A review: a comparison of different adsorbents for removal of Cr (VI), Cd (II) and Ni (II)

A review of the studies dealing with the removal of chromium, cadmium, and nickel ions with different adsorbents published in the literature between 2014 and 2018 is given in tabular form, along with the adsorption conditions, adsorption isotherm, and kinetic models applied by the authors to model the experimental data and adsorption capacities. The review focuses on the efficiency of ion removal.

solutions, leather tanning, pigment, and wood preservatives; it can be used as a corrosion inhibitor and as a catalyst. It has many oxidation states, but the most toxic form is hexavalent chromium. It causes severe health problems, especially due to its oxidating properties and its high solubility in water, which makes it available for biological uptake [14,15].
Cr (VI) likes to bond with hydroxyl groups; in most studies, the adsorption of Cr (VI) was successful when pH value was rather low, especially if pH value was below pHpzc. With a pH value below pHpzc, Cr (VI) is pulled toward the negatively charged surface of the adsorbent. If the pH value is above pHpzc, chromium ions are repelled by the adsorbent surface and adsorption is decreased. The adsorption selectivity at high and low pH values can be explained by different forms of chromium in the aqueous environment. At pH values below 1, chromium exists as H 2 CrO 4 . When the pH value is between 2 and 6.0, the dominant species HCrO − 4 and Cr 2 O 2− 7 form; with pH values above 6.0, CrO 2− 4 is dominant, which is harder to bind [10,15].
Most of the reviewed experiments were conducted at pH value 3. In some studies, experiments were performed at higher pH values primarily to prevent the adsorbent from dissolving. Higher pH values (above 6) will also cause chromium precipitation.
By increasing the initial Cr (VI) concentration and maintaining the adsorbent dose, the percentage of adsorbed Cr (VI) may be lower because the number of active sites is limited; when they are occupied, adsorption is complete. In addition, the reason for lower adsorption may be found in the formation of a film around the adsorbent, which prevents the adsorbate from accessing the surface. At the same time, the adsorbent capacity may be higher, because the higher initial concentration provides the necessary driving force to overcome liquid resistance [8,16].
Depending on the adsorbent, a higher adsorbent dose will increase adsorption because of the higher number of active sites. If the adsorption is slower, the main cause is the aggregation of adsorbent particles, which resultes in fewer active sites [8].
The overview of the adsorption studies of chromium (VI) is summarized in Table 1.
A review of studies with synthetic adsorbents showed that 100% chromium removal was achieved with the use of magnetic multiwall carbon nanotubes (MWCNT) [17] and nanomagnetite particles, which were synthesized in a low-pressure procedure [18]. The use of BaFe 12 O 19 magnetic nano powder [19] and ionic liquid functionalized oxidized multiwall carbon nanotubes (IL-oxi-MWCNT) resulted in 99.5% removal [20].
The maximum adsorbent capacity of 293.3 mg/g was determined with the use of a magnetic composite of reduced graphene oxide, polypirrole, and Fe 3 O 4 nanoparticles (Ppy-Fe 3 O 4 /rGO) [10]. The surface area of Ppy-Fe 3 O 4 /rGO was 80 m 2 /g. In the same study, the comparison was made with Fe 3 O 4 /rGO. The adsorption capacity of Ppy-Fe 3 O 4 /rGO was higher, even though the surface area of Fe 3 O 4 /rGO was 126.42 m 2 /g. It was established that the surface area does not represent a decisive parameter in the adsorption process. Ppy-Fe 3 O 4 /rGO showed excellent adsorption properties. Chain-like Ppy forms a 3-dimensional network with Fe 3 O 4 nanoparticles and with rGO sheets. The adsorption process happens because of electrostatic attraction, ion exchange, and chemical reduction of Cr (VI) to Cr (III) [10]. Natural organic/inorganic adsorbents  Similar adsorption capacities of 236.9 mg/g [15] and 232.51 mg/g [2] were determined with the use of ethylenediamine-functionalized magnetic polymer microspheres (EDA-MPMs) [15] and amino-functionalized , respectively. By increasing the temperature to 318 K, the EDA-MPMs achieved a maximum adsorption capacity of 253.2 mg/g. Synthesis of EDA-MPM's adsorbent starts in the presence of Fe 3 O 4 nanoparticles. After polymerization, the particles are modified with ethylenediamine [15]. Through synthesis of Fe 3 O 4 -NH 2 , amino groups were fixed to the surface of Fe 3 O 4 nuclei and a coreshell structure was created. The core-shell structure is considered to have unique properties such as stability and chemical capability, which are important properties of the adsorbent [2]. Both adsorption procedures were conducted in an acidic environment and both adsorbents had similar chemistry behind their successful adsorption capacity. Because of the acidic conditions in which both experiments were conducted, the amino groups were protonated to NH 3+ , resulting in an even stronger electrostatic attraction between NH 3+ and Cr (VI) [2,15]. In the natural organic and inorganic adsorbent section, Cr (VI) removal of 100% [21], 99.08% [22], and 99% [23] was achieved with the use of microporous activated carbon from almond shell [21], activated carbon from fox shell (FNAC) [22], and with modified activated carbon [23]. Among the 3 adsorbents, microporous activated carbon had the highest maximum capacity of 195.34 mg/g [21].
Astonishing results were achieved with the graphene-sand composite (GSC) [24], where the maximum adsorption capacity calculated with the Langmuir model was 2859.38 mg/g. In the study, the high adsorption capacity was attributed to the graphene sheets on the sand surface used for preparation of the composite [24].
For the adsorbents mentioned above with the highest adsorption capacities, we found that the researchers proposed a similar binding mechanism for chromium adsorption. Adsorption is more successful at low pH values, from pH 1 to 4. In this case, there are many H + ions in the solution that protonate the surface of the adsorbent.
At low pH levels, the chromium in the solution is in the form of HCrO − 4 and Cr 2 O 2− 7 and the attraction between the ionic species and the surface of the adsorbent is large; thus, the adsorption is high. As the pH increases, the adsorption capacity drops as the charge on the surface of the adsorbent changes, and chromium species, which are more difficult to bind, form in the solution.

Cadmium
It is estimated that only 0.1 ppm of cadmium is present in Earth's crust. It is usually found near sphalerite as a mineral, greenockite. Cadmium or its compounds are mainly used in battery manufacturing, electroplating, pigments, metal finishing, fertilizers, tanneries, and plastic manufacturing [11,25].
Cadmium, which occurs in industrial effluents, usually accumulates in soil. Soil contamination is also accelerated by the use of artificial phosphate fertilizers. Due to soil contamination, the most common cadmium exposure is through food intake. Inhalation of fine cadmium powder or vapor and ingestion of its soluble compounds is hazardous. Cadmium is toxic and carcinogenic even at low concentrations [11,25].
Cadmium exists in several different states, at pH levels below 7 as Cd 2+ , between pH 7 and 11 as Cd(OH) + , and above pH 11 as Cd(OH) 2 . Low pH values cause low adsorption of cadmium. If a higher adsorbent dose is added, the adsorption is enhanced because of the higher number of active sites. When the maximum dose of adsorbent is reached, the capacity starts to decrease again. Furthermore, it was observed that temperature also has an impact on adsorption; at higher temperatures, the adsorption of cadmium is reduced. Because copper has similar chemistry to cadmium, the adsorption capacity of cadmium decreased in the presence of copper [11].
Adsorption capacities of different adsorbents for cadmium removal are shown in Table 2. Natural organic/inorganic adsorbents   S M -specific surface area in a unit of mass, D50 -average particle size, D The best cadmium removal was achieved with the use of manganese oxide, with 98% cadmium removal [26]. Experimental data showed that the adsorption capacity of manganese oxide was 82.7 mg/g, and the maximum adsorption capacity calculated with the Langmuir model was 104.17 mg/g [26]. Functionalized multiwall carbon nanotubes (FMWCNT) [26] and copper oxide nanoparticles (CuO NP) [3] exhibited similar adsorption capacities, which were 83.33 mg/g [27] and 84.75 mg/g [3], respectively. Both adsorbents had similar surface areas of 206.45 m 2 /g for FMWCNT [27] and 220 m 2 /g for CuO NP [3].
With natural organic and inorganic adsorbents, the best experimental results were reached with chitosaniron oxide nanocomposite (CH-FeO nanocomposite), where the adsorption capacity of 201.84 mg/g was achieved experimentally [28]. This adsorbent was also effective in real wastewater samples, where 99.91% of cadmium was removed [28]. Chitosan consists of amino and hydroxyl groups that can bind heavy metals and form chelates.
Chitosan itself is soluble under acidic conditions; with chitosan modification, a wider pH range can be obtained [29]. Furthermore, with modification procedures, adsorption properties and selectivity can be enhanced. The maximum adsorption capacity achieved with modified chitosan was 405 mg/g [30]. This value was attained with chitosan grafted with itaconic acid and crosslinked with glutaraldehyde (CS-g-IA(G). The results are not presented in this study, as Kyzas and Bikiaris have already conducted a detailed analysis in their review [30].
A maximum adsorption capacity of 143.4 mg/g was calculated for nitrilotriacetic acid anhydride-modified lignocellulosic material (NTAA-LCM) [31]. Experimental data showed that cadmium ions reacted with the carboxyl groups of adsorbents. It was established that sodium was exchanged with cadmium after adsorption took place. The adsorption process of cadmium onto NTAA-LCM occurred through surface chelation and ion exchange [31].
In general, it can be concluded that cadmium binding is not very successful at low pH values. The reason is that there are many H + ions present in the aqueous solution which compete with Cd 2+ for binding to the surface of the adsorbent. The adsorption of cadmium increases with increasing pH (pH from 4 to 8.5) as the surface of the adsorbent deprotonates and begins to attract cadmium ions. In accelerated adsorption, a mechanism of cation exchange has been shown to occur in several cases.

Nickel
It is estimated that around 10% of the earth's core is made of nickel. Much nickel is dissolved in water and accumulates in coal and oil. It occurs in millerite with sulphur, and in niccolite with arsenic. It can also be found in ores such as pentlandite. Nickel is a good conductor of heat and electricity. It is mainly used for alloys, as an additive for increasing the hardness of stainless steel, in the production of batteries, porcelain enameling, electroplating, dyeing, and pigments. A large amount of nickel is used in galvanization procedures because of its stability when in contact with atmospheric air [32].
Nickel only occurs in nature in low concentration levels. Nickel began to accumulate in the soil due to industry. Nickel present in soil can adsorb in sediments and become immobilized. In acidic soil nickel becomes mobile, which can result in it dissolving into groundwater. If groundwater is contaminated, exposure can happen with drinking. In addition, plants are subjected to nickel accumulation, exposing animals and humans to nickel through ingestion. It is an essential element in small quantities, but in high quantities, it can cause numerous negative consequences, such as lung, nose, or larynx cancer, lung embolism, asthma, chronic bronchitis, heart disorders, etc. [32,33]. Table 3 summarizes the studies on nickel adsorption. A review of synthetic adsorbents showed that the best nickel adsorption and maximum adsorbent capacity were achieved with amino-functionalized magnetic nanoadsorbent (Fe 3 O 4 -NH 2 ) [2]. The percentage of nickel removed was above 96%, and the maximum capacity was 222.12 mg/g. In the study conducted with this adsorbent, removal of Cr (VI) was examined; high capacities were reached for Cr (VI) removal as well. In both cases, amino groups played a decisive role. The study of kinetic parameters confirmed the theory which suggested that adsorption might be achieved through chemical processes such as sharing or exchange of electrons. The difference between the two ions was in the optimal adsorption pH value. Nickel adsorption was more efficient at higher pH values than Cr (VI) adsorption. By increasing the pH value from 2 to 9, nickel removal increased from 46.21% to 93.03%, although it was also observed that nickel started to precipitate at a pH around 8.5. The study compared the results with other adsorbents such as Fe 3 O 4 -CNTs, Fe 3 O 4 , and Fe 3 O 4 -GS with maximum adsorption capacities of 65.96 mg/g, 38.3 mg/g, and 158.5 mg/g, respectively. According to the comparative results, nanomagnetite functionalized with amino groups achieved the highest maximum adsorption [2].
Furthermore, natural organic and inorganic adsorbents also showed good experimental results. Removal rates of 98.78% [34] and 98.6% [35] were attained with activated carbon from Cucumis melo peel (CMAC) [34] and aminobenzoic grafted crosslinked chitosan (FGCX) [35], respectively. Activated carbon (AC) is a widely used adsorbent. Its main feature is a porous structure which enables a large surface area. The surface area of AC can range from 300 to 4000 m 2 /g. Additional surface functional groups can greatly improve adsorption [6]. According to the analysis of the data that was obtained for CMAC, it could be concluded that the adsorption process can be attributed to a chemical reaction with functional groups on the surface [34].
Chitosan itself is a suitable adsorbent because it contains amine and hydroxyl groups [35]. Its disadvantage is that it is soluble at low pH values. Crosslinking helps overcome this disadvantage [29]; however, some studies have noted that crosslinking can reduce the overall adsorption capacity of an adsorbent. Furthermore, grafting allows the formation of functional derivatives with more established adsorption sites, which compensates for possible diminishment in capacity on account of crosslinking. The study of Igberase indicates that the binding of metal ions can occur through chemisorption, electrostatic attraction, or ion exchange [35].
A maximum adsorption capacity of 94.86 mg/g was achieved with lignocellulose/montmorillonite nanocomposite (LNC/MMT) [12]. By analyzing the adsorption procedure, it was shown that the main mechanism of adsorption was chemical adsorption. Additional adsorbent studies showed that hydroxyl and carboxyl groups were involved in adsorption process. Furthermore, in the results of a desorption study, LNC/MMT showed stability after desorption, indicating that the adsorbent is stable and has possible applications in industry [12].
The most successful results for nickel adsorption come from binding to amino groups at pH values between 6 and 8. At lower pH levels, many H + ions that are present in solution are protonating -NH 2 to NH + 3 . This causes a repulsive force between the positive surface and the nickel ions. When pH is increased, deprotonation of amino groups occurs, thus allowing nickel binding, which is reflected in higher adsorption. D50 -average particle size, pHpzc -point of zero charge, W -thickness, S M -specific surface area in a unit of mass, V P -pore volume, D -particle size, rp -radius, c0 initial concentration, m ads /V -adsorbent dose per sample volume, tcont. -the time of contact od solution and adsorbent, qm -maximum adsorbent capacity (Langmuir), K f -adsorption capacity (Freundlich), q e,cal -calculated adsorption capacity, qe,exp -experimentally determined adsorption capacity, R% -adsorption percent.

Discussion
Generally good results for chromium removal were obtained in an acidic environment that extended from pH value 1 to pH value 4. Low pH values can protonate the adsorbent surface. A positively charged surface is optimal for the removal of Cr species present in solution at this pH. The binding expires with electrostatic attraction. Experiments were conducted at pH value 7 in only one study out of all of the studies that we reviewed regarding chromium removal. The adsorbent used at these higher pH values was red mud modified by lanthanum (La-RM) [36]; the experimentally achieved adsorption capacity was 17.35 mg/g. The optimal pH value for Cr (VI) adsorption was adsorbent-dependent. Furthermore, a review of various studies has shown that specific surface area is not a determining factor for the adsorption process. It has also been shown that amino and hydroxyl groups can be strongly involved in the adsorption process and are usually responsible for enhancing chromium adsorption. Furthermore, the results of natural organic and inorganic adsorbents showed the great potential of adsorbents functionalized with amino and hydroxyl groups, and of graphene sand composite.
Cadmium adsorption was achieved with acidic and neutral solutions. The pH values of the various experiments ranged from 3 to 8, although the best results were obtained at slightly higher pH values, e.g., 6. The reason for the better adsorption at higher pHs lies in the deprotonation of the surface of the adsorbent, which in turn allows electrostatic attraction of cadmium. In addition, cadmium removal studies have shown that there is no correlation between specific surface area and adsorption. More accurate information about surface properties (e.g., micropore volume, total pore volume, macropore volume, pore diameter) would be required to properly evaluate the effect of a given surface area on adsorption.
Due to similar properties of heavy metals and similar removal mechanisms, nickel removal studies have shown similar results to those of chromium and cadmium removal. Amino and hydroxyl groups show potential for nickel adsorption. Moreover, chitosan and graphene showed high potential for removal of all examined ions. It was also seen that nickel binding performed more successfully in a less acidic environment between pH values 6 and 8, as the deprotonated surface of the adsorbent is important for electrostatic attraction. As the pH of the solution increased, nickel adsorption increased.
Comparing metal removal, natural adsorbents performed better in all 3 cases.