroscopic analyses of unloaded, and toxic metals/dyes loaded TISW were carried out by Perkin Elmer FTIR Spectrophotometer (Spectrum Version 2) at the scanning frequencies of 400–4000 cm−1. The Perkin Elmer thermogravimetric/differential thermal analysis (TG/DTA) was carried out by Perkin Elmer TG/DTA thermal analyser. Furthermore, the surface microstructures of TISW were carried out by Scanning Electron Microscopy (SEM) instrument, coupled with Energy Dispersive Analysis of X-ray (EDAX), JEOL JSM-6390 LV Model.

All the experimental details and mathematical equations used for these adsorption and desorption studies are shown in the Electronic Supporting Information (ESI) of this manuscript.

Results and discussion

Surface characterization of TISW microstructures

Proximate analysis, bulk density, PZC, SSA, FTIR, TG/DTA, SEM and EDAX of TISW

The percentage proximate compositions of TISW on dry weight basis are depicted in Table 1. The determined bulk density of TISW was 0.05 g cm−3. The SSA of TISW was 240.6 m2 g−1. The PZC of the TISW was 6.44.

Table 1 Proximate analysis of Terminalia ivorensis seed waste
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The PZC or (isoelectric pH) of biosorbent is the pH at which the density of anionic moieties is equal to that of the cationic moieties on a biosorbent surface. At pH < PZC, the surface of a biosorbent is predominantly cationic, while at pH > PZC, its surface is predominantly anionic. The functional moieties on TISW surface probably acquire negative or positive charges depending on the solution pH [55]. The PZC result of TISW suggests that it possesses a broad solution pH range to retain its positive charges (see Fig. 1).

Fig. 1
figure 1

The pH of point of zero charge plot for Terminalia ivorensis seed waste

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The FTIR spectra of TISW, with its loaded CR, MB, Pb2+ and Cd2+ analogues are shown in figures S1a-S1e of the ESI. The free O–H stretch for TISW appeared at 3871 and 3748 cm−1. After loading TISW with CR, MB and Pb2+, there was no shift in the vibration frequency of the free O–H stretch. For Cd2+-loaded TISW, the vibration frequency shifted to 3759 cm−1. The N–H stretch of TISW was observed at 3456 cm−1. The N–H stretch for CR, MB, Pb2+ and Cd2+-loaded TISW shifted to 3445, 3438, 3447 and 3443 cm−1, respectively. These shifts in the vibration frequencies might be due to binding of these toxic ions onto the functional moieties in the cell walls of TISW [29]. The C–H of CH2 and CH3 for TISW was observed at 2935 cm−1. No significant shift was observed for MB, Pb2+ and Cd2+-loaded TISW. But for CR-loaded TISW, the C-H vibration frequency decreased to 2924 cm−1. The –C=O stretch for TISW was found at 1648 cm−1. For MB-loaded TISW, the vibration frequency decreased 1623 cm−1. There were negligible shifts in the vibration frequency of –C=O stretch for TISW loaded with CR, Pb2+ and Cd2+. The aromatic –C=C– stretch for TISW appeared at 1544 cm−1. Also, there was no significant change in the –C=C– vibration frequency when TISW was loaded with CR, MB, Pb2+ and Cd2+. The C–H bend for TISW appeared at 1359 cm−1. This vibration frequency increased to 1376 and 1384 cm−1 for CR-loaded TISW, and Pb2+ and Cd2+-loaded TISW. For MB-loaded TISW, this vibration frequency decreased to 1348 cm−1. The –C–O bend for TISW was observed at 1057 cm−1. However, there were negligible shifts in the vibration frequency of –C–O bend for TISW loaded with MB, CR, Pb2+ and Cd2+. For TISW, the –C–O out of plane deformation bend was observed at 582 cm−1. This vibration frequency shifted to 571 and 568 cm−1 for CR and MB-loaded TISW, and Pb2+ and Cd2+-loaded TISW, respectively. The various shifts in the stretch and bend vibration frequencies after loading TISW with the studied pollutant ions/molecules might result from chelation, precipitation and ion exchange reactions that was involved in the biosorption process (see Table 2) [29].

Table 2 Fourier transform infrared spectra characteristics of Terminalia ivorensis seed waste before and after biosorption of Methylene Blue, Congo Red, Cadmium and Lead
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The TG of TISW (see Fig. 2) showed that <220 °C, ca. 11% weight loss was observed due to the loss of surface water from TISW. At temperature >220 and <550 °C, ca. 55% weight loss was observed, which was assigned to the loss of the volatile components of lignocelluloses and hemicelluloses of TISW [20, 21]. Above 550 °C, TISW was thermally stable. For DTA (also see Fig. 2) of TISW, exothermic heat flow assigned to 1% decrease in derivative weight was observed from 33 to 150 °C, endothermic heat flow assigned to 1.5% increase in derivative weight was observed from >150 to 180 °C, another exothermic heat flow assigned to 5% decrease in derivative weight was observed from 180 to 350 °C and two endothermic heat flows assigned to 3.5 and 0.5% increases in derivative weight were observed from >350 to 400 °C and 400 to 580 °C, respectively. The exothermic heat flows led to the dissociation of some chemical bonds of lignocelluloses and hemicelluloses of TISW. Also, the endothermic heat flows led to the association of some chemical bonds to form small molecules such as CO, CO2, NH3, etc. [20, 21, 25, 58].

Fig. 2
figure 2

Thermogravimetric/differential thermal analysis for Terminalia ivorensis seed waste

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The SEM micrographs of TISW show tiny irregular, scattered and spaced fibrils of the lignocelluloses that are contained in the cell walls that comprise lumped particles of unconnected and broken walls. The EDAX of TISW showed the elemental compositions of TISW, which was an indication of the constituents of the functional moieties in TISW (see Fig. 3a–d).

Fig. 3
figure 3

a–d Scanning electron micrographs and energy dispersive analysis of X-ray for Terminalia ivorensis seed waste

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Influence of pH on the removal of sorbate ions onto TISW

The influence of solution pH on the biosorption capacity of TISW largely depends on the speciation of the sorbate or solute ions/molecules at different pHs of the aqueous solutions. The removals of CR and MB onto TISW were highest at pH 2.0 and pH 12.0, respectively. These were obtained as 3.34 and 12.36 mg/g, respectively. The biosorption of CR was optimum at pH 2.0 due to increase in the proton cloud densities of TISW (see Fig. 4). This enabled negatively charged CR molecules to bind easily to the surface of TISW. On the other hand, the removal of MB onto TISW was found to be optimum at pH 12.0 due to the low proton cloud densities on the surface of TISW. This accounted for the increase in the removal of positively charged MB molecules onto TISW [21, 29].

Fig. 4
figure 4

The plots of the amounts of Methylene Blue, Congo Red, Cadmium and Lead biosorbed by Terminalia ivorensis seed waste, \(q_{\text{e}}\) (mg/g) against pH (pH 2.0–7.0 for Cadmium and Lead; pH 2.0–12.0 for Methylene Blue and Congo Red); adsorbent dose = 25 mg; agitation speed = 200 rpm; initial metal ion concentration = 100 mg/L; agitation time = 180 min; temperature = 300 K)

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Furthermore, the removal of Cd2+ onto TISW was highest at pH 6.0. This was obtained at 10.60 mg/g. Also, >pH 7.0, there was a very slight increase in the removal of Cd2+ to 10.64 mg/g. The negatively charged TISW surface at weak acidic and neutral pH favoured the high uptake of Cd2+ due to deprotonation (low proton density). The very negligible increase for the removal of Cd2+ by TISW observed at >pH 6.0 might be due to the likelihood of the precipitation of Cd2+ on the functional sites on the cell walls of TISW. On the other hand, the removal of Pb2+ by TISW was highest at pH 6.0. This was obtained at 12.13 mg/g. At pH >6.0–7.0, Pb2+ no longer exists in the solution. The speciation of Pb2+ at this pH led to the formation of Pb(OH)+, Pb(OH)2, Pb(OH) −3 and Pb(OH) 2−4 . From the PZC value of TISW, which was 6.44, it suggested that TISW retained positive charges on its surface over a pH range <6.44. Due to this, TISW would biosorbed high amount of CR molecules than the positive pollutant ions/molecules through ion exchange mechanism and electrostatic attraction. The adsorption of Pb2+ continued until precipitation occurred at pH >6.0. Hence, at pH >6.0–7.0, the binding of Pb(OH)+ species to the TISW biosorbent surface occurred by complexation mechanism and, at this point, further biosorption of Pb2+ ceased. Hence, the main mechanisms by which MB and CR molecules interact with TISW were by electrostatic attraction and π–π hydrophobic interactions. These π–π hydrophobic interactions are higher in CR molecule than MB molecule due to the many aromatic rings in CR thus it has higher propensity to bind to the negatively charged functional moieties on the surface of TISW by π–π hydrophobic interactions than MB. Electrostatic attraction occurred when MB or CR molecules bound to the negatively charged functional moieties on the surface of TISW depending on the pH of the aqueous solutions. On the other hand, the main mechanisms for the uptake of Cd2+ and Pb2+ by TISW were by electrostatic attraction and complexation, which are dependent on the pH of the working solutions as expounded above [29].

Influence of adsorbent dose on the removal of sorbate ions onto TISW

The amount of sorbate ions biosorbed decreased with increase in biosorbent dose from 10 to 200 mg. This decrease became more pronounced as the weight of the biosorbent increased, resulting in the agglomeration or aggregation of biosorbent particles, probably blockage of biosorption sites, reduced surface area for sorbate–sorbent interaction and the lengthening of diffusion path [55]. Figure 5 shows the plots of the amount of solute ions adsorbed at equilibrium, \(q_{\text{e}}\) (mg/g) against adsorbent dose (mg).

Fig. 5
figure 5

The plots of the amounts of Methylene Blue, Congo Red, Cadmium and Lead biosorbed by Terminalia ivorensis seed waste, \(q_{\text{e}}\) (mg/g) against adsorbent dose (adsorbent dose = 10–200 mg; agitation speed = 200 rpm; initial metal ion concentration = 100 mg/L; agitation time = 180 min; temperature = 300 K)

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Influence of initial sorbate ions concentration on the biosorption capacity of TISW

The initial sorbate ions concentration is a cogent factor that determines the mass transfer and diffusion dynamics of the biosorption of sorbate or solute ions onto biomasses. Also, increase in the initial sorbate ions concentration is the driving force that is needed to overcome the mass transfer resistance of the uptake of solute ions onto the surface of the biomass [59].

As the initial concentrations of CR and MB increased from 25 to 750 mg/L, there was a commensurate rise in their removal from 6.30 to 362.80 and 10.17 to 362.04 mg/g respectively. Also, the increase in the initial concentrations of Cd2+ and Pb2+ from 25 to 750 mg/L also amounted in the proportionate increase in Cd2+ and Pb2+ biosorbed by TISW from 11.55 to 146.89 and 10.48 to 176.22 mg/g, respectively. It is noteworthy to mention here that the increase in the initial concentration of sorbate ions led to the corresponding increase in their uptakes, as they occupied the functional moieties on the surface of TISW. The binding of these solute ions to these functional moieties in the active sites occurred in quick successions until they were occupied [25, 29].

Influence of agitation time on the removal of sorbate ions onto TISW

The residence time of solute ions plays a salient role in the solute ions uptake onto the biosorbent surface, due to its influence on the sorbent–solute interface. Ho [60] and Onal et al. [61] proposed that biosorption consists of three steps, which are: (a) the diffusion of adsorbate through the solution to the external surface of the adsorbent or the boundary layer diffusion of the solute molecules, (b) the gradual adsorption step, in which intraparticle diffusion may be rate limiting and (c) the diffusion of adsorbate particles to adsorption sites either by pore diffusion through the liquid-filled pores or by a solid diffusion mechanism.

The biosorption of CR by TISW at temperature of 300 K increased up to 15 min. Thereafter, equilibrium was attained. Also, the biosorption of CR by TISW increased up to 30 and 60 min at 320 and 340 K respectively, after which equilibrium was reached. It could be said that increase in temperature led to an increase in the time equilibrium was attained for the biosorption of CR onto TISW. At 300 K, CR molecules rapidly occupied the available functional moieties on the active sites in 15 min of biosorption process. At 320 and 340 K, the available functional moieties on the active sites were occupied at 30 and 60 min, respectively. Although, increase in the temperature from 300 to 340 K was meant to increase in the driving force (mass transfer) of CR molecules onto the functional moieties in the pores of TISW due to the increase in the collision frequency and mobility of CR molecules [29, 62]. This temperature increase might have led to the rupture of some chemical bonds of the functional moieties of TISW, distorting the π–π stacking from hydrophobic interactions of CR aromatic rings. Hence, this phenomenon decreased the propensity of CR molecules to bind to TISW surface [29]. The biosorption of MB onto TISW at 300 and 320 K increased up to 60 min. Thereafter, a slight increase in the uptake of MB was observed up to 120 min, after which equilibrium was attained. At 340 K, the biosorption of MB increased up to 120 min, after which equilibrium was reached. Increment in the temperature made MB to exhibit a dissociated form that decreased the rate at which it bound onto the functional moieties in the pores of TISW [29, 62].

The biosorption of Cd2+ onto TISW at 300 K increased up to 30 min, respectively, and  >30 min of Cd2+ uptake culminated into equilibrium. For the biosorption of Pb2+ by TISW at 320 and 340 K, the uptake of Pb2+ increased up to 15 min, after which equilibrium was observed. The increase in the mass transfer of Pb2+ onto the functional moieties in the TISW pores was due to the fast diffusion of Pb2+ from the solute phase onto the sorbent [61, 62].

Equilibrium, kinetics and thermodynamics of CR, MB, Cd2+ and Pb2+ removal by TISW

To know the biosorption capacity of TISW for CR, MB, Cd2+ and Pb2+, the experimental data were modelled with some equilibrium models. Langmuir isotherm fits the experimental data for the removal of CR, MB, Cd2+ and Pb2+ by TISW better than the Freundlich and Temkin isotherms. The correlation co-efficient values for the Langmuir isotherm were higher than those for Freundlich and Temkin isotherms. The saturation monolayer adsorption capacities, \(q_{{\max_{L} }}\), of TISW for CR, MB, Cd2+ and Pb2+ were 85.47, 175.44, 12.58 and 52.91 mg g−1, respectively (see Table 3). Lucidly, it is noteworthy to say that the homogeneous nature of TISW surface favoured the uptake of MB molecules more than the other pollutant ions of interest in this work. From Freundlich isotherm, it can be deduced that the values for the biosorption affinity, \({1 \mathord{\left/ {\vphantom {1 n}} \right. \kern-0pt} n}\), for the uptake of CR, MB, Cd2+ and Pb2+ onto TISW were 0.93, 2.41, 0.43 and 0.53, respectively. This suggests that the binding of MB onto TISW would be least favourable by heterogeneous surface, while the binding of Cd2+ onto TISW would be most favourable by heterogeneous surface (\({{0 < 1} \mathord{\left/ {\vphantom {{0 < 1} n}} \right. \kern-0pt} n} < 1\)-favourable, \({1 \mathord{\left/ {\vphantom {1 n}} \right. \kern-0pt} n} > 1\)-non favourable).

Table 3 Linear equilibrium parameters for the removal of Methylene Blue, Congo Red, Cadmium and Lead onto Terminalia ivorensis seed waste
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The nonlinear plots (Langmuir, Freundlich and Temkin isotherms) for the removal of CR, MB, Pb2+ and Cd2+ by TISW are shown in Fig. 6a–c. From these nonlinear isotherm plots, it was deduced that the equilibrium data better fit the nonlinear isotherms than the linear isotherms due to the higher correlation coefficient values (\(R^{2}\) values) obtained for these plots, when compared with those obtained for linear isotherm plots. For the nonlinear isotherm plots, the reduced Chi square values (\(\chi^{2}\) values) and the residual sum of squares values indicated that Langmuir isotherm better fits the equilibrium data than Freundlich and Temkin isotherms (see Table 4).

Fig. 6
figure 6

a Nonlinear Freundlich isotherm plots of Methylene Blue, Congo Red, Cadmium and Lead biosorbed by Terminalia ivorensis seed waste (temperature = 300 K; adsorbent dose = 25 mg; agitation speed = 200 rpm; initial metal ion concentration = 25–500 mg/L; agitation time = 180 min). b Nonlinear Langmuir isotherm plots of Methylene Blue, Congo Red, Cadmium and Lead biosorbed by Terminalia ivorensis seed waste (temperature = 300 K; adsorbent dose = 25 mg; agitation speed = 200 rpm; initial metal ion concentration = 25–500 mg/L; agitation time = 180 min). c Nonlinear Temkin isotherm plots of Methylene Blue, Congo Red, Cadmium and Lead biosorbed by Terminalia ivorensis seed waste (temperature = 300 K; adsorbent dose = 25 mg; agitation speed = 200 rpm; initial metal ion concentration = 25–500 mg/L; agitation time = 180 min)

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Table 4 Nonlinear equilibrium parameters for the removal of Methylene Blue, Congo Red, Cadmium and Lead onto Terminalia ivorensis seed waste
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The kinetics of biosorption gives a good idea of how the experimental timing controls the mechanism, chemical reaction and the bulk transfer of the solute ions onto the biosorbent surface. Table 5 indicates that pseudo-second-order kinetic equation gave better fits to the experimental data than pseudo-first-order kinetic equation for the removal of CR, MB, Cd2+ and Pb2+ by TISW. The correlation co-efficient values for pseudo-second-order kinetic equation were higher than those obtained for pseudo-first-order kinetic equation at the experimental temperatures. This suggests that the mechanism for the biosorption process is chemisorption. The pseudo-second-order rate constants, \(k_{2}\), for the removal of CR, MB, Cd2+ and Pb2+ by TISW at 300–340 K were \(k_{2} \; \le \;\) 0.417 g mg−1 min−1, while the pseudo-first-order rate constants, \(k_{1}\), for the removal of CR, MB, Cd2+ and Pb2+ by TISW at 300–340 K were \(k_{1} \; \le \;\) 0.026 min−1. The Weber–Morris intraparticle diffusion equation indicated that the plots did not pass through the origin, due to the fact that the values of the boundary film or the thickness of the boundary layer were \(c \ge\) 0 in all cases. This implies that intraparticle diffusion process was not the only rate controlling or limiting step for the biosorption process, but pore diffusion and film diffusion might have taken place in the rate controlling or limiting step [29, 63, 64].

Table 5 Linear kinetic parameters for the removal of Methylene Blue, Congo Red, Cadmium and Lead onto Terminalia ivorensis seed waste
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The nonlinear plots (pseudo-first-order, pseudo-second-order and intraparticle diffusion equations) for the removal of CR, MB, Pb2+ and Cd2+ onto TISW are shown in Tables 6 and 7. It was deduced that the kinetic data better fit the nonlinear kinetic equation than the linear kinetic equations. This resulted from the fact that the correlation coefficient values (\(R^{2}\) values) obtained for these plots were higher than those obtained for the linear kinetic equations. For the nonlinear kinetic plots, the reduced Chi square values (\(\chi^{2}\) values) and the residual sum of squares values suggested that pseudo-second-order equations gave better fit to the kinetic data when compared to pseudo-first-order and intraparticle diffusion equations. Figure 7a–c depicts the nonlinear kinetic plots for pseudo-first-order, pseudo-second-order and intraparticle diffusion equations for biosorbed CR, MB, Cd2+ and Pb2+ by TISW. But this work has shown that low-cost TISW demonstrated remediation potential for both toxic industrial dyestuffs and toxic metal ions. Hence, TISW has a wide array of applications for environmental remediation.

Table 6 Nonlinear kinetic parameters for the removal of Methylene Blue and Congo Red onto Terminalia ivorensis seed waste
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Table 7 Nonlinear kinetic parameters for the removal of Cadmium and Lead onto Terminalia ivorensis seed waste
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Fig. 7
figure 7

a Nonlinear pseudo-first-order kinetic plots of Methylene Blue, Congo Red, Cadmium and Lead biosorbed by Terminalia ivorensis seed waste (temperature = 300–340 K; adsorbent dose = 25 mg; agitation speed = 200 rpm; initial metal ion concentration = 100 mg/L; agitation time = 0.5–180 min). b Nonlinear pseudo-second-order kinetic plots of Methylene Blue, Congo Red, Cadmium and Lead biosorbed by Terminalia ivorensis seed waste (temperature = 300–340 K; adsorbent dose = 25 mg; agitation speed = 200 rpm; initial metal ion concentration = 100 mg/L; agitation time = 0.5–180 min). c Nonlinear Weber–Morris intraparticle diffusion kinetic plots of Methylene Blue, Congo Red, Cadmium and Lead biosorbed by Terminalia ivorensis seed waste (temperature = 300–340 K; adsorbent dose = 25 mg; agitation speed = 200 rpm; initial metal ion concentration = 100 mg/L; agitation time = 0.5–180 min)

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From thermodynamic standpoint (see Table 8), +ΔG° (kJ/mol) values increased from 6.53 to 7.40, 7.67 to 8.69, 23.87 to 27.05 and 13.31 to 15.09 as TISW biosorbed CR, MB, Cd2+ and Pb2+ from 300 to 340 K, respectively. The ΔH° (kJ/mol) values were −0.27, −0.40, +0.09 and +0.22 for the biosorption of CR, MB, Cd2+ and Pb2+ by TISW, respectively. The −ΔS° (J mol−1 K−1) values for CR, MB, Cd2+ and Pb2+ biosorbed by TISW were 217.72, 255.71, 79.56 and 44.38, respectively. Table 8 depicts that the values of Gibb’s free energy, (ΔG°) for CR, MB, Cd2+ and Pb2+ biosorbed by TISW were non-spontaneous at all temperatures. The values of enthalpy change, (ΔH°) for CR and MB biosorbed by TISW were exothermic, while those for Cd2+ and Pb2+ biosorbed by TISW were endothermic. The values of entropy change, (ΔS°) for CR, MB, Cd2+ and Pb2+ biosorbed by TISW showed decreasing disorderliness or chaos of the biosorption process. Table 9 shows the adsorption capacities, \(q_{{\max_{L} }}\)(mg/g) of some adsorbents for CR, MB, Cd2+ and Pb2+ used by researchers as reported in literature.

Table 8 Thermodynamic parameters for the removal of Methylene Blue, Congo Red, Cadmium and Lead onto Terminalia ivorensis seed waste at different temperatures
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Table 9 The adsorption capacities, \(q_{{\max_{L} }}\) (mg/g) of some adsorbents used by researchers
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Desorption kinetics

Figure 8 shows that desorption of MB from MB-loaded TISW was maximum at 86.37% for 0.1 M HCl desorbent and also maximum at 79.77% for 0.1 M HNO3 desorbent after 30 min. Similarly, desorption of CR from CR-loaded TISW was maximum at 92.73% for 0.1 M HCl desorbent and likewise maximum at 55.65% for 0.1 M HNO3 desorbent after 30 min. The desorption kinetics of Cd2+ showed that the maximum of 34.89 and 53.01% of Cd2+ was desorbed from Cd2+ loaded TISW using 0.1 M HCl and 0.1 M HNO3 desorbents, respectively, after 30 min. Also, the maximum of 39.5 and 65.16% of Pb2+ was desorbed from Pb2+ loaded TISW using 0.1 M HCl and 0.1 M HNO3 desorbents, respectively, after 30 min. Conversely, desorption of CR and MB from TISW was kinetically faster than that of Cd2+ and Pb2+ from TISW. This desorption kinetics result reveals that TISW can be regenerated and reused for another cycle of biosorption.

Fig. 8
figure 8

Desorption kinetics of Methylene Blue, Congo Red, Cadmium and Lead loaded-Terminalia ivorensis seed waste using 0.1 M HCl and 0.1 M HNO3 (adsorbent dose = 25 mg; agitation speed = 200 rpm; agitation time = 1–30 min; temperature = 300 K)

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Conclusion

For the first time, Terminalia ivorensis seed waste (TISW) demonstrated a good potential for the removal of CR, MB, Cd2+ and Pb2+. Proximate analysis, bulk density, specific surface area, pH of point of zero charge, Fourier transform infra red spectroscopy, scanning electron microscopy, thermogravimetric/differential thermal analysis and energy dispersive analysis of X-ray were used to study the surface texture or morphology of TISW. The equilibrium data best fit the Langmuir isotherm, with a maximum Langmuir monolayer saturation adsorption capacity, \(q_{{\max_{L} }}\) = 175.44 mg MB per g of TISW. The kinetic data best fit the pseudo-second-order equation for the removal of CR, MB, Cd2+ and Pb2+ onto TISW. This study revealed that TISW, a benign agricultural seed waste, could be recommended as ubiquitous and cheap biological adsorbent for environmental remediation of toxic industrial dyestuffs and toxic metals.