Management of cadmium contamination in water by biochar and hydrochar modified from Chlorella vulgaris (Beijerinck) Open Access

The increasing concern regarding water pollution presents a significant risk to human health, with data indicating a rising toll of fatalities each year due to tainted water. Experts project that by 2050, the demand for freshwater will have skyrocketed, even though access to less than 1% of the world's freshwater remains a challenging reality. Given water's status as a universal solvent, it is inherently susceptible to pollution. This pollution primarily stems from the discharge of chemicals into water bodies, originating from industries, laboratories, farms and various other sources. Diseases such as cholera, typhoid and giardia have been directly linked to polluted water, affecting both people and animals. Additionally, it exerts a significant toll on the environment, leading to the degradation of marine life (Liu et al. 2015; Li et al. 2023; Thillainayagam & Nagalingam 2023). Cadmium poses a substantial risk to water quality owing to its toxicity and potential health hazards. It is a non-essential trace element that can be extensively dispersed in the environment, with elevated concentrations in soils and groundwater resulting from both natural geological processes and human activities (World Health Organization 2004; Kubier et al. 2019). Cadmium, a profoundly perilous heavy metal, has been designated as a carcinogenic substance by both the United States Environmental Protection Agency and the International Agency for Research on Cancer. As per the most recent standards set by the World Health Organization, the maximum permissible concentration of cadmium in drinking water is 0.005 mg/L (Galal-Gorchev et al. 1993). This metal exhibits significant solubility in aquatic ecosystems and possesses a strong tendency to accumulate in various aquatic species (Liu et al. 2013). Cadmium poisoning can lead to a range of adverse effects in animals, from iron deficiency to liver diseases and damage to the brain and nerves (Ghiasi et al. 2010). Various methods exist for the removal of heavy metals from water, such as sedimentation, ion bleaching, surface absorption, membrane filtration and chemical coagulation. Among these techniques, the absorption process is notable as one of the most effective methods for removing low concentrations of heavy metals from water. This method garners attention due to its remarkable removal efficiency, high purification capacity and rapid reaction rates (Butter et al. 1998). Notably, biological adsorbents have gained significant recognition for their accessibility, ease of use, utilization of natural and biological materials, cost-effectiveness and potential for reusability (Kanel et al. 2005).

Microalgae have been recognized as a highly efficient and environmentally friendly approach for purifying and eliminating heavy metals from water sources, such as cadmium. Microalgae encompass both prokaryotic and eukaryotic photosynthetic microorganisms known for their rapid growth rates in both indoor and outdoor environments (Pérez et al. 2017). Their significance has grown considerably due to their accelerated metabolism and exceptional efficiency in converting sunlight into chemical energy through photosynthesis. In optimal conditions, they demonstrate remarkable growth, effectively reducing the time required for biomass harvesting (Gui et al. 2022). Chlorella vulgaris is a photosynthetic microorganism belonging to the eukaryotic family Chlorellaceae. This unicellular green microalga holds significant importance among green algae, owing to its rapid growth rate and resilience to manipulation in agricultural systems. Additionally, its uncomplicated and cost-effective production process makes it a valuable asset in wastewater treatment (Kabir et al. 2017). The robustness of C. vulgaris, coupled with its high oil content, ability to thrive in mixotrophic culturing conditions and its capacity for rapid growth even in harsh environments, including tolerance to high levels of heavy metals, positions it as a promising candidate for future bioremediation efforts across a range of wastewater qualities (Znad et al. 2018).

Biochar, a carbon-rich substance produced through biomass pyrolysis in an oxygen-limited environment (Yu et al. 2018; Alazaiza et al. 2023), has garnered widespread attention for its cost-effectiveness and high efficiency as an adsorbent in the removal of heavy metals (Hu et al. 2019; Yang et al. 2020; Lee et al. 2021) and organic pollutants (Nguyen et al. 2020; Melo et al. 2021). Currently, in studies concerning the adsorption of these pollutants, raw materials for biochar preparation primarily encompass straw (Medyńska-Juraszek et al. 2020; Mao et al. 2021), livestock manure (Yu et al. 2018; Huang et al. 2020), sludge (Diao et al. 2021) and other natural resources. However, it is important to note that biochar's surface structure and physicochemical attributes can vary significantly depending on the raw materials used. These properties serve as controlling factors that influence biochar's adsorption performance, leading to considerable disparities in its pollutant adsorption properties (Chen et al. 2015a, b; Yang et al. 2020).

Hydrochar has attracted considerable interest from the scientific community owing to its notable characteristics as an eco-friendly and cost-effective adsorbent for removing pollutants from aqueous solutions (Kambo & Dutta 2015; Chen et al. 2019; Jiang et al. 2019; Masoumi et al. 2021). Various biomass feedstocks have been employed in hydrochar production, encompassing a range of agricultural residues (Khushk et al. 2019), sewage sludge (Zhang et al. 2018), manure (Gascó et al. 2018) and microalgae (Saber et al. 2018). Hydrochar is formed via hydrothermal carbonization of biomass at temperatures ranging from 180 to 240 °C, in the presence of hot compressed water. This process leads to the creation of various surface functional groups on hydrochar, which originate from the hydrolysis and recombination reactions of biomass monomers (Gascó et al. 2018), thereby enhancing its efficacy in removing water pollutants through adsorption. Notably, hydrochar derived from rice straw has exhibited high efficiency in adsorbing various organic compounds and heavy metals (Abaide et al. 2019; Li et al. 2019; Peng et al. 2019). This efficiency can be further enhanced by increasing the oxygen-containing functional groups on its surface through techniques such as microwave oxidation (Wang et al. 2019) or by tailoring its physicochemical properties (Peng et al. 2019). However, it is worth noting that some studies have indicated that hydrochar may have a lower sorption capacity for heavy metals when compared to pyrolyzed biochar, attributed to its reduced mineral component content and lower concentration of oxygen-containing functional groups (Demirbas 2008; Inyang et al. 2012; Xu et al. 2017). In light of these findings, significant research efforts have been dedicated to improving the adsorption performance of hydrochar, yielding promising results, particularly when heavy metals are employed to modify its surface properties (Çatlıoğlu et al. 2020; Feng et al. 2020; Teng et al. 2020).

Biochar and hydrochar, derived from C. vulgaris, are considered innovative and potentially sustainable materials for the removal of cadmium. It is crucial to understand their effectiveness in both batch and column studies in order to develop environmentally friendly and cost-effective methods for water purification. The inclusion of both batch and column studies demonstrates a comprehensive approach to comprehending the removal process. Batch studies provide insights into the initial interactions, while column studies simulate real-world scenarios and offer a more practical evaluation of the materials' performance. If biochar and hydrochar derived from C. vulgaris prove to be effective in removing cadmium, it could lead to the development of sustainable and accessible solutions for water purification. Therefore, this study aims to conduct batch and column experiments to assess the effectiveness of biochar and hydrochar, both derived from C. vulgaris, in removing cadmium from aqueous solutions. The specific objectives are as follows: (1) to evaluate the batch adsorption capacity of biochar and hydrochar for cadmium removal, (2) to investigate the breakthrough behavior and sorption kinetics in column studies using biochar and hydrochar, (3) to compare the performance of biochar and hydrochar in terms of cadmium removal efficiency and (4) to investigate surface adsorption isotherms and thermodynamics.

The C. vulgaris stock was obtained from the Gohar Sabz Algae Cultivation Laboratory and has been consistently preserved under controlled laboratory conditions. The preservation process involved maintaining a temperature of 25 °C and a photoperiod of 12 h of light followed by 12 h of darkness. The microalgae were cultivated using the F/2 culture medium. Due to the sensitivity of algae to environmental conditions, all procedures were carried out with great care, following strict cleanliness and isolation protocols. A dedicated, fully isolated room was specifically designated for algae cultivation to ensure optimal conditions and prevent any external contaminants. The cultivated cultures were exposed to a light intensity of 2,500 lux, under a 12-h light and 12-h darkness photoperiod, and kept at a temperature of 25 °C. The initial cultivation of microalgae lasted for 7 days. Once the microalgae entered the logarithmic growth phase, they were transplanted to a larger container, and a new batch of culture medium was introduced (Pena-Castro et al. 2004). After a growth period of 2 weeks, the biomass was harvested. To eliminate any potential impact from residual salts, the algal biomass underwent five thorough washes with distilled water. It was then dried in an oven set at 40 °C, milled, sieved, and subsequently employed as an adsorbent in the ensuing stages of the experiment (Abdel-Aty et al. 2013).

The dried biomass of C. vulgaris underwent crushing in a mill and subsequently passed through a 2 mm sieve. To initiate the biochar production process, the samples were meticulously placed in a fully compressed state within a metal cylinder equipped with a lid, all under conditions of low oxygen. To achieve this, the samples were first accurately weighed and then carefully loaded into the cylindrical containers with securely closed lids. They were then compressed entirely to create an environment with minimal or no oxygen. In order to further minimize oxygen exposure, both the container lid and the oven door were tightly sealed, ensuring the establishment of low oxygen conditions necessary for the pyrolysis process. The algae samples were then introduced into an electric furnace set at a temperature of 450 °C for a duration of 2 h, requiring a total of 3 h for the complete biochar production process (Yang et al. 2019).

The algae biomass was initially ground into a powder and subsequently dried to a 2 mm size in an oven at 105 °C for a period of 24 h. In the subsequent step, the powdered algae was carefully placed into specialized lidded containers and subjected to a heating process in a stainless steel autoclave, utilizing deionized water at a temperature of 200 °C for a duration of 2 h. Following this stage, once the container housing the hydrochar reached room temperature, its contents were passed through a filter paper. The solid particles that were trapped on the filter paper were subjected to several rinses using deionized water and subsequently dried in an oven for a duration of 16 h at a temperature of 80 °C (Elaigwu & Greenway 2016).

Biochar and hydrochar were activated by means of potassium hydroxide. The produced biochar and hydrochar were used with potassium hydroxide at a ratio of 0.5–1. For this, a concentrated solution of potassium hydroxide was prepared and mixed well with biochar and hydrochar manually. Biochar and hydrochar impregnated with potassium hydroxide solution were dried overnight at 100 °C. After weighing, it was kept in ceramic crucibles for 1 h when the temperature reached 600 °C under vacuum conditions and the presence of nitrogen gas. Activated biochar and hydrochar were extracted from the plant material after it was cooled to room temperature. The extracted biochar and hydrochar were then subjected to multiple washes using a semi-normal hydrochloric acid (HCl) solution. Subsequently, they were rinsed with hot distilled water. The biochar and hydrochar, which had been in the oven overnight, were then dried and their weights were measured again.

The specific surface area (SSA) of biochar, biomass, hydrochar, and activated biochar and hydrochar was initially estimated using the methylene blue absorption method (Kaewprasit et al. 1998), and then precisely determined using nitrogen absorption isotherms through the Brunauer–Emmett–Teller (BET) method. The surface morphology of the samples was evaluated using scanning electron microscopy. The pH value was determined by placing 0.1 g of samples in 20 mL of distilled water, shaking for 1.5 h and measuring with a pH meter (Godlewska et al. 2017). Additionally, the quantities of carbon, nitrogen and hydrogen in the prepared samples were determined using a CHN analyzer (ThermoFinnigan Flash EA 1112 Series).

To establish the equilibrium time, a 300 mL beaker was filled with 50 mL of a cadmium solution containing 20 mg of cadmium per liter. Following that, 80% of the adsorbent was introduced into the solution, and samples were taken at different time intervals: 15, 30, 60, 90 and 180 min, all at room temperature. Subsequently, the collected samples underwent centrifugation for 10 min, and the resulting supernatant solution was meticulously separated and filtered through a filter paper. The concentration of cadmium in the extracts was determined using an atomic absorption device.

Once the equilibrium time of the adsorbent was determined, various quantities of the absorbent (0, 0.1, 0.2, 0.4, 0.8, 1 and 2 g) were introduced into centrifuge tubes, while maintaining a constant cadmium concentration of 20 mg/l in the flask. Subsequently, the samples underwent centrifugation for 10 min, and the resulting supernatant solution was separated and filtered through a filter paper. The concentration of cadmium in the extracts was measured using an atomic absorption device.

Several mathematical models have been proposed to explain the equilibrium in sorption processes, including the Langmuir, Freundlich and Temkin isotherm models, which are widely used. The Langmuir model, specifically designed for heavy metal adsorption on microalgae, is based on three main assumptions: (1) The sorption process continues until heavy metal ions form a uniform coating on the outer surface of the microalgae absorbent. (2) The surface of the microalgae absorbent is uniform, with all binding sites occupying the same positions. (3) Each heavy metal in the aqueous solution interacts with a corresponding binding site on the microalgae surface, and this interaction is not affected by the extent of site occupation (Pehlivan & Arslan 2007; Bulgariu et al. 2013). To investigate surface adsorption isotherms, the previously determined optimal amount of adsorbent was added individually into Falcon tubes. Cadmium concentrations of 0, 2, 5, 10, 20 and 40 mg/L were then added to the Falcons. The mixtures were adjusted and cooled for the specified equilibration time determined in prior experiments. Adsorption isotherms were utilized to quantify the mass of adsorbent material absorbed per unit mass of adsorbent material. The removal efficiency of cadmium was calculated, and subsequently, the experimental data were fitted to various surface adsorption isotherms (including Langmuir, Freundlich and Temkin). The characteristics of the investigated isotherm equations are presented in Table 1.

Table 1

Isothermal models studied

Model .	Equation .	Parameters .
Langmuir (1918)		a and b are Langmuir isotherm constants
Freundlich (1906)		kf is the Freundlich isotherm constant
Temkin & Pyzhev (1940)		A and B are the equilibrium constant (L/g) and Temkin isotherm constant (L/mol), respectively

Model .	Equation .	Parameters .
Langmuir (1918)		a and b are Langmuir isotherm constants
Freundlich (1906)		kf is the Freundlich isotherm constant
Temkin & Pyzhev (1940)		A and B are the equilibrium constant (L/g) and Temkin isotherm constant (L/mol), respectively

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In these relationships, q_e represents the adsorption capacity of the adsorbent at equilibrium conditions (mg/g); and C_e stands for the equilibrium concentration of cadmium after reaching equilibrium (mg/L).

The surface adsorption isotherm test was measured at pH levels of 6, 7 and 9.5.

To ascertain the breakthrough curve time and absorption capacity for refinery applications, the adsorbents were assessed in a refinery test column featuring continuous solution feed. The column, with dimensions of 20 cm in length and 2.5 cm in diameter, was packed with the selected adsorbents. Cotton and netting were placed at both the top and bottom of the absorbent bed to secure the material in place. A solution with the concentration at which the highest absorption was achieved (adjusted to a pH of 6.5) was introduced into the absorber using a constant flow rate (approximately 10 mL/min) facilitated by a pump. The initial solution's concentration was determined based on prior experiments. At eight specified time intervals, samples of the output solution were collected and the cadmium concentration was measured (Adhikari et al. 2016).

In these equations, q_e represents the amount of solute absorbed in the equilibrium state (mg/g), q_t represents the amount of solute absorbed over time (mg/g) and K₁ and K₂ are the equilibrium rate constants for first- and second-order kinetics, respectively.

In the pH measurement process, 1 g of ball-milled biochar or hydrochar was mixed with 20 mL of deionized water and stirred for 1.5 h to establish equilibrium between the surface of the biochar or hydrochar and the solution, following the procedure described by Benavente et al. (2022) and recommended by the International Biochar Initiative (IBI 2015). pH determination was conducted using a Mettler Toledo FE 30 device. Subsequently, the electrical conductivity (EC) of the biochar or hydrochar was evaluated using an HM digital EC probe (model COM-100).

The results of elemental analysis performed on biochar and hydrochar modified from C. vulgaris are presented in Table 2. Biochar modified from C. vulgaris is primarily composed of carbon, as the pyrolysis process used to create it involves the removal of volatile components like hydrogen. This results in a carbon-rich material that often shows a reduced hydrogen content in comparison with the initial biomass. In contrast, hydrochar is produced via hydrothermal carbonization, a process conducted in an environment abundant in water. This process tends to retain more of the original hydrogen content from the biomass compared with pyrolysis. Therefore, hydrochar generally has a higher hydrogen content compared with biochar. Biochar (modified from C. vulgaris) typically has a lower nitrogen content compared with the original biomass. This is because nitrogen-containing compounds are often among the volatile components that are released during the pyrolysis process. Hydrochar (modified from C. vulgaris) tends to retain more of the original nitrogen content from the biomass compared with pyrolysis. Therefore, hydrochar generally has a higher nitrogen content compared with biochar. In the context of oxygen content, both biochar and hydrochar (modified from C. vulgaris) demonstrate different quantities, a characteristic that is influenced by factors such as the type of material used and the specific methodology employed during their production.

In Table 3, the amount of potassium in biochar (modified from C. vulgaris) is higher than that in hydrochar (modified from C. vulgaris). Furthermore, in Table 3, the calcium content of biochar (derived from C. vulgaris) (2.9%) is lower than that of hydrochar (derived from C. vulgaris) (3.7%). These elements are among the elements of micronutrients and macronutrients and plants absorb more than micronutrients, so they have more in biochar and hydrochar production. Comparing the amount of iron and aluminum in biochar and hydrochar, it can be seen that these elements are more abundant in biochar than in hydrochar, which is related to the production process of these two materials. The amount of silicon, zinc and magnesium in both materials is relatively similar.

The coefficients for the first-order and second-order kinematic models have been outlined in Table 6. Notably, the adsorption kinetics in this study demonstrate a strong alignment with the second-order model, evidenced by R² values of 0.983 and 0.987 for biochar and hydrochar (modified from C. vulgaris), respectively. This implies that cadmium likely undergoes chemisorption onto the surface of the adsorbents (Lu et al. 2012; Kim et al. 2013). The results show that the q_e values of the pseudo-second-order kinetic model with biochar and hydrochar were 13.67 and 23.52 mg/g, respectively. Previous studies have also reported that the second-order kinetic model effectively describes the retention of metals on biochar and activated carbon (Inyang et al. 2012). In this study, the results indicate that chemisorption may be the rate-controlling mechanism, suggesting the possible sharing of electrons between cadmium ions and the absorbent (Usman et al. 2016). Apiratikul & Pavasant (2008) conducted a study on the biosorption of heavy metals by the macroalgae Caulerpa lentillifera. Their results indicated that the absorption of cadmium and lead followed the quasi-quadratic kinetic model, with R² values of 1.00 and 0.999, respectively. Similarly, Sulaymon et al. (2013) investigated the kinetics of cadmium absorption and found that it followed the pseudo-quadratic model, reporting an R² value of 0.998 for their study. Overall, these findings highlight the applicability of different kinetic models in understanding adsorption processes for various substances.

The investigation of the adsorption thermodynamics of biochar and hydrochar (modified from C. vulgaris) played a crucial role in evaluating the influence of adsorption temperature on removal efficiency and determining the endothermic or spontaneous nature of the adsorption process. Thermodynamic parameters, including Gibbs free energy (ΔG°), enthalpy change (ΔH°) and entropy change (ΔS°) associated with cadmium adsorption, have been meticulously investigated and are detailed in Table 7. The estimation of ΔG° is particularly important as it provides insights into the spontaneous nature and the specific type of adsorption process involved in ion removal (Hu et al. 2016). Additionally, this parameter helps differentiate between physisorption and chemisorption processes that underlie the surface complexation process. The negative ΔG° values for cadmium metal at all temperature levels indicate the spontaneous nature of cadmium ion adsorption onto the surfaces of both hydrochar and biochar. The significantly more negative ΔG° for biochar suggests a higher degree of spontaneity compared with hydrochar during the adsorption process (Rehman et al. 2018). Furthermore, the positive ΔH° value suggests that the adsorption was an endothermic process, implying that increasing the system's temperature, up to a certain limit, could enhance the cadmium removal efficiency (Yoo et al. 2016a). Given that the ΔG° values for cadmium were below −40 kJ/mol, it suggests the potential involvement of a physisorption process after the establishment of a surface monolayer on both biochar and hydrochar surfaces. The positive ΔS° value in Table 6 signifies a robust attraction between hydrochar and cadmium metal ions. The adsorption of these metal ions onto the hydrochar surface may induce structural alterations (Yoo et al. 2016b). Moreover, the notably elevated positive ΔS° observed in biochar, in contrast to hydrochar, indicates a heightened affinity of cadmium ions to the biochar surface. This implies an augmented degree of unpredictability at the solid/liquid interface during the cadmium ion sorption process onto both biochar and hydrochar surfaces. The amplified level of randomness is particularly accentuated in biochar as compared to hydrochar, attributed to the higher ΔS° value associated with cadmium adsorption. These results align cohesively with previously documented research outcomes pertaining to the thermodynamics of heavy metal adsorption (Elaigwu et al. 2014).

The paper provided a detailed discussion of various experiments and analyses related to the adsorption of cadmium by biochar and hydrochar (modified from C. vulgaris). Due to the abundance of the raw material, biochar and hydrochar were rich in calcium and potassium. Biochar had higher levels of iron and aluminum compared to hydrochar. Silicon, zinc and magnesium levels were relatively similar in both materials. In general, biochar had a higher pH compared to hydrochar. Also, biochar typically had a higher SSA than hydrochar, and GCV was lower in biochar compared to hydrochar. Both types of modifiers (biochar and hydrochar) showed similar effectiveness in cadmium removal over time. Due to improved binding and removal, the percentage of cadmium removal increased over time. Increasing the dosage of both biochar and hydrochar led to higher efficiency in cadmium removal, up to a certain concentration threshold. pH significantly influenced cadmium removal, with higher pH levels leading to increased removal efficiency. Beyond a certain pH, efficiency started to decline. Increasing the dosage of the absorbent (biochar and hydrochar) led to higher cadmium removal due to more active adsorption sites.

The results demonstrated that the Langmuir model offered the most accurate fit for the data, suggesting that cadmium absorption was more closely aligned with this model. Second-order kinematic models aligned well with the data, showing high R² values for both biochar and hydrochar. Adsorption of cadmium onto both biochar and hydrochar surfaces was a spontaneous process. The process was endothermic, which means that the removal efficiency could be increased by increasing the temperature, up to a certain limit. The biochar showed a higher degree of spontaneity in comparison to the hydrochar. During cadmium ion sorption, both biochar and hydrochar exhibited increased randomness at the solid/liquid interface.

The study provided comprehensive insight into the adsorption behavior of cadmium on biochar and hydrochar, including factors such as dosage, pH, concentration and thermodynamics. The results suggest that these materials may have potential applications in the remediation of environmental contamination caused by heavy metal pollutants. The results of this research indicated that the activation of the biochar and hydrochar increased their effectiveness in the removal of cadmium. Activation resulted in the formation of a more extensive and porous surface structure, which increased the surface area available for the adsorption of cadmium ions. The activation also created a network of pores within the biochar and hydrochar. This improved the accessibility and trapping of cadmium ions. Greater interaction between the adsorbent and the contaminant was facilitated by this improved pore structure. The activation process introduced or enhanced functional groups on the surface of the biochar and hydrochar that could form strong bonds with cadmium ions, thereby increasing the overall adsorption capacity. In addition, activated biochar and hydrochar exhibited higher reactivity with contaminants, resulting in a faster and more effective adsorption process. The activation process introduced specific surface functionalities with a high affinity for cadmium ions, promoting more effective and selective adsorption.

The subject of plagiarism has been considered by the authors and this article is without problem.

J.S. and M.B.S.: Conceived of the presented idea. M.B.S., A.A. and S.M.: Developed the theoretical framework. J.S., A.A. and M.B.S.: Developed the theory and performed the computations. M.B.S. and A.A.: Verified the analytical methods. J.S., A.A. and S.M.: Carried out the experiments. All authors discussed the results and contributed to the final manuscript.

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

The authors declare there is no conflict.