Waste to resource: Utilization of waste bagasse as an alternative adsorbent to remove heavy metals from wastewaters in sub-Saharan Africa: A review

The scarcity of fresh water is one of the 21st century's global challenges for developing countries and is increasing alarmingly due to improper disposal of urban and industrial wastes (Anastopoulos et al. 2017; Harripersadth et al. 2020). Currently, in developing countries such as in sub-Saharan Africa water consumption is increasing substantially to achieve sustainable economic development from different economic sectors in terms of both adequate quantity and acceptable quality for end-users. Reduction of freshwater consumption needs holistic approaches for the effective utilization of existing freshwater and quality enhancement strategies for used water to satisfy global water demands (Jones et al. 2021). Currently, water pollution by heavy metals (HMs) is increasing alarmingly day by day due to urban runoff, agricultural activities, mining, and industrial and domestic discharges into the aquatic environment (Agoro et al. 2020).

Heavy metals are chemical elements that have an atomic density greater than 4 g/cm³ (Aprile & Bellis 2019; Ayob et al. 2021). Among the heavy metals, cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), zinc (Zn), and iron (Fe) are today's primary concerns in the aquatic and terrestrial environment due to their toxicity, high mobility and solubility (Kong et al. 2014; Ali et al. 2019a). The unique features of heavy metals, such as bioaccumulation, non-biodegradability, and biological toxicity, make the existential threat very complex (Dong et al. 2016; Tran et al. 2017; Nkutha et al. 2020; Tejada-Tovar et al. 2020). The high concentration of heavy metals can be toxic or carcinogenic in nature and can cause severe problems to humans and aquatic ecosystems (Renu & Singh 2017; Jamshaid et al. 2018; Omran et al. 2019).

The most challenging issues for sustainable water management in sub-Saharan African are mainly rapid population growth (869 million in 2010 to 1.1 billion in 2019) (UN 2018), unplanned urbanization (Gashaye 2020), and inadequate policies and political commitments (Bhari et al. 2008; Bishoge 2021). Currently more than 77 million people live under critical water stress around the globe and the majority of them live in sub-Saharan Africa (GIZ 2019; Olagunju et al. 2019). In addition, the majority of sub-Saharan Africans exploit only up to 5% of annual renewable water resources due to the low level of investments in conservation, treatment technologies, and other infrastructures (United Nations General Assembly 2016; UN & WHO 2021).

Currently several robust and sophisticated technologies are utilized for heavy metals removal from wastewater such as ion exchange, reverse osmosis, chemical precipitation, electrocoagulation, electro-dialysis and adsorption (Harripersadth et al. 2020; Iwuozor et al. 2021a; Hamad & Idrus 2022). However, the aforementioned treatment methods are challenged by high initial costs, membrane fouling and high sludge production. In comparison with other processes, adsorption is a potential alternative treatment to existing technologies due to its low cost, simplicity of operation, low energy demand, recovery of heavy metals, good selectivity, and adsorbent regeneration options (Yu et al. 2020; Hassan et al. 2021; Younas et al. 2021).

In recent decades, various adsorbents including activated carbon (AC), biochar, clay, silica gel, zeolite, graphene oxide, nanomaterials, nanocomposites, polymers, and activated alumina have been widely utilized to remove organic and inorganic contaminants including heavy metals ions (Ali et al. 2018, 2019b; Hassan et al. 2020). Recently, there has been considerable interest in the gradual replacement of conventional materials with green and low-cost adsorbents. Among these adsorbents, biological materials – agricultural by-product residues as adsorbents to remove metals ions from aqueous solution – have had a lot of attention (dos Santos et al. 2019; Harripersadth et al. 2020; Hamad & Idrus 2022).

Bagasse constitutes the largest amount of solid waste from sugar factories (Saccharum officinarum) (Anastopoulos et al. 2017; Raza et al. 2021) and has huge potential in developing countries. Bagasse is one of the largest agricultural residues and 1,869.7 million tonnes of bagasse was harvested worldwide in 2020 (Faostat 2022). In Ethiopia, annually about 4.6% of total bagasse produced from sugar industries is dumped in the compounds around the factory, posing serious environmental problems, including fire hazards (Getu et al. 2021; Adane et al. 2022). Despite the Ethiopian government's waste product reduction/reuse polices for sugar processing industries, there are still many unresolved environmental issues that are to be addressed through valorization of waste bagasse in the circular economy.

The circular economy offers various tools for raising awareness of the recovery of waste through mitigation of excessive raw material consumption and reducing the disposal of wastes (Athira et al. 2021; Osorio et al. 2021). In most sugar industries, bagasse is mainly utilized in boilers to provide heat in the distillation process and ethanol production (Mbohwa & Fukuda 2003; Cueva-Orjuela et al. 2017). Although the energy from renewable biomass fuel is carbon neutral, when biomass fuels are burned in unventilated kitchens using smoky and inefficient conventional stoves with poor combustion, this results in a significant concentration of hazardous pollutants, primarily carbon monoxide and particulate matter, as well as nitrogen oxides and poly-aromatic hydrocarbons (Formann et al. 2020; Benti et al. 2021). Thus, a twofold problem in developing countries will be solved by integrated approaches of converting bagasse waste to value-added adsorbent for heavy metal removal from wastewater. This brings many advantages to bagasse waste management and provides low-cost, locally available alternative adsorbents (Anastopoulos et al. 2017; Sarker et al. 2017; Ungureanu et al. 2022).

The adsorption capacity of sugarcane bagasse (SCB) is due to its high porosity, high surface area, carbon-oxygen containing functional groups, and good stability for reuse (Ighalo et al. 2022; Mondal et al. 2022). Surface modification methods for bagasse increase its adsorption capacity towards heavy metals (Wang et al. 2017; Sarker et al. 2017; Tejada-Tovar et al. 2020; Irawan et al. 2021; Younas et al. 2021). The main objective of this review paper is to reveal comprehensive overviews on selective HMs adsorption capability of SCB for its future potential utilization in real wastewaters of low-income countries of sub-Saharan Africa.

In most developing countries, sugar industries were designed only for production in sugar mills and account for the generation of large quantities of the by-products bagasse (25–30%) after crushing sugarcane, press mud (3–5%) after clarification, and molasses (3.5–5%) after centrifuge (Botkin et al. 2012; Formann et al. 2020). In a highly competitive environment, the non-utilization of SCB ends up with a loss of resources without generation of revenues. However, most sugar industries burn bagasse in the furnace to produce electric power for the sugar factory (Thangavelu et al. 2016; Ajala et al. 2021). Open dumping of bagasse as waste is a common practice of sugar industries in most developing countries, including Ethiopia, after fulfillment of hot utilities such as steam and electric power. Thus, bagasse accounts for serious environmental problems including fire hazards (Fito et al. 2019; Getu et al. 2021).

In contrast to the traditional take-make-use-dispose strategy of a linear economy, a circular economy minimizes waste production, creates wealth from these by-products and maximizes reuse (Hysa et al. 2020; Osorio et al. 2021). Also, several environmental impact assessment reports show that sugar industries can be considered as zero discharge industries if proper waste conversion technologies are implemented (Sahu 2018). A great deal of research shows that SCB can be used for different applications such as in adsorbent, ion exchange resin, ceramics, concrete, cement and polymer composites (Ajala et al. 2021) formations. Consequently, SCB is a biomass with great potential to meet global energy demand and encourage the waste to wealth conversion principle for economic and environmental sustainable development (Ajala et al. 2021). Energy security and environmental conservation issues are likely to remain two of the major long-term contradictory challenges facing human existence globally. Therefore, for a sustainable environment and to engage with greener circular economy principles, integrated approaches of renewable resources utilization should be required instead of waste disposal (Formann et al. 2020).

Among sources of bio-adsorbents, agro-industrial waste and polysaccharides materials are the most widely reported types for adsorptive removal of heavy metals. In contrast to conventional adsorbents that contain single types of binding sites, the multi-electron density functional groups of bio-sorbents have good potential for metal ion binding by a variety of adsorption mechanisms. The effective removal of heavy metals from wastewater using bio-adsorbents such as bagasse depends on its unique surface chemistry and functional groups (Hamad & Idrus 2022). Among agro-industrial wastes, bagasse has carbon-oxygen containing functional groups like carboxyl, ketone, ester, aromatic rings and hydroxyl that have good capacity for metal ion adsorption (Anastopoulos et al. 2017; Siqueira et al. 2020).

The main potential heavy metal adsorption capacity of SCB is due to its highly porous nature and presence of carbon–oxygen, which account for pore filling and/or binding heavy metals by changing their hydrogen ions for metal ions or giving an electron pair to form complexes with the metal ions (Shah et al. 2018; Ambaye et al. 2021). However, raw sugarcane bagasse and its main functional components show good adsorption capacity towards heavy metals; they can satisfy the water quality need for low concentrations of heavy metals (Ighalo et al. 2022). Thus, to meet water quality need at high pollutant loads and to enhance selectivity towards the target adsorbent, further modification of bagasse was needed (Irawan et al. 2021). Various modification techniques such as chemical modification by addition of functional groups, acid hydrolysis, carbonization of SCB to derive activated carbon are among those widely reported (Xavier et al. 2018).

Surface modification of bagasse increases its heavy metals adsorption capacity (Yu et al. 2015; Wang & Wang 2018). In addition, surface modification of SCB based bio-adsorbents is easy due to the abundant hydroxyl, phenolic and carboxylic functional groups of cellulose and hemicellulose (Ighalo et al. 2022). Chemical modification of SCB enhanced HMs bio-adsorption capacity; this may be attributed to the increase in functional properties and binding sites of the bio-adsorbent by the modifying agent and therefore, higher sorption affinity (Li et al. 2013; Romero-Cano et al. 2017; Irawan et al. 2021). As presented in Table 2, SCB chemical modification with EDTA di-anhydride and pyromellitic di-anhydride provides additional carbonyl functional groups on the bagasse surface that account for enhanced Pb(II) removal capacity (Moyo et al. 2017; Tang et al. 2018). In another study by Shah et al. (2018) and Al-Saidi et al. (2022) acid modifications of SCB surface hydrolysis ester functional groups converted them to carboxylate groups, which accounts for the highly coordinated interaction with HMs through electrostatic interaction as shown in Table 2.

Sugarcane bagasse based adsorption removal shows selectivity based on physicochemical properties of HMs (Homagai et al. 2010; Saxena et al. 2017; Hassan et al. 2021; Mondal et al. 2022). Also, the literature shows that SCB adsorption removal of different heavy metals under similar experimental setups, shows different adsorption capacity (Kong et al. 2014; Tran et al. 2017; Ighalo et al. 2022). Recently, Harripersadth et al. (2020) reported in the same working environment that at 1.0 g of SCB adsorbent dose and solution pH = 5.5, the adsorption capacity of bagasse for two different heavy metals ions, namely Pb²⁺ and Cd²⁺, are different, 31.45 and 19.49 mg/g respectively. Higher adsorption capacity of Pb²⁺ than Cd²⁺ in the same working environment was due to high ionic radius (0.118 nm) and electron negativity (1.8) of Pb²⁺. Whereas, Cd²⁺ has relatively smaller ionic radius and electronegativity, 0.097 nm and 1.7.

In general, highly electronegative metal ions show relatively high affinity to attract electrons and are thereby adsorbed at surface of bagasse or trapped by surface pores. As shown in Table 3, Ighalo et al. (2022) reported low-density polyethylene hybrid bagasse biochar for adsorption removal of four heavy metals, Cu²⁺, Pb²⁺, Zn²⁺ and Fe²⁺. The bagasse biochar low-density polyethylene hybrid adsorbent shows higher adsorption capacity for Zn²⁺ than other heavy metals (Pb²⁺, Cu²⁺, and Fe²⁺) because of its greater hydrolysis constant, higher ionic radius, and larger softness for inner-sphere surface complexation or adsorption reaction mechanisms. Heavy metals with greater hydrolysis constant, electronegativity, higher ionic radius, and larger softness value show greater tendency for adsorption at bagasse bio-adsorbents surface (Ighalo et al. 2022). Similarly, most literature reported that chemical modification of SCB enhances adsorption capacity. This may be explained as addition of further functional groups and binding sites that accounts for better adsorption tendency towards target heavy metals.

However, in some instances, the adsorption capacity of the unmodified sugarcane bagasse appeared to be significantly higher than that of the modified bagasse. As shown in Table 3, Ighalo et al. (2022) reported 17.83 mg/g Pb²⁺ adsorption of low density polyethylene hybrid biochar modified SCB and Tran et al. (2017) reported 19.3 mg/g of Pb²⁺ removal capacity of ZnCl₂ derived activated carbons from SCB at 500 °C carbonization temperature. Whereas, Harripersadth et al. (2020) reported 31.45 mg/g of Pb²⁺ adsorption removal of unmodified bagasse. Similarly, Tejada-Tovar et al. (2020) reported 37.88 mg/g of Pb²⁺ adsorption removal of unmodified bagasse. This may be due to differences in growth of SCB with different climatic conditions that may affect its chemical composition, and SCB bio-adsorbent preparation techniques may account for variable percentages of functional groups and surface chemistry nature. In addition, a plausible reason for low adsorption capacity of modified bagasse as reported by Tran et al. (2017) and Ighalo et al. (2022) may be the high temperature carbonization methods that account for removal of functional groups and pore volume of bagasse.

It is worth mentioning that bagasse adsorption removal of heavy metals happens through physical processes (pore filling, hydrogen bonding, Van der Waals force, etc.), chemical processes (inner-sphere complexation (–OH) and surface complexation (–COOH)), and electrostatic interaction (Hamad & Idrus 2022). Adsorption capacity of SCB is not only charge dependent but it also shows adsorption at the positive adsorbent surface (acidic medium) due to the pore nature of bagasse, and physical adsorption has a significant role in HMs adsorption removal capacity. In addition, the main chemisorption adsorption mechanisms of bagasse are surface complexation, inner sphere complexation and electrostatic attraction (Kumar et al. 2014; Harripersadth et al. 2020).

Surface adsorption is a physical process that involves formation of covalent bonds with relatively weak forces through diffusion of metal ions into pores of the adsorbent surface (Madeła & Skuza 2021). The pore volume and surface area of adsorbent depends on methods of bagasse synthesis. Different literature reports acid treatment of bagasse for adsorbent shrinks the SCB cell wall, largely reducing the specific surface area and pore volume (Ighalo et al. 2022).

The mechanisms of metal complexation include the arrangement of multi-atom formation through the interaction of specific metal ligands to form a complex (Madeła & Skuza 2021). Sugarcane bagasse functional groups, which contain carbon–oxygen in their structure, such as phenolic (lignin), hemicellulose, and cellulose, bind with heavy metals. Thus, carbon–oxygen content can increase surface oxidation of the SCB leading to enhanced metal complexation (Ambaye et al. 2021). Consequently, heavy metal ions can be removed by inner-sphere complexation (by interaction with –OH) or by surface complexation (interaction with –COOH) of bagasse functional groups (Sarker et al. 2017).

Adsorption is a two-way process, where adsorption and desorption equilibrium occurs simultaneously. An adsorption isotherm is the equilibrium relationship between the concentration of adsorbate in solution and the adsorbate retained in the adsorbent at a given temperature (Iwuozor et al. 2021b). The adsorption isotherm is more concerned with how the adsorbate molecules are distributed between the liquid and solid phase when the sorption process attains equilibrium (Hashem et al. 2010; Abonyi et al. 2019; Al-Ghouti & Da'ana 2020). Analysis of isotherm data by fitting different isotherm models is a crucial step to find an appropriate model that represents adsorbate molecules at equilibrium. Most literature reports that Langmuir and Freundlich are the most appropriate isotherm models to describe heavy metal ion adsorption by bagasse (Ezeonuegbu et al. 2021; Iwuozor et al. 2021a). Hence, these isotherm models give detailed information about the nature of adsorption of HMs on bagasse surface. Adsorption could happen either through monolayer adsorption with affinities over homogeneous surface or multilayer adsorption with high affinities over heterogeneous surface. The Langmuir model assumes all forces that act on adsorption processes are similar to chemical reactions and there is no interaction among adsorbed species only between adsorbate and adsorbent (Langmuir 1918). In addition, the Langmuir isotherm confirms the positive interaction between the adsorbate and SCB, which is valid proof for an ion-exchange type adsorption mechanism (Çelebi et al. 2020; Shafiq et al. 2021). On the other hand, the Freundlich isotherm model assumes adsorption on a heterogeneous surface with various adsorption sites, which can hold more than one metal ion at a time; the strong binding sites are occupied first and the amount of adsorption is the summation on all surface sites (Freundlich 1907).

As most literature reports, the adsorption isotherm of SCB fits Langmuir and/or Freundlich, which refers to Langmuir at low concentration and Freundlich at high concentration of HMs (Ighalo et al. 2022). In fact, such empirical isotherm models may often not give insights on the mechanism of sorption when multiple models based on different assumed adsorption mechanisms can fit the same experimental data as shown in Table 5. Thus, the goodness of the fit alone cannot be used to conclude the superior mechanism of one model over the other. However, as presented in Table 5, the adsorption isotherm parameters of Langmuir such as separation factor (R_L), in the range of 0 < R_L < 1, shows favorability of the adsorption process, while R_L > 1 shows unfavorability of the Langmuir adsorption isotherm and R_L > 1 shows the physical nature of adsorption (Hashem et al. 2021; Ragadhita & Nandiyanto 2021). Sugarcane bagasse shows different adsorption isotherm models as shown in Table 3; this variation may be due to different concentrations of heavy metals, adsorbent synthesis and the nature of surface modification (Tejada-Tovar et al. 2020; Ezeonuegbu et al. 2021). Similarly, from the Freundlich isotherm, the slope was also reported as in the range of 0 < 1/n < 1 and shows normal adsorption process (Salman et al. 2016). In addition, most literature reported both the Langmuir parameter (R_L) and Freundlich isotherm parameter (1/n), showing favorability of Langmuir (R² = 96.9–99.4%) and Freundlich (R² = 95.6–99.9%) for the same adsorption data as shown in Table 5.

Table 5

Adsorption isotherm parameters of adsorption data

Adsorbent .	Pollutant heavy metals .	Isotherm parameters .					Reference .
		Langmuir (qe = ⁠) .		Freundlich (qe = KFCeq1/n) .
		RL .	R2 .	n .	.	R2 .
SCB	Pb2+	0.001146	0.9692	2.6462	0.378	0.991	Hashem et al. (2021)
SCB	Pb2+	1.035	0.9819	2.3050	0.434	0.9401	Ezeonuegbu et al. (2021)
SCB	Pb2+	1.604	0.974	6.410	0.156	0.964	Tejada-Tovar et al. (2020)
SCB	Cd2+	0.029	0.9904	1.34	0.746	0,9902	Harripersadth et al. (2020)
Citric acid modified SCB	Cr3+	–	0.994	1.580	0.633	0.965	dos Santos et al. (2019)
Acrylic modified SCB	Pb2+	0.07289	0.995	1.105	0.905	0.956	Kong et al. (2014)
Acid assisted pyrolyzed SCB	Pb2+	–	0.9006	1.257	0.795	0.999	Shah et al. (2018)

Adsorbent .	Pollutant heavy metals .	Isotherm parameters .					Reference .
		Langmuir (qe = ⁠) .		Freundlich (qe = KFCeq1/n) .
		RL .	R2 .	n .	.	R2 .
SCB	Pb2+	0.001146	0.9692	2.6462	0.378	0.991	Hashem et al. (2021)
SCB	Pb2+	1.035	0.9819	2.3050	0.434	0.9401	Ezeonuegbu et al. (2021)
SCB	Pb2+	1.604	0.974	6.410	0.156	0.964	Tejada-Tovar et al. (2020)
SCB	Cd2+	0.029	0.9904	1.34	0.746	0,9902	Harripersadth et al. (2020)
Citric acid modified SCB	Cr3+	–	0.994	1.580	0.633	0.965	dos Santos et al. (2019)
Acrylic modified SCB	Pb2+	0.07289	0.995	1.105	0.905	0.956	Kong et al. (2014)
Acid assisted pyrolyzed SCB	Pb2+	–	0.9006	1.257	0.795	0.999	Shah et al. (2018)

Langmuir parameter (K_L), adsorption capacity at equilibrium (q_e), equilibrium concentration (C_e), Freundlich constant (K_F), Freundlich adsorption isotherm slope (1/n), and linear regression coefficient (R²).

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Similarly, the Freundlich adsorption isotherm slope gives some hints that the isotherm process is more favorable in cooperative adsorption (physi-sorption and chemisorption) (Ragadhita & Nandiyanto 2021). The chemisorption occurs at the first layer occupation of all functional group active sites on the SCB surface (Chaiwon et al. 2017). After all chemically active sites of adsorbent are saturated with heavy metal, a multi-molecular layer of HMs is formed by physical adsorption such as pore filling, and intermolecular Van der Waals forces result in attraction on the SCB surface (Wang et al. 2017). Also, the equilibrium adsorption capacity (q_e) increases with increasing initial pollutants (HMs) which refers to multilayer adsorption (Freundlich) at high pollutants concertation.

In addition, cooperative adsorption is applicable to both specific and non-specific interactions between sorbate and interface (Shimizu & Matubayasi 2021). Furthermore, some literature reports have been supported with chemical instrumentation from scanning electron microscopy (SEM) analysis: the adsorbed heavy metals ions on the surface of bagasse are uniformly distributed on bagasse surface and interior of the pores (Akanni et al. 2019) and this may suggest chemical and physical adsorption mechanisms of bagasse for heavy metals adsorption removals.

Adsorption kinetics is one of the adsorption processes used to understand the rate at which metal ions are transferred from bulk solution to the adsorbent surface (dos Santos et al. 2019). In general, adsorption kinetics occurs through two steps. The first step assumes the transfer of the adsorbate from the bulky solution to surface of the adsorbent through the solid–fluid boundary layer known as film. Such surface diffusion adsorption kinetics is governed by hydrodynamic effects such as agitation speed and fluid flow rates. It is fast and results in chemical entrapment of HMs on the bagasse functional groups (Iwuozor et al. 2021a). However, the second step of adsorption kinetics assumes pore diffusion of the adsorbate into the porous adsorbent. In particular, such adsorption kinetics occurs at high concentration of pollutant dose after saturation equilibrium of chemically active sites. However, pseudo second order kinetics of initial rapid adsorption is due to the large binding sites of bagasse functional groups. In contrast, slow adsorption onto residual binding sites and diffusion of heavy metals into the porous surface follows pseudo first order kinetic models (Hassan et al. 2021). Most adsorption kinetics of modified and unmodified SCB for metal ion removal have been reported as pseudo first and pseudo second order kinetic models (Irawan et al. 2021; Iwuozor et al. 2021a). Pseudo first order kinetics assumes that the adsorption kinetics is directly proportional to the available number of unoccupied sites and governed by a physical adsorption process that is diffusion-controlled; this can occur at high concentration of pollutant dose after all the chemically active adsorption sites are occupied (Shah et al. 2019). However, in pseudo second order adsorption kinetics it is assumed that adsorption rate and reaction mechanism are controlled by chemical entrapment processes including ion-exchange between adsorbate and functional groups of adsorbent or inter-valence force (Liu et al. 2018; Irawan et al. 2021). The pseudo second order kinetics model may be the rate-dominating mechanism at appropriate ratios of SCB chemically active sites to pollutant heavy metals dose. Adsorption kinetics deals with the uptake rate of unmodified and/or modified bagasse with time and rate constant calculated with pseudo first and pseudo second order kinetics (Hashem et al. 2021) as shown in Table 6.

Table 6

Kinetic adsorption model equation and parameters

Model .	Equation .	Parameters .
Pseudo first order	Ln(qe − qt) = lnqe − kt	qe (mg/g): adsorption capacity at equilibrium
		qt (mg/g): adsorption capacity in a time t
		K1 (1/min): adsorption rate constant for pseudo first order
Pseudo second order		K2 (g−1min−1): adsorption rate constant for pseudo second order

Model .	Equation .	Parameters .
Pseudo first order	Ln(qe − qt) = lnqe − kt	qe (mg/g): adsorption capacity at equilibrium
		qt (mg/g): adsorption capacity in a time t
		K1 (1/min): adsorption rate constant for pseudo first order
Pseudo second order		K2 (g−1min−1): adsorption rate constant for pseudo second order

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The effective adsorbents should have good adsorption potentials for the removal of HMs and also a good recovery capacity of metal ions and reusability (Kołodynska et al. 2017; Ezeonuegbu et al. 2021).

Most literature reports show batch adsorption is a dominant process for SCB bio-adsorption; there are only limited reports on dynamic adsorption, column and pilot-scales. This review may provide some hints for the dynamic adsorption possibilities of SCB-based bio-adsorbents for practical uses. In addition, research on SCB-based adsorbent for removal of HMs has failed to report the effect of coexisting ions and a disposal strategy for used adsorbent for a truly sustainable approach. Moreover, the current literature lacks reports on the treatment cost analysis of SCB. Thus, a cost analysis should be undertaken to show the real cost effectiveness of SCB for initial and running costs for wastewater treatment. In addition, several researchers have tried to explain the HMs adsorption mechanisms of SCB. However, detailed understanding of SCB for HMs adsorption removal needs knowledge of molecular level study such as DFT of Gaussian view and/or other molecular level study such as Monte Carlo simulations.

Global water pollution is increasing alarmingly. Most sub-Saharan African countries are forced to live under economic water stress due to lack of finance and skilled staff to reuse the abundant renewable water resources. The agricultural bio-waste sugarcane bagasse is eco-friendly, locally available and has a large surface area, high porosity and carbon–oxygen containing functional groups that account for heavy metal adsorption capacity. The HMs adsorption mechanism of both modified and unmodified bagasse involves inner sphere complexation, surface adsorption, pore filling, electrostatic interaction, ion exchange and precipitation. This review paper has revealed that adsorbent bagasse showed great adsorption potential towards heavy metals with high electronegativity, hydrolysis constant, higher ionic radius, and larger softness. It was inferred that cooperative (physical and chemical) adsorption processes are appropriately represented by Langmuir and/or Freundlich adsorption isotherms for HMs adsorption removal over SCB-based bio-adsorbents. The kinetic adsorption equilibrium explained by pseudo-first and pseudo-second-order kinetic models depends on initial HMs concentrations. The short adsorption equilibrium nature of bagasse at low concentration of HMs follows pseudo second order kinetics with chemical adsorption and first order kinetics through pore filling and other physical adsorption processes at high HMs concentrations. The bio-adsorbent SCB-based adsorption process is promising in terms of economic viability and easy regeneration for practical applications in real wastewater in developing countries such as in sub-Saharan Africa.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.