Cactus material-based adsorbents for the removal of heavy metals and dyes: a review

Cactus is cultivated in many regions over the world. Because of its chemical composition and its valuable nutritional and biological characteristics, cactus finds applications in different sectors such as the pharmaceutical and the food industries. Interestingly, cactus materials (cladodes, fruit seeds, peel, etc) have been explored for their probable use as adsorbents for the removal of toxic heavy metals and dyes from wastewater. Various preparations methods were used to produce cactus material-based biosorbents. These biosrbents have been investigated and successfully used for the elimination of both heavy metal and dyes from aqueous solutions. Related results showed very promising pollutant removal efficiency associated with an interesting adsorption capacity similar to other materials from various origins. This paper explores various cactus biosorbents preparations. Furthermore, their efficiency in depollution and factors controlling the adsorption capacity will be discussed.


Introduction
Water pollution is an increasing environmental problem associated mainly to industrial development. For their processes, industries utilize large quantity of water generating contaminated effluents. Because freshwater resources are limited, the use of treated wastewater for industrial processing is gradually becoming a familiar practice worldwide (Vishali and Karthikeyan, 2015). The reuse of wastewater through the employment of various treatment methods is beneficial to resolve water shortage, to preserve the high-quality resources for potable use only and to decrease pollution of surface waters. Heavy metals and dyes are two examples of the largely widespread pollutants in industrial effluents that may have severe problems for the environment including the human health (Jaishankar et al 2014, Tchounwou et al 2012. Decontamination processes involved physical, biological and chemical techniques. The available technologies are classified into conventional methods (particularly activated sludge, coagulation-flocculation, chemical precipitation, adsorption and filtration), established recovery methods (ion exchange, oxidation, solvent extraction, electrochemical treatments, membrane bioreactors, etc) and emerging methods (advanced oxidation, biosorption, adsorption into non-conventional materials, nanofiltration, etc) (Crini and Lichtfouse 2019). Because of diverse factors related to economical and technological considerations, limit number of wastewater treatment processes is universally used by various industries to treat their effluents. Interestingly, the adsorption, which is considered as safe, clean, efficient and technical feasible process, is frequently employed to remove various pollutants such as heavy metals and dyes. Therefore, the adsorption method is an interesting separation technique using suitable material called absorbent characterized by high surface area and porosity and allowing rapid adsorption equilibrium kinetics (Crini and Badot 2010, Amari et al 2018). Various factors (high efficiency, ecofriendly, low cost, capacity to remove different inorganic and organic pollutants, absorbent material resistance to toxic substances, high effectiveness, simple design and easy operation) make this technology the most widespread method for water depollution (Farooq et al 2010, Salleh et al 2011, Bazrafshan et al 2016. The adsorption is a procedure based on pollutant transfer from aqueous solution to the adsorbent (Crini and Badot 2010). Using experimental data, equilibrium modelling, kinetics and thermodynamic factors should be studied to determine the suitability and the applicability of an adsorbent in removing pollutant (Douven et al 2015, Allen et al 2004). Generally, data of the equilibrium adsorption were analyzed using mainly the Langmuir, Freundlich, Temkin, Dubinin-Radushkevik and the Halsey isotherm models. However, kinetics results were described commonly via pseudo first order, pseudo second order, Elovich equation and intra particle diffusion models (Malkoc and Nuhoglu 2007, Mahamadi and Nharingo 2010, Nharingo and Hunga 2013. Activated carbon is mostly applied as adsorbent for pollutants removal from wastewater (Agrawal et al 2017). Nevertheless, carbon adsorption is very expensive and the process cost is related to the used material and its regeneration after exhausting (Li et al 2015). The loss of adsorption efficiency after regeneration, may limit its use. Consequently, the substitution of activated carbons with other useful material was studied by many investigators. Excellent alternative material of activated carbon should be available, inexpensive and chemically regenerable with efficient quantitative recovery. I this perspective, various green adsorbents materials were investigated (Kyzas andKostoglou 2014, Mahfoudhi andBoufi 2017). These new materials included natural materials, agricultural by-products and industrial wastes (Ali and Gupta 2006, Bhatnagar and Sillanpaa 2010, Sharma et al , Abdolalia et al 2014. Among the studied materials, cactus based materials reached large consideration in water treatment due to its abundance accessibility, biodegradability and safe behaviour. In this context, it was reported its successful use and its efficient removal rate of various pollutants (dyes, pesticides, turbidity, carbon, heavy metals, etc) by treated or untreated cactus materials. This paper will review the use of cactuses and their modified materials as adsorbents for the removal of toxic heavy metals and dyes.

Cactuses: origin and characteristics
Cactus is a member of the plant family Cactaceae originated from arid and semi-arid zones. The Cactaceae family consists of large number of genera and species, which were found in many regions over the world (South . Cactus parts mainly, cladodes, fruits, and flowers of different species have been well studied and characterized. Table 1 summarized the proximate composition protein, lipid, ash and carbohydrates), the major minerals and fatty acids content of cactus samples collected from various regions. Generally, cactus contains protein and lipid at low concentrations, however carbohydrates represent the major component for all samples (percentage on dry weight base ranged between 40 and 93%). According to Hernandez-Urbiola et al (2011), this amount increased with cactus age. Besides, cactus is a good source of minerals mainly Ca, K, Mg, Mn and Na which represent the major minerals. Concerning the lipid content, palmitic (C16:0), linoleic, linolenic and oleic were the main fatty acids recorded in samples. In addition to data presented in table 1, it was demonstrated by many researchers that cactus species contain various bioactive molecules with interesting nutritional and biological properties (antimicrobial, antioxidant, etc)  The data reported by the literature showed slight variation of cactus chemical composition depending on diverse factors (sample handling and preparation, species, location, age, etc). Because of their chemical compositions, cactuses were used mainly in food for humans and forgage for animals. Moreover, cactus could be used for wastewater treatment processes including the coagulant/flocculant, the adsorption, the biofiltration, sludge conditioning, etc (Ben Rebah and Siddeeg 2017).

Heavy metal removal
The majority of heavy metals are poisonous or carcinogenic even at low concentrations. Their presence represents an important environmental health problem. Exposition to heavy metals attacks the function of the central nervous and damages the blood content and many other organs such as lungs, liver, etc Moreover, exposure may cause other health problems such as muscular and neurological degeneration (Jaishankar et al 2014, Tchounwou et al 2012. Consequently, the elimination of metals, mainly from aqueous solutions via   (table 2). Generally, results varied depend mainly on the used preparation and the operating conditions (pH, temperature, heavy metal concentrations, biosorbent dosage and size, etc). In order to describe the adsorption equilibrium data and selecting optimum operating conditions, kinetic study is required. Generally, several models could be used to investigate the adsorption kinetics of heavy metals on cactus-based materials. The pseudo first-order kinetic model, the pseudo-second-order model, the intra-particle diffusion model and the Elovich model are the most commonly used to provide the mechanism involved in the sorption process ( . As indicated in table 2, these two models were the most adopted models for heavy metal removals by cactus-based materials. The Langmuir model assumes that the adsorption occurs on a specific homogeneous surface by monolayer adsorption. It is also considering that the coverage of adsorbate is of equal energy of adsorption on the surface of adsorbent. The binding sites have equal affinity and can be either chemical or physical. The Freundlich isotherm is an empirical model that assumes adsorption occurs on a heterogeneous surface as well as multilayer adsorption (Pelaez-Cid et al 2013).
As reported in table 2, both cladodes and fruit ectodermis of O. ficus-indica catus were evaluated as biosorbents (Fernandez-Lopez et al 2014). These materials were washed, cutted, dried (48 h at 60°C), crushed and sieved (<18 mesh) to be used to remove Cr(VI) from aqueous solution. A higher level of biosorption (>80%) was achieved at 1 g l −1 of biosorbent, pH 2 and at Cr(II) initial concentration of 2 mg/l. Fruit ectodermis and cladodes showed maximum adsorption round 5 mg g −1 . Similarly, dried and powdered cactus (O. ficus-indica) cladodes allowed high maximum adsorption ability of both Pb (98.62 mg g −1 obtained with a dosage of 10 mg l −1 and pH 3.5) and Cd (30.42 mg g −1 obtained with a dosage of 4 g l −1 and pH 3.5) (Barka et al 2013a). Dried cactus peels with different sizes (10-20 mm) were also tested for the removal of Mn. Interestingly, the highest removal level (36.02%) was obtained with 0.5 g of 10 mm particle size of cactus peels (Belayneh and Batu 2015).
The effect of the operating conditions including the contact time, adsorbent dose and temperature on the adsorption of Pb and Cd ions by cactus powder was performed by Derbe et al (2015). Generally, Pb and Cd removal rates increase by rising the contact time and adsorbent dose. For example, the highest rates were obtained after incubation time of 120 min (58% for Pb and 43% for Cd) at a dose of 1 g l −1 . This fact is related to the end point at which adsorption phase reached the equilibrium. However, the temperature affects negatively the removal capability of cactus powder and significantly decreases both Pd and Cd removals, which could be explained by the physicosorption process. Moreover, it was reported that NaCl interact with the functional group of cactus powder limiting the adsorption of heavy metals and consequently metal removal capability decreases by increasing the NaCl dose (Derbe et al 2015).
For many reasons (low cost, abundance, sustainability, reliability, renewable, environmental safety, etc), cactus materials ensure the environmental rules for the treatment of contaminated water with heavy metals. This ability involves chemisorption exchange between metallic ions and functional group including mainly carboxyl, carbonyl and hydroxyl groups present in cactus materials as demonstrated by spectroscopic studies. In some cases, cactus materials can be easily used as biosorbent without chemical addition. At the same time, various preparations using high temperature and chemicals were evaluated. The majority of experiments were conducted to remove heavy metal from aqueous solution. As far as we know, few data describing the employment of cactus based-biorsorbents for heavy metal elimination from real wastewaters was reported. For example, Fe and Cr were significantly reduced at acceptable levels while treating tannery wastewater using sundried cactus cladodes (Swathi et al 2014).
Generally, the results presented in this revue confirm that biosorbents obtained from cactus materials exhibit reasonable heavy metal adsorption capacity while compared to many other cheap materials (table 3). However, variations among results can be explained by the used biosorbent preparation processes and by the operating conditions (biosorbent dose, heavy metal initial concentration, pH, temperature, contact time, etc), which varied between experiments. Hence, is very important to point out the importance of the operating conditions, which should be optimized for each material used as biosorbent. Generally, pH is an important parameter that control the biosorption process (Barka et al 2011). According to the literature, there is no range or fixed value of pH at which the maximum adsorption capacity is attained, and this is related to the nature of adsorbent materials and the adsorbate. The difference in biosorption trend for the same pH range may be attributed to the differences in behaviour among metals and their ions in solution. Also, the pH affects the speciation of the metal (metal distribution, precipitation and complexation), its stability and the chemical state of its reactive groups (protonation/deprotonation) (Fernandez-Lopez et al 2014). Interestingly, the removal efficiency of metals is highly dependent on the quantity of the biosorbent. The initial dose is a key parameter to overcome mass transfer resistance between the aqueous and solid phases. Moreover, the removal rate increases with the increasing of adsorbent mass until an appropriate dose, and further increasing did not show any significant change on heavy metal removal. This due to the fact that active site of the biosorbent materials already occupied by adsorbent and the solution reaches equilibrium between the heavy metals and the used biosorbent materials (ALOthman et al (2013)). Also, the particle size of the materials influences slightly the biosorption process. The decrease in particle size increases the biosorption yield at equilibrium. Small particule size allowed higher biosorption capacity. This could be explained by the fact that smaller particles offer larger surface area of the biosorbent (Barka et al 2013b). The effect of the contact time was also investigated by allowing the solution to agitate for different periods. Sufficient contact time and stirring rate allow good mass transfer by minimizing the boundary layer width involving the adsorbate and the adsorbent (Barka et al 2011). Concerning the temperature effects, the decrease of the adsorption with temperature is due to the weak binding interaction between the active site of the used material and the metal ions which support physicosorption process (Derbe et al 2015). Furthermore, increasing the temperature could cause more pores expansion that can lead to leaching the heavy metal adsorbed (Uwah et al 2013).

Dye removal
Different industrial activities discharge large quantity of coloured effluents in the environment, which may cause health and ecological problems as reported above. Human health problems varied depending on dye nature, time contact and concentration. In the aquatic environment, dyes avoid light penetration decreasing the photosynthetic activities (Hassaan and El Nemr 2017). Generally, dyes such as azo dyes are found to be toxic for flora and fauna. They cause the decline of microorganisms in soil affecting the agricultural activities. Dyes were also recognized by their poisonous and mutagens effects on organisms. To reduce the harmful impact of water polluted with chemical colorants, various treatment processes were commonly applied by the industries. These treatments included the coagulation-flocculation, bioprocess, membrane filtration, advanced oxidation and adsorption (Venkatesh et al 2017, Robinson et al 2000. Recently, the adsorption technique is frequently used and many researches were done in order to select a new suitable material useful as absorbent. In this context, various preparations of cactus materials (fruit peel, mucilage and cladodes) were evaluated as biosorbents for decolorization. Interestingly, cactus based biosorbents exhibit very high maximum adsorption capacities when applied for the dyes removal from aqueous solution or from real wastewaters. Before use, cactus materials were subject to treatments including simple sun-dehydration, heat treatment and/or chemical treatments. As reported in table 4, sun-dried cactus cladodes were subject to dehydration at 60°C (for 24 h) before being used for decolorization of solutions containing Methylene Blue, Eriochrome Black T and Alizarin S. Depending on pH, the biosorption capacity reached 190 mg g −1 , 118 mg g −1 and 200 mg g −1 respectively for Methylene Blue (at pH basic), Eriochrome Black T (at pH acid) and Alizarin S (at pH acid) (Barka et al 2013b). Advantageous results were also obtained with activated carbon obtained using cladodes (activation with phosphoric acid at 450°C) while applied for the removal of Methylene blue and Iodine (Ouhammou et al 2019).
In addition to cladodes, cactus fruit peel was subject to various treatments. For example, fruit peels sample was boiled in distilled water, sun-dried (20 days), washed with bidistilled water, redried (40-50°C) and crushed (< 0.315 mm). The obtained powder showed high potential as biosorbent for Methylene blue, with sorption capacity of 222 mg/l (Seghier et al 2017b). Likewise, an efficient biosorbent was obtained by treating dried fruit peels with sulfuric acid (1 M) and sodium perchlorate (1 M). The allowed Brilliant Green adsorption capacity achieved 167 mg g −1 at 20°C and pH 3 (Kumar and Barakat 2013). Similarly, sodium hydroxide and sodium perchlorate was also used to treat separately other fruit peel samples. Interestingly, sodium perchlorate and sodium hydroxide enhanced significantly the removal rate (up to 96%) of basic dye and anionic dye, respectively . Using other preparation methods, granular activated carbon was prepared using cactus pear peels. In this process, cactus residue was sun-dried, cutted into strips and activated with phosphoric acid for 24. After carbonization (673 K, 3 h), the material was washed with water, dried (393 K, 38 h) and grounded. The obtained materials were applied to remove various dyes (Methylene Blue, Solophenyl Blue, Indigo Carmine and Crystal Violet) from water. Based on Langmuir isotherms, the obtained granular activated carbon with size (0.25-0.841 mm) reached adsorption capacity ranged of 284 mg g −1 (for Indigo carmine), 909 mg g −1 (for Solophenyl blue) and 416 mg g −1 (for Methylene blue.) These results were comparable to that obtained for white sapote seeds and broccoli stems (Pelaez-Cida et al 2016). Interestingly, these materials were also useful for textile wastewaters with removals rates (76%-90%) comparable to that obtained with commercial powdered carbon (Pelaez-Cid et al 2016). In the same context, a real tannery wastewater was used to perform the feasibility of dried cladodes adsorbent as treatment option. The cactus material was able to remove up to 70% of both COD and BOD, 90% of sulphate and 98% of iron. In addition to that, it was demonstrated that the biosorbent capacity tolerates pH ranged from 6 to 10 and increases with working temperature (Swathi et al 2014). More recently, dried and powdered biomass of Tacinga palmadora cactus showed decolorization rate of 93% (adsorption capacity of 228.74 mg g −1 ) from simulated textile wastewater loaded with crystal violet. This result was obtained with adsorbent dose of 0.5 g l −1 and at pH 10 (Georgin et al 2019). Nevertheless, is very important to point out the increase of dye sorption by the dosage allowing high number of reactive vacant sites, high transfer and high gradient concentration (Ghaedi et al 2015). Generally, cactus based-material offered an adsorption ability comparable to other biosorbents as summarized in table 5. This indicates that adsorbents prepared from cactus are useful for the removal of various dyes, with fast kinetics (Georgin et al 2019). However, decolourization rates are managed by various paramaters such preparation methods and the operating conditions (pH, temperatures, contact time, dosage, etc) as reported for heavy metals. However, the pH remains the most important factors affecting dye adsorption. Depending on dye nature, high pH is favorable for cationic dye adsorption and low pH is favorable for anionic dye adsorption. Moreover, the pH affects the surface behavior of the adsorbent (Salleh et al 2011). Consequently, it is important to optimize the pH value for each adsorption experiments.
As reported for heavy metals, functional groups of cactus are involved in the sorption process. Furthermore, chemical treatments permit the conversion of cactus functional group, which may enlarge the biosorbent specific surface area Barakat 2013, Pelaez-Cid et al 2013). Each activated cactus material is marked by its particular pore surface and its index of adsorption. The two properties varied depending on the activation methods and on the used biomass. Although the similarity in structure, rigidity and porosity, differences were observed while compared cactus to other materials (such as date, pumpkin seed shell, etc) (Li et al 2015).

Conclusion
This study summarized recent reports in which cactus materials (cladodes, fruit seeds, peel, etc) have been employed to produce efficient adsorbents. Cactus-based biosorbents were prepared using various methods including heat and chemical treatments. The obtained materials (untreated, sun-dried, thermally and chemically treated materials) were tested for decolorization and metal removal. The removal efficiency and biosorption capacity were controlled by various factors including the preparation methods, the pollutant subject to removal and the operating conditions (dosage, pH, temperature, contact time, etc). Based on the experimental data, promising removal efficiencies were observed for both heavy metals and dyes. Generally, kinetic results fit well with Langmuir isotherm. However, the majority of experiment was conduced for pollutants in aqueous solutions and only few data dealing with real wastewater were reported. Therefore, more researches are needed to evaluate the process efficiency using real wastewater at large scale. Moreover, in order to evaluate the competitive applicability of cactus materials, economical and environmental study should be addressed taking in consideration the adsorbents properties (alteration, bacterial degradation, regeneration, life cycle, etc) and the disposal of the generated wastes including the loaded pollutants and chemical related to the adsorption/desorption process.