A state-of-art review on the sustainable technologies for cadmium removal from wastewater

Heavy metals are unquestionably both transitional and post-transitional metals, lead, thallium, cadmium, and antimony are among the hazardous heavy metals that are frequently found in industrial settings and are major environmental polluters. Both natural and human-made processes are the sources of heavy metals in the surface environment. Parent rocks and metallic minerals are examples of natural sources. Agriculture (fertilizers, pesticides, etc.), metallurgy (mine, smelting, etc.), energy production (power plants, leaded gasoline, etc.), and sewage disposal are all examples of anthropogenic causes (Odika et al. 2020; Mitra et al. 2022).

Heavy metal-induced toxicity and related repercussions have grown to be a significant problem for human health where heavy metals in living systems enter the food chain and are not biodegradable. Increased concentrations and buildup of heavy metals can have extremely harmful effects where they have been reported to be highly carcinogenic, mutagenic, and teratogenic depending on the species, dose, and exposure time (Aendo et al. 2022; Dasharathy et al. 2022; Kumar et al. 2022). This is because they are extremely soluble in aquatic environments and so rapidly absorbed by living organisms through water consumption and domestic use. Workers in the industries, as well as individuals who live near an industrial region, are in danger of heavy metal exposure and contamination.

A high degree of toxicity causes harm to the renal, skeletal, and cardiovascular systems, affecting their physical development in the future. It may also cause malignancies of the kidneys, prostate, and stomach. Meanwhile, it might also affect children's intellectual development (Kinuthia et al. 2020). On the other hand, for aquatic plants and animals, the heavy metals establish oxidative stress which disrupts immune systems, damages tissues and organs, causes defects in growth and reduces the reproductive ability of the flora and fauna. As an example, humans consume fish as a major energy source as it is rich in vitamins and Omega-3 fatty acids. As a result, the heavy metals that previously existed in the fish are now transferred to the human bodies (Garai et al. 2021).

To preserve the health of humans and aquatic creatures, heavy metal concentrations in the environment must be regularly monitored. The United States Environmental Protection Agency (US EPA), the World Health Organization (WHO), and most countries' Departments of Environment (DOE) have issued allowable heavy metals limits for industries to follow (Kinuthia et al. 2020). The inorganic, organic, and toxic elements of industrial effluent require extensive treatment before disposal to avoid physical, chemical, and biological pollution of the limited water resources. Various treatment procedures are employed to improve the elimination of pollutants from wastewater. Table 1 shows different technologies such as coagulation and flocculation, adsorption, chemical precipitation, membrane filtration, and reverse osmosis (RO) membrane filtration have been employed to eliminate heavy metals from wastewater (Shrestha et al. 2021; Saleh et al. 2022). Every technique has pros and cons, and the decision is influenced by several elements like cadmium content, wastewater properties, financial constraints, and legal mandates.

Cadmium is corrosion-resistant, inflammable and insoluble in water but it can form cadmium oxide while burning in air, furthermore, it reacts with HCl, H₂SO₄ and HNO₃ then forms CdCl₂, CdSO₃, Cd(NO₃)_2, respectively (Genchi et al. 2020). Cadmium enters the environment through various mediums from different sources. The sources can be divided into natural and anthropogenic sources (Saini & Dhania 2020). Cadmium has been transported continuously among the main environmental components: soil, water, and air. The metal moves around by natural means such as natural phenomena and is commonly found in zinc ore as a byproduct. On the other hand, cadmium can be easily found in metal industries, smelters of nonferrous metals and electronic waste recycling centres. Since the twentieth century, there has been a rapid increase in the usage of cadmium as cadmium is one of the components required in the production of batteries, foods, soil, cigarette smoke and PVC products (Suhani et al. 2021). It also acts as a corrosive reagent, stabilizer, color pigment and neutron poison in nuclear fission to control neutron flux (Genchi et al. 2020). Besides, cadmium can also be detected in industrial waste, such as gas produced by the combustion of fossil fuels and municipal trash incineration.

Cadmium has adverse effects on animals and human beings upon consumption. Cadmium is a developmental toxicant in animals. A high concentration of cadmium in animal bodies can cause fetal malformations. In addition, cadmium is also toxic to human bodies which can lead to several respiratory diseases resulting in acute pulmonary irritation (Ferro et al. 2021). Some of them enter via the gastrointestinal tract and accumulate in the kidneys, liver, and gut (Ebrahimi et al. 2020). The effect of metal accumulation in human bodies can be severe and chronic, cadmium tends to accumulate in the kidney due to the tissues' ability to manufacture metallothioneins (MT), a substrate protein with a high affinity for cadmium ions. Metal ions are prevented from entering the cells through binding (Nordberg & Nordberg 2022). Furthermore, cadmium poisoning in humans causes bone illnesses such as osteoporosis and osteomalacia, renal and hepatic failure, pulmonary edema, testicular damage, and cancer (Genchi et al. 2020).

Cadmium causes apoptotic and necrotic processes, hence it has been confirmed to be a human carcinogen according to the International Agency for Research on Cancer's group 1 classification, and the EPA has categorized it as a potential human carcinogen in Group B1 (Mezynska & Brzóska 2018; Farimani Raad et al. 2021). Therefore, it is significant to include the process of heavy metals removal in water and wastewater treatment. According to the Environmental Quality Regulations 1979 and 2009, the acceptable conditions of cadmium concentrations in sewage and industrial effluents are 0.01 and 0.02 mg/L for standards A and B, respectively (Abdullateef et al. 2020).

The field of study devoted to the removal of cadmium from water bodies is not new. However, there is a need to keep updating the existing data and expanding the frontiers of knowledge, as the existence of cadmium pollutants in aquatic environments is still a global issue. The novelty of this work is to provide updated information on the utilization of different technologies for removing cadmium from wastewater, such as precipitation process, coagulation and flocculation, flotation, adsorption and filtration. The limitations and future prospects of technologies used for cadmium removal from wastewater are also discussed. The study's findings will advance wastewater treatment and improve clean water availability for many purposes.

To begin with, the ‘precipitates’ are often sparingly soluble and develop under relatively high supersaturation conditions (Rolf et al. 2022). Rapid precipitation is typically unaffected by the presence of solute crystalline material and so, does not necessitate secondary nucleation. It is caused by homogeneous or heterogeneous nucleation mechanisms. Second, because of the high supersaturation, nucleation plays a significant part in the precipitation processes. As a result, a significant number of tiny crystals are created. The particle concentration is between 10¹¹ and 10¹⁶ particles per cm³, while the crystal size ranges between 0.1 and 10 μm. Third, secondary processes such as Ostwald ripening and aggregation might happen because of the high particle concentration and small crystal size, drastically affecting the characteristics of the resultant precipitates (Janusz et al. 2022). As a result, colloidal systems must be developed to manage these secondary processes and obtain the desired precipitate quality. Fourth, the supersaturation required for precipitation is often caused by a chemical reaction; precipitation is sometimes referred to as reactive crystallization (Das et al. 2020). Chemical reactions can entail the mixing of concentrated chemical reactants quickly, and they are usually quite rapid. As a result, both macro- and micro-mixing play an essential role in precipitation processes.

Hydroxide precipitation is relatively simple, low cost, and easy to pH control; it is the most extensively employed chemical precipitation. The solubilities of different metal hydroxides decrease dramatically in the pH range of 8–11. A few studies have been seen to imply this hydroxide precipitation mechanism in cadmium removal. In a recent study, cadmium hydroxide precipitates fill the holes between Fe₃O₄ nanoparticles on the surface of the coal waste and settle down on the nanocomposite's surface in adsorption study (Mirshrkari et al. 2022).

However, hydroxide precipitation represents several including disruption of chelating agents, relatively high residual metal concentrations in wastewater (0.5–2 mg/L), and unsuitable reuse of sludge when many metals are present (Prokkola et al. 2020). Second, some metal hydroxides are amphoteric, and combined metals make hydroxide precipitation more difficult since the correct pH for one metal may cause another metal to dissolve back into the solution. Finally, the presence of complexing agents in the wastewater prevents the precipitation of metal hydroxide. The high amount of background chloride that acts as a complexing agent has been investigated in a theoretical and experimental manner and showed that hydroxide precipitation is inhibited due to exceeding the amount of chloride concentration ∼0.1 mol water (Stec et al. 2020).

Nowadays, microbially induced calcium carbonate precipitation (MICP) has drawn a lot of interest because of its potential in geotechnical and building applications. This method has been used to remove the ions of heavy metals from water including cadmium. The strength, durability, and capacity for self-healing of MICP products are frequently improved (Chuo et al. 2020). The MICP technique can be used to improve sustainability, particularly in the construction sector where a significant amount of the materials utilized are not sustainable. The MICP process occurs in the presence of bacteria where the formation of carbonate ions is facilitated by appropriate compounds, alters the microenvironment to facilitate the precipitation of calcium carbonate and serves as sites for the precipitation of calcium carbonate crystals.

Due to its high efficiency, reliability, and cost-effectiveness, various studies have been conducted on MICP and gradually received great attention (Konstantinou et al. 2021; Wang et al. 2023b). Cd-tolerant ureolytic bacterium DL-1 (Pseudochrobactrum sp.) and Brevundimonas diminuta uncovered the entire micro dynamic process of cadmium removal by MICP (Ali et al. 2022; Sheng et al. 2022). Nonetheless, MICP also represents several drawbacks such as the high pH value of the treated wastewater and the ammonia nitrogen produced by the MICP process (Song et al. 2022).

Compared to hydroxide precipitation, sulfide precipitation is a more effective alternative approach for the treatment of poisonous heavy metal ions (Esperi 2022). Metal sulfide precipitants also have better thickening and dewatering properties than metal hydroxide. Some studies reported that sulfide precipitation utilizing Na₂S is highly efficient for the removal of Cd, Zn, Cu, Pb, As and Se from intricate wastewaters. Sulfides are less soluble than metal hydroxides when used. A higher level of metal reduction can be accomplished more quickly in sulfide precipitation compared to hydroxide precipitation. The technique's drawbacks include the extremely low solubility of metal sulfides, sensitivity to precipitation agent dosage, and potential for harmful hydrogen sulfide emissions (Pohl 2020). The sulfide precipitation procedure to clean wastewater contaminated with cadmium is investigated in a study where cadmium sulfide precipitation was improved by a higher sulfur/Cd²⁺ molar ratio. The findings indicated that at 25 °C, a residence period of 40 min was required to eliminate Cd²⁺ by 85.6% (Kouzbour et al. 2022).

Co-precipitation or combined chemical treatment is the simultaneous precipitation of one component with one or more additional compounds to form mixed crystals. Co-precipitation is an increasingly essential technology for distributing ingredients and precursors utilized in a process to make a necessary substance. Co-precipitation is used to manufacture multicomponent materials by forming intermediate precipitates, mainly hydrous oxides, or oxalates so that an intimate combination of components forms during precipitation and chemical homogeneity is preserved during calcination (Bajaj & Joshi 2021). At suitable temperatures, aqueous metal salts are combined with a base, which functions as a precipitating agent, in the conventional co-precipitation process. The co-precipitation approach has the benefit of producing crystalline sizes in the tiny range when compared to other synthesis procedures, depending on the precipitating agent used during the reaction (Abid et al. 2022). Furthermore, by utilizing capping agents, the crystallite size and shape of the material generated using this process can be regulated. However, the co-precipitation process has significant drawbacks such as frequent washing, drying, and calcination to create a pure phase of phosphor (Patil et al. 2022).

The precipitation process is pH sensitive, and each metal has a maximum pH at which it will precipitate out of the solution. This drop was caused by the breakdown of heavy metal compounds in precipitates and the production of soluble metal hydroxyl complexes, which can increase metal solubility. The maximum removal effectiveness of mixed metals cannot be attained with a single precipitation pH. From the point of view of real treatment of wastewater loaded with a variety of heavy metals, it may necessitate many stages of precipitation at varying pH levels. Besides, sludge problems containing heavy metals from precipitation should be managed properly such as treated using cement-based solidification/stabilization before being disposed of in a landfill in industrial procedures, and it required strict leaching test results to meet landfill criteria.

As a conclusion, precipitation processes are crucial for the treatment of heavy metal-laden wastewater in industrial settings. While hydroxide precipitation is the most commonly used method due to its cost-effectiveness and simplicity, it has several limitations that may necessitate alternative methods like sulfide precipitation or innovative techniques like MICP. Co-precipitation offers advantages for producing specific material properties but requires meticulous processing. Each method has its specific applications, benefits, and challenges, often requiring tailored approaches to optimize heavy metal removal and manage resultant sludge.

Heavy metals are removed from wastewater via coagulation and flocculation, followed by sedimentation and filtering. Coagulation is the process of destabilizing colloids by removing the forces that hold them apart. When the precipitation decreases, coagulation can help to promote the separation of mud and water, as well as remove pollutants from the system and lower the turbidity of the solution (Cai et al. 2018). Many coagulants, such as aluminum, ferrous sulfate, and ferric chloride, are widely used in traditional wastewater treatment processes, resulting in the effective removal of wastewater particulates and impurities by charge neutralization of particles and enmeshment of the impurities on the formed amorphous metal hydroxide precipitates. Nonetheless, both flocculation and coagulation produce a considerable amount of sludge as waste, which is why it is considered an expensive method of purifying water. Electrocoagulation on the other hand combines the advantages of coagulation, flotation, and electrochemistry and shows a promising technology in cadmium removal wastewater (Alameen & Majeed 2020; Yang et al. 2020; Bajpai et al. 2022). The removal of Cd²⁺ ions from the water was accomplished using modified cellulose nanocrystals (CNC) made from sawdust with experimental absorption values of 2,207 mg Cd/g (Oyewo et al. 2019) and along with the removal of the cadmium by 79% using extremely active absorber coagulates/precipitates generated in FeOOH, 5Fe₂O₃·9H₂O, MnO₂, and Mn₃O₄ (Bora & Dutta 2019).

To further enhance the ability of electrocoagulation in the treatment of cadmium in wastewater, electrocoagulation is combined with other technologies such as other membrane techniques, oxidation or ozonation that are highly effective in removing various contaminants in a short time (Bilińska et al. 2019; Almukdad et al. 2021) or with more sustainable technologies such as photovoltaic or biological treatment.

Flotation has been widely utilized in wastewater treatment to remove heavy metals. The primary flotation procedures for removing metal ions from solution are foam flotation, dissolved air flotation (DAF), ion flotation, and precipitation flotation. Table 3 summarizes the different types of flotation methods for the removal of cadmium from water bodies. The introduction of a surfactant causes a non-surface-active material to become surface-active, resulting in a product that is removed by bubbling a gas through the bulk solution to produce foam. Adsorption occurs on a precipitate (coagulant) that functions as a carrier in the adsorbing colloid flotation process. The carrier is then floated, usually with an appropriate ‘collector’ surfactant accompanying it. Ion flotation is a potential technique for removing heavy metal ions from wastewater. The ion flotation technique is based on the idea of making the ionic metal species present in wastewater hydrophobic by using surfactants and then removing these hydrophobic species with air bubbles (Hoseinian et al. 2020).

Precipitate flotation is based on precipitate production and subsequent removal by attachment to air bubbles. However, a precipitation–flotation combination system offers the opportunity to execute both an effective removal of metal ions and the recovery of solids. To do this, surface charge, composition, and precipitate type must be considered (Serrano et al. 2021). Flotation has several advantages, including excellent metal selectivity, high removal efficiency, high overflow rates, short detention times, and the creation of concentrated sludge. There are some downsides as well, such as the high original cost as well as the high operations and maintenance expenditures. Many studies have been conducted utilizing flotation in cadmium removal from wastewater. The removal of Cd(II) by flotation using micro-bubbles and DAF was 98.44% (Pooja et al. 2021) and 97.39% (Abd Alhuseen & Abdulrazzaq 2023) effective, respectively. DAF has become an excellent choice for the clarification process, design criteria during pilot studies must be taken into high consideration to optimize flocculation mixing time, energy (velocity gradient), loading rate (gpm/ft2 or m/hr), and recycle rate for optimal downstream equipment performance in gravity media, pressure or membrane filters, and solids handling processes.

Traditional methods, such as coagulation and flocculation, use coagulants like aluminum but produce significant sludge and are costly. Electrocoagulation, combining multiple processes, is promising for cadmium removal, especially when paired with membrane techniques, oxidation, or sustainable methods. Flotation methods, including foam flotation and DAF, efficiently remove contaminants with surfactants and air bubbles, achieving up to 98.44% cadmium removal. In conclusion, while traditional methods are effective, they have drawbacks.

The water purification technology has been enhanced by the discovery of carbon nanotubes (CNTs) along with their adsorption properties. Because of their huge specific surface area, mechanical stability, and high pore size distribution, CNTs have been further studied for the removal of heavy metal ions from aqueous environments (Bankole et al. 2019; Khan et al. 2021). There are many pores and sites where CNTs can be used as an adsorbent for adsorption techniques, including the exterior surface of individual nanotubes, the exterior surface where two adjacent parallel tubes meet, the groove present on nanotube periphery bundles, the interstitial spaces between individual nanotubes in the bundles, and the cylindrical interior of individual nanotubes (Adil et al. 2020). There are several ways to synthesize CNT, the chemical vapour deposition (CVD) method produces CNTs with the largest aspect ratio, with the lowest integrated intensity ratio and diameter size variation, and their purity is remarkable (Manawi et al. 2018).

The adsorption capabilities and efficiency of cadmium ions are also affected by the kind of CNTs used, such as SWCNTs and multi-walled carbon nanotubes (MWCNTs). MWCNTs are made up of a concentric arrangement of numerous cylinders, while SWCNTs are made from a single rolled-up graphene sheet. Both are synthesized in the presence of metal catalysts. MWCNTs made from cobalt-ferrite catalyst on AC (from castor seed) were utilized to adsorb cadmium resulting in maximum adsorption capacities of 404.858 mg/g for Cd(II) (Obayomi et al. 2020). At low pH it showed a reduction in the adsorption capacity of these MWCNTs because of electrostatic repulsion, thus its maximum adsorption capacity was achieved at pH 8.

Recently, an interesting synergistic effect of microorganisms and CNTs has been explored. The pathogenic fungus Chrysosporium tropicum was safely loaded onto carbon CNTs in dead form to efficiently remove cadmium in wastewater (Naseri et al. 2023). This system showed a significant potential for application as a sorbent for preconcentration and detection of ultra-trace levels due to its high affinity for fast sorption (flow rates less than 7.0 mL/min) of the cadmium ion at pH 6.1.

Interestingly, although they have a higher surface area than their derivatives, the MWCNTs have a weaker adsorption potential than the modified or functionalized ones (Alimohammady et al. 2017) due to MWCNT's poor dispersibility, hydrophobic surface, lack of functional groups, and other factors. To enhance their adsorption characteristics, CNTs' surfaces must therefore undergo a covalent alteration. The adsorption capacity of CNTs can be enhanced after going through some modification on the surface which can be done in two ways: (i) chemical bond formation by physical adsorption, and (ii) chemical bond formation where the modification could be a benefit in the stabilization of dispersion in solvents and provides good solution stability (Petrushenko & Petrushenko 2019; Karimzadeh et al. 2021; Gao et al. 2023). Hence, resolving the instability problem of nanotubes in polar solvents prevents them from agglomerating in the solution. Various chemical groups could be modified into both SWCNTs and MWCNTs, but it depends on temperature, pH, the strength of bonds between ions, concentration of metal ions, and the number of modified CNTs.

To enhance the uptake of analytes from the solution, the surface of CNT can be easily modified by chemical (i.e. via the rich chemistry of OH and COOH groups put into the nanotube surface by oxidation) or physical functionalization. Modified CNTs were established by the reaction of saccharine (i.e. secondary amine by which its chemical structure is abundant in atoms that may remove heavy metal ions) and CNTs to produce carboxylated MWCNTs (Raoof Mahmood et al. 2019). Full removal of cadmium entails using pH 7 and can be reused up to 10 times at room temperature without needing to be washed frequently.

A more advanced method is utilizing magnetic CNTs as adsorbents benefiting through easy production methods, minimal matter loss, and exceptionally high adsorption effectiveness. Magnetic CNT modification with citric acid was conducted and showed no improvement in the removal of Cd(II) from wastewater compared to unmodified magnetic CNTs and pH 8.2 is favored by the system with full recovery (El-Sheikh et al. 2019). An affordable, ecologically friendly, and highly stable adsorbent for cadmium was produced by using aloe vera leaf powder-treated carboxylated CNTs using sulfuric acid and nitric acid, the maximal adsorption capacity reported was 46.95 mg/g (Mahmoodi et al. 2020). Magnetic MWCNTs covered with polythiophene (PT) were created as a magnetic sorbent and undergo modification with nitric acid to produce carboxylated magnetic MWCNTs (Dehghani et al. 2020) with adsorption capacity of 73.6 mg/g of Cd(II) observed.

Magnetic MWCNT functionalized with 3-mercaptopropyltrimethoxysilane was designed resulting in an analytical signal recovery rate (93.58–106.61%) in ten cycles, the nanocomposite's adsorptive capacity had not significantly decreased, ensuring stability and a high likelihood of its reuse in extraction procedures (dos Santos Morales et al. 2022). The removal of Cd(II) from industrial wastewater was accomplished using nickel nanoparticles (NiNPs) supported on activated MWCNTs with surface activation using potassium hydroxide as an adsorbent, which showed a promising reusability for eight cycles where the maximum adsorption capacity was 190.8 mg/g (Egbosiuba et al. 2022). Besides, 0.1 wt% of MWCNT were covalently linked to melamine-based polyamine polymers using alkyldiamines of 1,6-hexadiamine showing the polymer's high functionality allowed for the greatest amount of adsorption to occur compared to other polyamines (i.e. 1,10-diaminodecane, 1,12-diaminododecane and 1,8-diaminooctane) (Adelabu et al. 2020). Table 4 shows the adsorption properties of modified CNTs-based adsorbents used in cadmium removal.

In summary, carbon-based nanosorbents, especially CNTs, show great promise for wastewater treatment. Both SWCNTs and MWCNTs exhibit high adsorption capacities for cadmium due to their extensive surface areas and tunable properties. Modified CNTs enhance adsorption through functionalization, improving stability and efficiency. Although chemical modifications can significantly enhance adsorption, challenges like dispersibility and agglomeration remain. Overall, CNTs, particularly when functionalized, offer an effective solution for cadmium removal in wastewater, but optimization and stability improvements are necessary for practical applications.

Solvothermal, hydrothermal, dispersion, and co-precipitation approaches were the most widely employed techniques for creating graphene composites for the removal of cadmium. Graphene is superior to CNTs; however, it is consistently difficult to create promising graphene complexes via surface modification employing different functional groups, nanoparticles, or polymers. Conventional graphene-based adsorbents, however, have a few inherent disadvantages, including a predisposition for stacking caused by π–π interactions and Van der Waals forces (Kong et al. 2021). Increasing the quantity and variety of active adsorption sites is a straightforward way to improve mass transfer overall and raise practical removal efficiencies. To enhance the adsorption characteristics of GO and rGO on cadmium removal they are often modified using transition metals such (e.g. titanium, nickel and iron), organic materials (e.g. chitosan), inorganic materials (e.g. silica and montmorillonite) and polymers (e.g. poly(2-hydroxyethyl methacrylate and β-cyclodextrin).

Titanium dioxide (TiO₂)/rGO nanocomposites were used to adsorb cadmium, advantaging through TiO₂ particles that are better attached because of GO's wavy and wrinkled structure as well as other elements like oxygen-containing groups on the surface of the nanocomposites, led to increasing electrochemical performance and metal ion adsorption (Vajedi & Dehghani 2019). GO modified with nickel showed a great reusability in four successive runs indicating high efficiency of adsorbent to be recycled with a high maximum capacity of 725 mg/g (Shivangi et al. 2021)

Chitosan, owing to its large surface area and high adsorption capacity, suitable pore size and volume, large number of functional groups, mechanical stability, compatibility, flexible structure of the polymer chain, high chemical reactivity, ease of regeneration, cost-effectiveness, environmental friendliness, and simple processing is a suitable organic material that can be used in modification of graphene (Ilyas et al. 2022; Omer et al. 2022; Nordin et al. 2023a). A study showed that graphene modification with chitosan showed an adsorption capacity of 125.5 mg/g (Al-Salman et al. 2023) and 35 mg/g (Mallakpour & Khadem 2019) in two separate studies. The thermogravimetric analysis curve in the same study demonstrated that the stability of pristine graphenes (400) was more stable than oxidized (250) and functionalized graphenes (300) (Al-Salman et al. 2023).

Nitrogen-doped graphene quantum dot-chitosan (CCS/NGQD) modified and montmorillonite rGO showed good reusability with adsorption capacity remaining up to 80% to 86% for three cycles (Mallakpour & Khadem 2019) and 80.83% on the fourth cycle (Zhou et al. 2022), respectively. Other modified graphene by inorganic functionalized materials by montmorillonite (Zhou et al. 2022) and silica (Mahmoudi et al. 2020) effectively adsorbed cadmium with a capacity of 262.79 mg/mg and 43.45 mg/g, respectively. Another interesting modification of GO is by utilizing the wealth of functional groups on GO that can adhere to bacteria and increase their metabolic rates. Thus in a study, rGO was synthesized using Lysinibacillus sphaericus able to produce extracellular polymeric substances (EPSs) that can combine with the surface of graphene and further improve the adsorption capability. Additionally, EPS can increase microbial communities' tolerance to harmful metals by offering metal cations biosorption sites when complexation between metal ions and EPS functional groups takes place (Huang et al. 2022; Pagliaccia et al. 2022).

Finally, graphene modification using polymer shows promising adsorbents for removing heavy metals from water. β-cyclodextrin/GO demonstrated a high adsorption capacity of 117.50 mg/g (Rathour et al. 2019) and 196 mg/g (Samuel et al. 2020) in two different studies where the capacity to be restored and reused was shown in three and five cycles, respectively. Another polymer used in graphene modification was poly(2-hydroxyethyl methacrylate) resulting in 79% removal of cadmium (Salawudeen et al. 2020). Table 5 shows the adsorption properties of graphene-based adsorbents used in cadmium removal.

While designing a novel adsorbent for cadmium treatment, the researcher must bear in mind the cost-effectiveness of the materials used and the implications of when the large-scale applications are applied. CNT-based hybrid adsorbents were nearly four times the cost of natural adsorbents (Shahadat & Isamil 2018). A kilogram of pure solid GO and melamine sponge sheets cost around $72.87/kg and $2.91/1 m × 1 m × 2 cm, respectively (Xu et al. 2020).

Graphene, with its exceptional thermal and mechanical properties, is highly effective for cadmium removal from wastewater. Modified forms like GO and rGO enhance adsorption through various mechanisms, including electrostatic attraction and hydrogen bonding. Despite its potential, challenges like stacking due to π–π interactions hinder graphene's effectiveness. Modifications using metals, organic, and inorganic materials, as well as polymers, significantly improve adsorption capacity and reusability. Notable examples include TiO2/rGO, chitosan-GO, and β-cyclodextrin/GO composites. While these modifications enhance performance, further research is needed to optimize stability and practical application for large-scale wastewater treatment.

Natural minerals are abundant in industrial and municipal wastes. Hence, industrial, and municipal waste are also used as adsorbents to get rid of heavy metals in wastewater. This approach has overcome the waste disposal issues in the industrial sector by converting waste into something more beneficial. Chitosan is another type of biosorbent. It is a linear polysaccharide that is obtained from the hard outer skeleton of shellfish namely shrimp, lobster and crab (Nordin et al. 2023a). In a previous study chitosan is mixed with starch in a 1:1 ratio in the presence and absence of glutaraldehyde cross-linker; it appears to be a great absorbent of cadmium(II) ions (Ramasubramaniam et al. 2014). Chitosan oligosaccharide-glycidyl methacrylate/polypropylene glycol-glutaraldehyde blend also shows excellent adsorption of cadmium(II) (Radha et al. 2021). The study suggested a 2.92% (w/v) crosslinking degree able to achieve a maximum adsorption capacity of 99.87 (mg/g) at 55 °C (Babakhani & Sartaj 2020). For the performance of GO coated with chitosan on the adsorption of cadmium(II), the maximum adsorption achieved is 107.8 mg/g using Langmuir isotherm (Azizkhani et al. 2018).

Ferromanganese (Fe-Mn) nodules are another technique used in biosorption. Fe-Mn nodules are marine sedimentary mineral deposits, composed of iron and manganese oxides. The adsorption and desorption reactions are followed by electrochemically controlled redox processes at a constant cell voltage. According to the findings, the adsorption capacity rose as the voltage was increased and eventually reached equilibrium with increasing pH (Qiao et al. 2020). Aside from that, palm oil mill sludge biochar (POSB) pyrolysed at low temperatures is also used for cadmium elimination. POSB at 400 °C demonstrated the best performance of 48.8 mg/g uptake of cadmium (Goh et al. 2019). Meanwhile, biochar produced by co-pyrolysis sewage sludge with tea waste has the highest cadmium removal using 4 g/L adsorbent dose and pH 6.0.

The mechanisms involved during the adsorption include ion exchange, surface complexation, electrostatic interaction, and surface co-precipitation (Fan et al. 2018). Oily-sludge-derived char generated from 500 °C pyrolysis achieved the highest cadmium capacity at 23.10 mg/g according to the Langmuir model, it also shows that chemical characteristics namely alkaline minerals and effective cation exchange capacity are the control variables of the biosorption (Tian et al. 2020). Furthermore, mesoporous treated sewage sludge is one of the inexpensive adsorbents for cadmium removal. The kinetic study also showed the adsorption capacity of 56.2 mg/g which is well expressed by the pseudo-second-order kinetic model (Ahsaine et al. 2017).

Sludge from a modified drinking water treatment facility was used to create a floating adsorbent. It is effective in cadmium removal. The optimum cadmium load increased from 25 to 40.3 mg/g after the sludge was modified by phosphorus acid. Whereas, the alginate gel-capsulated sludge adsorbs a slightly greater amount of cadmium, 30 mg/g more than unmodified raw sludge (Siswoyo et al. 2019). Anaerobic granular sludge managed to remove 90.7% of 5 mg/L Cd(II) in the absence of Se (VI) and the entrapment can be enhanced using EPSs. Besides, cadmium also shows a positive outcome on removal efficiency with the dominance of the Methanosaeta genus (Zeng et al. 2019).

Olive stone is commonly subjected to alkaline pre-treatment to enhance its reactivity toward maleic anhydride. Then, the sodium form of maleated olive stone (MOS), NaMOS has maximum adsorption of 250.96 mg/g (Belalia et al. 2018). Furthermore, olive stones are rich in elemental carbon (40–45 wt%), therefore they are used to produce AC that has reported high adsorption capacities (Saleem et al. 2019). Bentonite is an expanding clay that forms when volcanic ash is weathered in seawater, converting volcanic glass in the ash to clay minerals. It comprises aluminum phyllosilicate minerals which are in the form of microscopic platy grains. The microstructure has provided a large surface area for adsorption to occur, making bentonite an excellent adsorbent. In recent studies, the modified bentonite has demonstrated high removal potential in removing inorganic pollutants in wastewater (Prabhu and Prabhu). Besides, Dolochar, a solid waste product of the sponge iron industry, was found to result in the optimized value of 1.85 mg/L exhaustion capacity in 11.39 h for Cd(II) adsorption with 3.48 m bed height, 76.31m³/day flow rate and 10 ppm inlet concentration (Upadhyay et al. 2021).

Aluminum dross is a hazardous solid waste synthesized in aluminum production industries which poses hazards to the environment and public health. It can, however, be used to remove cadmium from aqueous solutions. It has a reaction time of 2 h at pH 4–9 and 42 °C, indicating that aluminum dross is an excellent adsorbent for heavy metals removal (Mahinroosta & Allahverdi 2018). A study on the characteristics of granulated red mud coupled with cement at various mass fractions of 2–8% is also available. After being stored for 6 days, granulated red mud with a 2% cement fraction was found to have better textural properties (Ju et al. 2012). Other researchers used microwaved olive stone activated carbon (OSAC) to adsorb Cd(II). The optimal parameters for making OSAC in a microwave are 565 W radiation power, 7-min radiation period, and a 1.87 impregnation ratio. The removal rate was found to be as high as 95.32% (Alslaibi et al. 2014). Table 6 shows the summary of cadmium removal by natural minerals.

Natural minerals offer promising, cost-effective solutions for cadmium removal from wastewater. Chitosan, Fe-Mn nodules, biochar, and modified sludges demonstrate significant adsorption capacities through mechanisms like ion exchange and surface complexation. Innovations such as crosslinking and functionalization with organic and inorganic materials enhance their efficiency. Despite their potential, variability in adsorption performance due to differing material characteristics and treatment conditions presents a challenge.

Agricultural waste has emerged as the most frequently tested biosorbent as it is cost-effective. Agricultural wastes (Table 7) contain considerable levels of cellulose, hemicelluloses, lignin, lipids, proteins, starch, and other functional groups, making them appropriate for usage as raw materials to make AC or adsorbents that undergo surface modification (Nordin et al. 2023b). These adsorbents are inexpensive, and generally non-hazardous even undergoing modification and chemical utilization where they produce minimal environmental impact (Pyrzynska 2019).

Zizania caduciflora's root (ZCR), stem (ZCS), and leaf (ZCL) biochars were used and batch tests showed Cd(II) adsorption capabilities, ZCL (59.8 mg/g) > ZCS (40.6 mg/g) > ZCR (36.2 mg/g) (Wang et al. 2023a). A study on pristine, carbonized and nanocomposite of hemp seeds (CHS) on Cd(II) removal resulted in 20.19, 40.16, and 42.12 mg/g adsorption capacity, respectively (Shooto & Thabede 2022). Various functional groups on adsorbents such as –OH, –COOH, –C = O and –NH₂ could easily bind to the Cd(II) ions. Biochar prepared from banana stem and leaf showed adsorption mechanisms through complexation with oxygen-containing functional groups, ion exchange and mineral precipitation accounted for 0.4%, 6.3% and 83.0% to Cd²⁺ adsorption, respectively where the major contribution of adsorption mechanism is by mineral precipitation (Liu et al. 2022). Adsorbent derived from Moringa oleifera seed biomass produced 81.77% removal and 5.03 mg/g adsorption capacity best fitted with Langmuir isotherm (Nwagbara et al. 2022), while bamboo stem biomass-based adsorbent obeys Freundlich isotherm with 80.98% removal of cadmium (Akinyeye et al. 2020).

A few types of biochar have been investigated whereby sludge biochar made up of decomposed corn straw showed the greatest capacity for Cd(II) adsorption (72.2 mg g^–1) which was 1.5 times higher than sludge corn straw biochar and 3 times higher than maize straw biochar (Chen et al. 2023). According to the fixed bed column modeling, the percentage of cadmium removal is 90% using sugarcane bagasse (Vera et al. 2019). Magnetic biochar has achieved the removal of 4.77 mg/g and the optimal pH found is pH 4.5 (Yap et al. 2017). The pyrolysis temperatures of biochar made from various types of pine tree wastes were examined. The Cd adsorption characteristics of all PRBs were accurately characterized by pseudo-second-order and Langmuir isotherm models, with pine bark biochar having the highest maximum adsorption capacity of 85.8 mg/g (Park et al. 2019). Aside from that, maize cob biomass chemically modified with alumina nanoparticles can be used as a biosorbent from plant biomass. In batch mode, the adsorption yield of cadmium ions was 91% (Herrera-Barros et al. 2018).

Ulva fasciata, known as limu palahalaha or sea lettuce, is a green algae that can be consumed. The biomass of this green algae was found to be useful in cadmium removal in the research of (El-Naggar et al. 2018). According to the findings, 4 g of biomass was able to remove 99.96% of Cd from aqueous solution with an initial Cd concentration of 200 mg/L, pH 5.0, 25 °C, and a contact time of 60 min. The adsorption of cadmium by grape stalk waste was also investigated. The maximum grape stalk adsorption capacity was 0.248 mmol/g for Cd(II) at pH 5.5. On the other hand, carbonaceous adsorbents prepared from sunflower waste were used as biosorbents and achieved Freundlich adsorption capacities of 1.22 and 1.48 mg/g using Sunflower Head Carbon (SHC) and Sunflower Stem Carbon (SSC), respectively (Jain et al. 2015). Removal of Cd(II) from wastewater can be done by Luffa cylindrical. The biosorbent gave the highest adsorbent capacity at pH 6 which, according to Langmuir's values, was estimated at 5.46–7.29 mg/g from a temperature of 10–40 °C (Ad et al. 2015). Orange waste was studied for its usage in cadmium removal from wastewater. Moreover, its seed powder and Phyllanthus acidus seed powder are also found to be useful in removing cadmium from wastewater (Devi et al.). Table 5 shows the summary of cadmium removal by plant-derived adsorbents.

It is worth taking advantage of the high effectiveness of biochar as an adsorbent compared to AC due to the low cost of biochar, it is reported that AC (PAC, 1.8–2.1 USD/kg) is higher in cost compared to biochar (0.35–1.2 USD/kg) (Thompson et al. 2016). In contrast to AC, which ranges in price from $1100 to $1700 per ton, biochar-based products are an economical option that is often less expensive, ranging from $350 to $1200 per ton (Rehman et al. 2023). With this regard, agricultural waste-derived biochars and adsorbents effectively remove cadmium from wastewater due to their abundant functional groups. Studies highlight varying adsorption capacities, with sugarcane bagasse biochar achieving the highest (90.9 mg/g). Biochars generally exhibit superior cost-effectiveness compared to AC, with comparable or better adsorption efficiencies.

A new type of biochar with a high mineral/ash concentration especially calcium hydroxyapatite (HAP) concentration was created, called animal-derived biochar (ADB), using the harmless pyrolysis method of animal-derived biomass. In addition to resolving the issue of properly discarding dead animals, it also created novel materials that could effectively absorb heavy metals. ADB generally contains calcium (29%), phosphorus (16%), and carbon (7%), in contrast to high-carbon biochar (Lei et al. 2019). Because of the HAP hexagonal crystal structure with the space group P63/m (Nguyen et al. 2022), which consists of an adjustable framework perforated by channels and facilitates the transport of ions through the tunnels, HAP may easily absorb ionic replacements, such as Cd(II), Cu(II), and Zn(II). Therefore, by exchanging ions with Ca(II), these heavy metal cations can be integrated into HAP.

Rendering animal carcass residue char (RACR-C) consists of bone and protein created at a pyrolysis temperature of 500 K that led production of carbon- and inorganic-rich adsorbents for cadmium with adsorption capacity of 73.5 mg/g (Park et al. 2021). This system obeys Langmuir isotherm and involves various adsorption mechanisms (i.e. adsorption by functional group (C = C and C–O), precipitation of Cd-P and ion exchange reaction). Biochar made up of frozen cattle carcasses produced HPA-rich biochar coated with nanoscale iron hydroxide aggregates serves as an adsorbent for cadmium (Hong et al. 2022), demonstrating 1.55 mmol/g adsorption efficiency and promising functional material for heavy metals immobilization.

Humic acid from horse dung powder which is called horse dung humic acid (HD-HA) was used to adsorb Cd(II) from the solution and the experiment was conducted in batch mode at pH 5.0. The uptake capacity by HD-HA was found to be 1.329 and 26.46 mmol/g for monolayer and multilayer adsorptions respectively with the application of the Langmuir model as it was the best fit among all (Basuki et al.). It is a beneficially important choice to maximize the use of biomass where it will reduce the negative impacts of biomass management such as air pollution, it is highly associated with the consequence toward bioeconomy development. Thus, it is critically important to create and deploy comprehensive assessment frameworks for tracking improvements in resource use efficiency, which would allow comparing the efficiency of biomass. Table 8 shows the summary of cadmium removal by animal biomass.

Microbial biomass has developed as a cost-effective and environmentally friendly heavy metal removal option, including Cd as tabulated in Table 9. The source of biomaterial is an important aspect that must be considered. Biomaterial is made up of (a) microorganisms that are a byproduct of the fermentation industry; (b) organisms that are abundant in nature in large numbers; and (c) organisms that are created or propagated for biosorption applications using low-cost media. Two main mechanisms can be used to explain the heavy metal binding capacity of microbial biomass: (1) Surface adsorption is a metabolism-independent process in which heavy metal particles are adsorbed onto cellular surfaces, and (2) surface accumulation is an active process that involves metal particles entering and accumulating in cells during metabolism.

Nostoc muscorum is also a biosorbent for cadmium removal. 92% of Cd was removed within 24 h using the cyanobacterium at pH 8.0 and 25 ± 2 °C (Ahad et al. 2017). Weissella viridescens (lactic acid bacteria) due to its detoxifying abilities, ZY-6 was used to remove Cd²⁺ from an aqueous solution producing a removal efficiency of 69.45–79.91% (Li et al. 2021). The majority of Cd²⁺ particles interacted with the hydroxyl, amino, carboxyl, and phosphoric groups on the cell wall. After Cd²⁺ binding, the cells' surface shape changed during bioaccumulation. Penicillium chrysogenum was discovered to be cadmium resistant, with a maximal tolerance level of up to 1,000 mg/l and a cadmium removal capacity of 49 ± 0.8% (Din et al. 2021).

Significant increase in sequestration of Cd(II) by fungal biomass of Aspergillus spp. may be attributed to the removal of surface impurities, rupture of cell membranes and exposure to available binding sites during pretreatment with formaldehyde: biosorption capacity of A. clavatus (99.72%), A. oryzae (99.12%) and A. fumigatus (99.24%). The application of microbial biomass however must be administered carefully looking at the possibility of contamination toward the clean water and the likely health danger toward humans. Several pathogenic bacteria in wastewater can cause diseases like dysentery, typhoid, gastroenteritis, and cholera. With heavy metals accumulating at an alarmingly high pace in densely inhabited areas around the world, it is critical to bring current microbial bioremediation tactics up to date by developing speedier methodologies and ecological tools. In wastewater treatment, several modeling approaches have been presented to simulate microbial activities in anaerobic environments. In anaerobic conditions, the majority of metabolic reactions occur near their thermodynamic equilibrium. As a result, it is crucial to include the implications of thermodynamic factors in the rate expression as an indication of product inhibition, because the buildup of products in such systems might limit the overall reaction.

UF is a group of membrane filtration techniques that uses a permeable membrane to separate suspended particles, macromolecules, and heavy metals utilizing pressure or concentration gradients as the driving factors (Yang et al. 2022). To remove heavy metals from an aqueous solution, UF must be changed because its typical pore diameters of 5–20 nm are bigger than dissolved metal ions, whether they take the form of hydrated ions or complexes with common ligands (Cao et al. 2020). There are two basic methods, polymer-enhanced ultrafiltration (PEUF) and micellar-enhanced UF (MEUF), for overcoming the limitations of removing metal ions using UF membranes.

Cd MEUF removal efficiency is determined by the (i) properties and concentration of surfactants, (ii) solution pH, and (iii) parameters related to membrane operations. However, the high surfactant consumption in MEUF (often over CMC) leads to secondary contamination in the permeate solution in addition to high treatment costs (Laksita & Aryanti Majewska-Nowak & Górna 2023). While MEUF is still being developed for wastewater treatment, this pressing issue needs to be addressed.

Membrane approaches appear to be very promising, however, they are coupled with significant problems, such as fouling (Mashhadikhan et al. 2022). Thus, a study has been conducted to attain a better understanding of the relationship between the rejection mechanism of metal ions and the fouling layer created by SDS adsorption and deposition on the membrane through the removal of metal ions during MEUF at low surfactant concentration levels. Sodium dodecyl sulfate (SDS) and cadmium solutions are ultrafiltered utilizing UF membranes under conditions of low SDS content where Cd rejection is connected to the generated adsorption layer and cake layer (Huang et al. 2019a). By foam fractionation with mixed anionic–nonionic surfactant, Cd(II) and surfactant recovery in permeate from MEUF were examined (Huang et al. 2019b). The findings demonstrated that a small amount of nonionic surfactant can significantly increase the removal percentage of Cd(II) by improving the foam properties of anionic surfactant, while an excessive amount of nonionic surfactant may decrease the removal percentage due to its detrimental impact on Cd(II) adsorption at the gas–liquid interface.

In wastewater treatment, both NF and RO are increasingly adopted because they are proven to have strong retention abilities for ion removal. A NF membrane is a pressure-driven separation method used in several industries for desalination, water treatment, and waste effluent treatment. The distinguishing qualities of RO and UF membranes are like those of NF membranes. The NF membrane is more dependable due to characteristics like a higher flux, a low rejection of monovalent ions and a high rejection of divalent ions. The exceptional hydrophilicity of positively charged NF membranes causes them to strongly reject multivalent cations. Important elements in the removal of heavy metals are the membrane charge and the transport mechanism (Jakhar et al. 2023).

Several studies have been conducted utilizing NF and RO technologies in cadmium removal. The surface-modified mesoporous silica nanoparticles/polysulfone (MSNs/PS) NF are very efficient in removing heavy metal ions from aqueous solutions because of the functional groups on the surface where it produced remarkable removal rates of Cd²⁺ by 82% (Alotaibi et al. 2023). Due to the phytic acid's strong negative charges, the resulting NF membrane has an extremely high binding affinity for heavy metal cations including Cd²⁺ where it removed Cd²⁺ record-breaking rates by 100.000% (Zhang et al. 2021). Besides, this NF membrane can be easily regenerated and reused without noticeably losing its initial Cd²⁺ removal rate even after six filtration cycles. The 1,2,3,4-cyclobutane tetracarboxylic acid chloride (BTC) monomer has been used to create a positively charged aliphatic polyamide NF membrane (PEI-BTC) and demonstrated good removal efficiency Cd (90.49%) at 1,000 ppm (Li et al. 2022).

Membrane technology is gradually transforming water and wastewater treatment. The area has seen a lot of work over the years. There is certainly opportunity for development in a variety of areas. Fouling and high-energy demand continue to be key issues in non-equilibrium pressure-driven systems, necessitating ongoing research to find a long-term solution, either through the implementation of stringent yet inexpensive pre-treatment techniques or the creation of fouling-resistant membranes. Continuous research is required in membrane distillation to thoroughly comprehend the notion of temperature polarization, and designing appropriate membranes will assist in making the procedure more viable for large-scale use.

Conventional methods as discussed have a few drawbacks, e.g., (i) costly maintenance, (ii) not eco-friendly, (iii) pollutants production for secondary treatment. Bioremediation is one of the eco-friendly ways to meet the need for innovative, low cost and sustainable methods scientists have been looking for in the last few years. Many bacteria species can resist a high concentration of cadmium with different mechanisms, e.g. cadmium accumulation, enzymatic detoxification, active efflux of cadmium and cadmium ions sequestration (Domen 2023; Mallick & Das 2023). These bacteria can be categorized into three different levels. Bacteria that can efflux cadmium from their cells are categorized as the main group. In an efflux mechanism, CadA and CadB gene systems are involved in encoding different efflux pump proteins (Tayang & Songachan 2021).

Microbial intervention is through bioaccumulation which directs deposition of cadmium ions into bacterial, extracellular binding of cadmium ions to functional groups (e.g. phosphate, carboxyl and amine) on the surface of the bacterial and enzymatic detoxification of active cadmium ion to inactive cadmium ion. Besides, efflux transportation through efflux transporter (e.g. CadCA protein) is facilitated by the conversion of ATP to ADP. Finally, intracellular sequestration where Smt metallothionein locus on bacterial DNA transcript and translate metallothionein (MT), which binds cadmium ion and provides protection to the bacterial cell against harmful effects of these ions.

Bioelectrochemical systems are new revolutionary bioremediation technologies in microorganisms or enzymes that are integrated with traditional electrochemical methods and can be applied to metals removal in wastewater (Zheng et al. 2020). This system can deal with the treatment of organic wastewater, at the same time offering metals recovery. The study has been done to find out the cadmium(II)/cadmium(0) couple's standard redox potential is at −0.40 V. Researchers have completed the removal of several metal ions including cadmium ions, however more research is needed to understand its mechanism to fully exploit its selective metal recovery. However, it is most likely not suitable for domestic wastewater treatment as normally the concentration of metals is too low for the system (Abbas et al. 2018).

In order to fully exploit the potential of technologies in cadmium wastewater treatment, it is crucial to address and take into consideration the future recommendations as follows.

• (I) Settling the issue of membrane fouling by pretreatment as the initial treatment of wastewater before the use of membrane separation techniques is needed. Feed pretreatment is critical to the effectiveness of the membrane process. Pretreatments not only minimize membrane fouling but also improve energy utilization. Technically, pretreatments change the physical, chemical, or biological qualities of wastewater to make membrane separation more effective. The development of fouling-resistant membranes is seen to be an effective way to extend the capability of this system. New membrane materials, such as PVDF-HFP and PVDF-TFE, have been developed for increased efficiency.

• (II) Critical issues that require addressing are production costs and the demand for new technology and equipment especially in poor countries where water pollution is extremely high. As a result, it is critical to research simple ways of producing and recycling adsorbents that use less energy and lower-cost feedstocks and do not require complicated infrastructure.

• (III) Adsorption capacity can be measured on actual wastewater to show how different contaminants and salt concentrations affect it. For example, actual wastewater should be used for research rather than wastewater that has been artificially treated to contain heavy metals. Multi-metal system testing can mimic actual conditions seen in industrial effluent.

• (IV) Root-adherent microbes and biochar have shown promise as a unique strategy for heavy metal and pollution cleanup, with enhanced removal rates. Biochar and microbial strains have been proven to have greater removal efficacy than either component alone. To reduce secondary contamination, microorganisms immobilized on biochar can increase heavy metal immobilization while decreasing bioavailability.

• (V) The further development of new biosorption technologies based on immobilized algae would necessitate extensive life cycle research to determine environmental implications, and field-scale investigation of algal immobilization could considerably advance the subject and provide techno-economic insights.

• (VI) Artificial intelligence (AI) solutions have enormous potential in wastewater treatment due to the availability of easily available data, sophisticated biological processes, and a myriad of assets that may be optimized. AI's applications in wastewater treatment are numerous and provide considerable benefits in terms of process optimization, energy economy, predictive maintenance, and water quality monitoring. By leveraging AI, wastewater treatment plants can operate more efficiently, cost-effectively, and sustainably while adhering to environmental requirements and most importantly seeking the most effective treatment for cadmium removal.

This review shows the potential of a wide range of treatments for cadmium in wastewater. When comparing the precipitation method, each method possesses benefits and limitations. The most anticipated study in terms of precipitation treatment is using MICP, one of the novel methods in carbonate precipitation. It provides a potential sustainable technique for cadmium removal in wastewater treatment. However, it is still in its early stages and lacks large-scale studies, but it could provide a remedy that is more environmentally friendly compared to hydroxide, sulfide precipitation and co-precipitation that often requires frequent washing. When comparing different techniques utilizing carbon-based nanoadsorbent, the maximum adsorption capacity produced by MWCNTs made from cobalt-ferrite catalyst on AC was 404.858 mg/g. It surpasses all the maximum adsorption capacity of modified CNTs (46.95–190.80 mg/g) but is lower compared to GO modified with nickel 725 mg/g. This concludes that the efficiency of carbon-based nanoadsorbents is different depending on the materials and modifications. Biosorption using natural minerals, plant-derived adsorbents, animal biomass and microbial biomass shows a high utilization with most studies reported for cadmium removal in wastewater due to the low cost and easily available materials. The transition from a fossil-based to a bio-based economy has become increasingly important in recent decades. Bioeconomy development has received significant policy backing and has become a focus of research. Future research is anticipated for the betterment of these techniques and to find the solution in providing treatment of wastewater on a greater scale while taking into consideration the cost-effectiveness of each of the methods.

The authors wish to thank Universiti Teknologi Malaysia for their financial support toward the project titled ‘The influence of chemical and fibre traits of Malaysian bamboos on their pulp and paper characteristics’, under grant number PY/2022/02318— Q.J130000.3851.21H99. The research has been carried out under the program Research Excellence Consortium (JPT (BPKI) 1000/016/018/25 (57)) provided by the Ministry of Higher Education Malaysia (MOHE). The authors also acknowledge the financial support funded by Kurita Water and Environment Foundation through Kurita Overseas Research Grant 2023 (23Pmy220 and PY/2023/02465). The authors would like to express gratitude for the financial support from the Universiti Pertahanan Nasional Malaysia (UPNM).

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

The authors declare there is no conflict.