Removal of A Cadmium from Wastewater Using Hydrochar-Supported Nanoscale Zero-Valent Iron (nZVI) on Composite Materials

doi:10.21203/rs.3.rs-3887095/v1

Download PDF

Article

Removal of A Cadmium from Wastewater Using Hydrochar-Supported Nanoscale Zero-Valent Iron (nZVI) on Composite Materials

https://doi.org/10.21203/rs.3.rs-3887095/v1

This work is licensed under a CC BY 4.0 License

Version 1

posted

You are reading this latest preprint version

Among the most hazardous heavy metals, cadmium can be found in various environmental compartments. Cadmium pollutants are ubiquitous in soil, surface water, groundwater, plants, vegetables, and the atmosphere. The focus of this research was specifically on cadmium pollution in wastewater. When present at high concentrations, cadmium can lead to a range of intricate health problems, including but not limited to cancer, kidney failure, high blood pressure, etc. Suppose industrial wastewater effluent is not treated to meet the World Health Organization (WHO) standards. In that case, cadmium can be released into the environment due to increased industrialization, originating from both point and nonpoint sources. Many new technologies have emerged in recent years to help solve the world's environmental concerns regarding heavy metal pollution. However, some of these technologies are challenging and expensive, while others have proven to exacerbate the problem of cleaning up our environment. Among the various available technologies, nanoscale zero-valent iron nZVI has gained significant attention for removing heavy metals from polluted soils and wastewater and even employing in situ remediation methods.

Nevertheless, nZVIs encounter easy oxidation and poor suspension stability, severely limiting their application in groundwater and industrial wastewater treatment. To increase the removal efficiency of nZVI for the heavy metal cadmium removal and improve its suspension stability in solution, this study focused on hydrochar-modified nZVI or hydrochar- nano-zero valence iron (H-nZVI). This study aimed to investigate this treatment's impact on wastewater by explicitly examining cadmium removal and suspension stability in different solutions. In the experiment as mentioned above, H-nZVI was characterized using techniques such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The characterization results indicated that nZVI was effectively loaded onto the hydrochar substrate, enhancing its absorption capacity. Adsorption experiments demonstrated that H-nZVI exhibited a high cadmium removal capacity and strong adsorption capability, with a maximum removal percentage of 97.56%.

Furthermore, the sedimentation kinetics experiment revealed that H-nZVI displayed superior suspension stability to nZVI. The sedimentation percentage of H-nZVI varied with the solution's ionic strength due to agglomeration changes, which depended on factors such as zeta potential and fluid dynamic radius. This research was a state-of-the-art contribution, especially for new researchers in environmental real-time research, whether at the undergraduate or graduate level. The experimental setups, steps, analysis, and explanations were so explicit that any researcher could easily follow them and replicate the same or related work. This research follows the paradigm of modern research work, explicitly explaining the ramifications of cadmium in our environment and its removal method using nZVI technology.

Earth and environmental sciences/Environmental sciences

Physical sciences/Nanoscience and technology

cadmium

H-nZVI

adsorption

sedimentation

zeta potential

Urbanization and industrialization adversely affect water and our ecosystems today ^1–3. These anthropogenic activities and geogenic sources release harmful and poisonous substances into the environment. Due to the degradation of water bodies, surface water, groundwater, and soil, scientists and engineers are becoming increasingly involved in the remediation of these toxic substances in the environment. The effects of these enormous amounts of natural activities are worsening global health conditions daily. According to the WHO, 40% of the world's population lacks clean water due to groundwater and surface water contamination ². Plants are equally affected by nutrient intake. Dissolved heavy metals in the form of contaminated water enter plants' tissues, stems, and leaves through osmosis. As humans feed on these plants, they are prone to chronic sickness and diseases caused by heavy metal contamination. Cadmium is a heavy metal with a high atomic weight and relatively high density ¹. It is abundant in nature through rocks and in combination with other minerals, primarily other heavy metals such as zinc and mercury. Its chemical makeup is somewhat complex, and it is also carcinogenic. Cadmiums find their way via industrial processes, rock weathering, or mining specific minerals. This study examined the processes through which cadmium is immobilized from point sources of pollution or nonpoint sources, as well as the resulting health problems. Wastewater and groundwater are the most polluted sources of water in the environment. Pollution originates from landfill leachate, industrial waste (solid and liquid forms), municipal waste, stormwater, and agricultural waste^3–5. These contaminants eventually enter surface water and percolate into groundwater through leaching or infiltration. Most of these debris substances can easily percolate through the soil and end up in the aquifer, contaminating groundwater and natural water bodies. Similarly, natural pollutants can also find their way into surface water and groundwater. Groundwater plays a crucial role in ecosystems as plants extract surface water and nutrients from the land, where these nutrients exist as dissolved substances ^6–8. Through osmosis and nutrient transport, plants can easily absorb harmful substances such as heavy metals, including cadmium, through their root systems, stems, and vegetative parts ⁹. Human and animal food, primarily fruits, vegetables, and cereals, is essential for human life. Furthermore, there is a hydraulic exchange phenomenon between groundwater and surface water, particularly when groundwater is contaminated with cadmium or other heavy metals¹⁰. Many people in developing countries rely on boreholes as their domestic or potable water source. If the groundwater in a borehole is polluted with cadmium, it can pose significant health risks at an alarming rate ¹¹ . Additionally, wastewater and groundwater can also serve as sources of surface water, meaning that groundwater pollution increases the likelihood of surface water pollution. Plants and vegetation obtain their water supply from surface water or groundwater, and if that source of water is polluted with the heavy metal cadmium, plants will eventually assimilate these toxic chemicals into their protoplasm. Humans and other lower organisms live on plants, and they tend to come into contact with contaminated sources of cadmium. This research aimed to improve the adsorption and removal capacity of nano zero-valent iron for cadmium and its stability. Priority pollutants are the collection of pollutants that receive the most attention because of their acute toxicity and cancerogenic, teratogenic, and mutagenic characteristics. According to statistics, there are 129 compounds on the USA Priority Pollutants list and 68 compounds on the China Priority Pollutants list, and sadly, cadmium falls within both the USA and China ¹². Cadmium is widely used worldwide even though it is not a precious metal like others are. However, it has high industrial potency in various areas despite its toxicity and high concentration ¹³. Cadmium is one of the principal heavy metals that is associated with health problems, such as blood pressure and kidney failure ^14–16. Anabolism and catabolism are also involved in other health impairments in higher organisms ¹⁷. The fungus can reach the environment through industrial wastewater discharge and natural sources, such as geogenic sources, weathering, and leachate ¹⁵. According to the literature, a cadmium dosage of 15 mg/L can cause nausea and severe vomiting ^18–20 Bioavailability and bioaccumulation occur throughout the food chain and include contaminated water consumption. Some books or the literature say that the acceptable dose of cadmium in the human system can be in the range of 0.05 mg/L or less; however, to be safer and secure, the required value according to the WHO standard is 0.005 mg/L for municipal drinking water, and for the industrial wastewater treatment standard, it should be 2 mg/L before discharge to natural water bodies ^21,22.

Research Objectives

After conducting a thorough literature review on removing cadmium from aqueous solutions, my research took a unique approach to address previous challenges and provide innovative solutions for future researchers. This study's novelty lies in synthesising composite materials consisting of nano zero-valent iron (nZVI) supported on modified biochar, specifically hydrochar, using environmentally friendly methods. The synthesis process involved the utilization of green tea leaves to produce nZVI, and the stability of the resulting composite material was assessed. The effects of pH on the adsorption capacity, removal efficiency, and particle mobility were also investigated as critical aspects of this study. Additionally, the investigation focused on analyzing the features of the composite materials to enhance the validity, effectiveness, and potential implications of the findings.
In conclusion, the synthesis process did not generate toxic waste, eliminating the need for external experimental conditions such as high energy or pressure. This outcome led to significant energy savings, reduced capital, and lower labour costs. The datasets were generated and analyzed during the Master's program (2017-2019)

This research involved the synthesis of hydrochar and, in comparison with the hypotheses of different materials (different carbon materials such as biochar and eggshell), synthesis via other methods was performed to determine the expected outcome for the adsorption of cadmium in the wastewater environment, which was the adsorbent used in this study. The materials employed in this research were synthesized from green tea, the background material for the composite of hydrochar and was the focus of this research. These materials were blended with nZVI for holistic adsorption and maximum removal efficiency. Green tea was purchased from Beijing Qingyuan Xianshan Tea Co. China

Green tea synthesis

Fifteen grams of green tea leaves were weighed by an electronic balance and placed in a 250 ml beaker with deionized water at a heating temperature of 80°C in a water bath for 30 minutes. Then, the solution was filtered through a vacuum filter and allowed to cool to room temperature. 0.04 mol/L Fe ₃+ was prepared (weighing 4 g of Fe₂(SO₄)_3). XH₂O and diluted to 250 ml with deionized water (heating at 60°C for 10 min). Then, green tea extract was added to the four-necked flask, nitrogen (N₂) was added, the fe3+ solution was added, and the mixture was stirred for 0.5 hours. After the suspension was obtained, the mixture was poured into a centrifuge tube and placed in an automatic balance centrifuge (BIORIDE TDZ5-WS) for centrifugation (speed: 4000 rev/min for 7 min). The suspension was poured off, and the solid was left to solidify. The resulting product was vacuum-dried overnight at 323 K to obtain nanoscale zero-valent particles. After drying, the particles were ground using a mortar and pestle and passed through a 0.18 mm pore eye-size sieve before being packaged in small sealable polyethylene plastic bags. Finally, the storage bags were stored in a desiccator purged with nitrogen gas before use.

Synthesis of Hydrochar

The rice mill waste was washed with deionized water, they were left at 80°C for 12 hours, 20 g was weighed, and the samples were placed in a 250 ml beaker. (The rice mill is a waste product from rice milling, which in most cases is disposed of in landfills, but where the demand for such research is high, it can be sold). Evenly pour into 200 ml of 72% (weight %) sulfuric acid (H₂SO₄), not stir, gently squeeze all the rice husk to the liquid, and place the mixture in a 50°C water bath for 10 min. Filter the mixture with a glass bar grate filter, and obtain approximately 150 mL of filtrate after removing the rice husk. The filtrate was filtered (with a 100 mL sand core funnel, which was cleaned with distilled water three times before use). The filtrate was prepared as 47% hydrochar (47:53 = filtrate: distilled water). 47% of the hydrochar was put into a 400 ml beaker; the surface was sealed with plastic wrap, and tiny/perforated holes were poked. Then, the mixture was stirred under magnetic force in a 95°C water bath and heated for approximately 6 hours (after which water was added). In the end, the hydrochar was filtered and rinsed thoroughly with distilled water three times. The final product was obtained after drying at 120°C for 12 hours.

Supplement: approximately 100 ml of prepared H2SO4.

Add 35 ml of distilled water, and slowly pour 65 ml of 95% H₂SO₄.
This process is relatively dangerous. The samples were then incubated in a water bath.
After drying, the particles were ground using a mortar and pestle and then passed through a 0.18 mm pore eye-size sieve before being packaged in small sealable polyethylene plastic bags. Finally, the storage bags were stored and kept in a vacuum drier. From this point, you can now use the prepared NZVI from green tea with your hydrochar composite material for adsorption.

Standard preparation of Cd2⁺at different concentrations for the standard line

Atomic absorption spectrometry (AAS 7000) was used to detect the presence and concentration of cadmium in the experiment. The standard line should be obtained, the benchmark wherein all other adsorptions fall within that armpit. If para-ventures exist, any range of values falls outside that preset value and cannot be detected by the AAS. The Cd2⁺ standard solution was prepared at different concentrations from 0 mg/L to 5 m/L.

Standard concentration preparation of cadmium for the standard calibration curve/line

Standard calibration curves should be utilized to determine the analyte concentration in the given samples before detection, especially when using AAS/UVS, since this approach provides a more accurate sense of detection. Standard calibration curves are frequently used in analytical chemistry for quantitative and qualitative analysis. The standard curve in this study was obtained in the following manner:

Cadmium standard storage solution: 0.5000 g of metal cadmium powder (spectral pure) was weighed and dissolved in 25 mL (1+5) HNO₃ (slightly thermally dissolved). The solution was cooled, transferred to a 500 mL volumetric flask, diluted with distilled deionized water, and fixed. This solution contained 1000 mg of cadmium per litre (this solution was the finished product we bought).
For the cadmium standard, 10.0 ml of cadmium standard storage liquid was added to a 100 mL volumetric flask, diluted with water to the marking line, shaken well, and set aside. This solution contained 100 mg of cadmium per litre.
The 5.0 mL diluted standard solution was drained into another 100 mL volumetric flask and diluted to the marked line to obtain a standard solution containing 5 mg of cadmium per litre.

In such studies, five (5) standard concentrations of cadmium were usually prepared from 0, 1, 2, 3, 4, and 5 mg/L, but 0 mg/L was used as a buffer solution or distilled water only.

The Standard Line of the Adsorption Isotherm

The sorption kinetics should always be standardized in agreement with the sorption data of the Langmuir (R² = 0.093-0.98) and Freundlich (R² = 0.90- 0.99) isotherms (Ahmad, Munir et al., 2018) for the calibration curve. The fig. 3 graph confirms that the analogy holds for the Langmuir and Freundlich isotherms.

An adsorption isotherm is an adherence phenomenon wherein an adsorbent at a particular given mass of solute can adsorb the adsorbate at a defined equilibrium concentration at a contestant temperature. The mathematical expression is expressed as follows:

q_e is the equilibrium adsorption capacity (mg/g^-1) and is the amount of metal absorbed per specific amount of adsorbent, according to these authors ²³. This research investigated the adsorption of cadmium pollutants by nZVI composites doped with hydrochar.

Selection of the Best Material for the Adsorption Capacity of Cadmium

However, some researchers may argue that this stage involves trial and error. However, this approach is very intuitive and provides a wealth of information regarding the research design. Proper material selection is one of the most essential aspects of any analytical and laboratory research. This approach is crucial for coherence, systematic data collection, and qualitative analysis.

A series of tests were conducted at various ratios and concentrations to determine the best material with the best sorption capacity and removal efficiency. After examining all the findings from many trials, the best material ratio was 1:4, which was the ratio of nZVI to hydrochar. This ratio was adopted for all the preceding experiments.

Fig. 1. graph clearly shows that the 4:1 ratio was the best ratio with a high adsorption capacity compared to the other ratios (4:1 or 1:4).

Preparation of the composites (nZVI/hydrochar)

After the preparation of hydrochar as described in the section above, the required ratio of the powdered hydrochar was synthesized together with the green tea in the correct proportion per the design of the experiment. These nZVI composites, as they are often called, were used in all the experiments for both adsorption and all the other characterizations. As explained above, when the nZVI/hydrochar composite was synthesized at a ratio of 1:4, the material was also converted to 4:1, as shown in the graph below. This ratio was calculated for every 1.1 g of nZVI, and the hydrochar yield was 0.275 g (1.1/4). The mass ratio of nZVI was calculated from the molecular weight of Fe₂(SO₄)₃. XH₂O. Approximately 1/3 of the biosorbent (hydrochar) was doped with nZVI in this study because of its maximum adsorption capacity and removal efficiency. One can deduce from this mathematical analysis that the hydrochar biosorbent is an efficient material for the environmental remediation of contaminated water and soil.

The quantity of the nZVI/hydrochar composite (concentration: 500 mg/L) for the adsorption capacity experiment.

The mass (0,025 g) of the nZVI/Hyrochar composites was used for all the preceding adsorption experiments.

Characterization of Materials

The main goal of the material matrix is to provide a thorough and valuable overview of the techniques used to classify and characterize a wide range of materials, inherently by emphasizing each technique's theoretical components, an in-depth analysis of how each grain of material is arranged in its morphological outlooks ^18,24,25as been performed. Material characteristics are the inherent nature of scientific materials being ascribed for research purposes. The behaviour of materials is dynamic, especially when subjected to mechanical or chemical stress. However, the innate properties of these laboratory materials are crucial for experimental purposes. The technical concept behind this elusory of material phenomenon is to reduce the failure time and keep equipment close to its design life span. The scanning rate was 10° for every minute, and the scanning angle ranged from 10° to 80°. The amorphous crystalline peak of both hydrochar and the magnetic nanoparticle composites was distorted. XRD can further reveal the chemical composition of known or unknown chemical constituents ²⁴. In such analytical studies, material composition plays a crucial role in determining the inherent traits of each studied sample.

SEM of Nanozero-Valent Iron Hydrochar

There is always a deep sense of analytical explanation of the structure of nanoscale zero-valent iron (nZVI) and how its particles evolve during the reaction. To understand how the removal process occurs, further investigations of the morphological orientation under different characteristic conditions are needed. SEM is a significant method for adequately investigating nZVI materials' dynamics. The scanning showed good electrical conductivity based on the images shown in Fig. 2.

Two different samples were photographed—the hydrochar and nZVI/hydrochar composites—using scanning electron microscopy (SEM). The hydrochar in Fig. 2(b) differs slightly from the starting material in Fig. 2(a). The clicking doom shape of the hydrochar indicated the type of process it underwent during hydrolysis and dehydration. After further grinding the probes in the figures for the composite, the hydrochar protruded deeper into the nZVI matrix, almost overshadowing the nZVI. The nanoparticles of the nZVI/hydrochar composites and hydrochar were studied under these conditions. The orientation of these samples clearly showed the crystal growth texture on the surface of these particles. Both of the particle images have platy dendrite structures. The dispersion further exhibited nanosized particles within 1 to 10 nm in diameter. According to some of the related literature. This result in Fig. 2. indicated that the average size of any nanosized particle ranges typically from 1 to 30 nm in diameter. Interestingly, the dendrite structures fade away as the SEM image size increases or the particle size increases. It is always good for the photographic image to be in a particular range for good imagery. The scanning results showed the dendritic texture of the nZVI composite. The pigmentation within the lattice structure further confirmed the magnetic nZVI composite indicated by nanozero-valent iron materials ^26,27.

Transmission electron microscopy (TEM)

The crystallographic orientation of the grains was determined, as was the arrangement of the atomic planes in the polycrystalline sample ²⁸. The nZVI particles had diameters ranging from 10–200 nm without the need for additional reducing agents based on the particular experiment at 25°C. In general, nZVI offers a suitable approach for improving the stability and reactivity of renewable resources, especially carbon materials, under the umbrella of hydrochar. The scanning analysis shows that nZVI aggregates and moves toward the targeted area during nanoremediation. The nanoparticles can agglomerate due to their metallic ability and emulsify contaminants into harmless substances. This is one of the reasons nanotechnology is used in groundwater remediation due to its unique characteristics.

Fourier Transform Infrared Spectroscopy (FTIR) Analysis of nZVI and Hydrochar

Fourier transform infrared spectroscopy (FTIR) is a sampling technique used to observe materials in their solid or liquid state ^29–32. Owing to the nature of the experiments, this research was limited to the observation of both hydrochar and nZVI composites in their solid-state about their adsorbent nanoparticles. Here, the principal goal of this experiment was to determine how the raw nZVI and hydrochar data are converted into spectral data when the source of light is focused on the principal angle of the observed solid materials (hydrochar and nZVI composite). The nature of the absorbance can be deduced by the wavelength of the light applied to the observed material. The vertical axis is the transmittance (T) as a percentage (%), and the horizontal axis is the wavenumber in cm^-1. The spectrum was described within the periphery of the boundary conditions.

FTIR typically has several points on the graph (sharp peaks) for characterising the adsorption band. However, before chemical analysis, the FTIR results of the nanoparticle composites. They are both nZVI/hydrochar composites, but their FTIR results might differ before and after adsorption, allowing us to determine the meteorological orientation of their particle sizes and bandwidths in wide spectral bands. Both experiments were performed under different conditions to assess the reliability and sorption capacity of the nZVI/hydrochar for pollutant removal. Fourier transform infrared spectroscopy was carried out to identify the functional groups in the adsorbents range of 500-4000 cm^-1. The shape of the particles plays a vital role in determining the position of this band. It was discovered that particle size explicitly influences the width and intensity of peaks in the infrared (IR) spectrum. The width of the peak narrows, and the intensity increases as the particle size increases. According to the figure above, the bandwidth corresponding to the wavenumber 3270 cm^-1 corresponds to the OH functional group, which participates in chemical bonding.

Zeta potential and particle size analysis

The zeta potential (ZP) helps to understand and control colloidal suspensions. Therefore, nanoparticles (ZPs) were tested using a zeta-steel meter (Malvern nano ZS90). Five milligrams of each nanomaterial was added to a 50 ml beaker containing 10 ml of distilled water containing 0.01 mM NaCl as a background solution to make a 500 mg/L Hydrochar-nZVI suspension. Because of the instability of the nZVI particles, the mixture was first sonicated, after which the pH was adjusted with HCl or NaOH to obtain the desired pH. The suspension mixture was vortexed for 10 min, after which the particles were allowed to settle before being injected into a zeta potential sample holder for testing. (ZP) was measured at pH values of 5.5, 6, and 7, and a ζ potential scatter diagram was finally generated to determine the isoelectric point and point of zero charge for the synthesized nanoparticles. Regardless of the technique used to create them, all nZVIs naturally possess the ZP. Every possible method generates nZVI with a unique ZP. How nanoparticles attract to one another and aggregate in the process depends on their (ZP) or electric potential. According to the output of the zeta sizer, particles are considered stable if their potential is more remarkable than +29.3 mV and less than -29.3 mV. The maximum instability and aggregation occur at zero as shown in Fig. 3(a), (b), (c).

Particle Size Distribution of nZVI

The same experiments, which were investigated for zeta potentials under different experimental conditions, further analyzed the zeta potentials to determine the size distribution of the particles in combination with nZVI in solution.

The same quantitative evaluation and analysis performed to determine the ZP concentration, intensity, and size using a zetazizer were also carried out for hydrochar alone with different concentrations of a background solution of NaCl: 1 mM, 5 mM, and 10 mM. The reason is that the particle sizes are significant when these particles are agglomerated in an aqueous solution, which indicates the effectiveness of their sorption capacity and removal efficiency in water remediation. Therefore, studying material kinetics in such experiments is highly important to ensure reliability and accuracy. Another reason that we wanted to be sure of compatibility is that both materials of the nZVI composite would have the same potential and dynamics shown in Fig.4 (^33–35.

Adsorption capacity at different concentrations but with a single pH of 6.0

Fig. 5 was the adsorption of cadmium by rice husk with the nZVI composite at different concentrations was carried out with the required adsorbate and an optimum pH of 6.0 at temperatures between 25 and 30°C, and a batch experiment was carried out. The adsorbate was the CdCl2 pollutant at different concentrations ranging from 5 mg/L to 200 mg/L. The cadmium concentrations in the aqueous solution before adsorption and after equilibrium were determined by an AAS 7000 instrument at a constant pH of 6.0. As explained in the previous sections, the adsorption isotherm depicts an equilibrium concentration curve of a solute on the surface of the adsorbent, which is designated q_{e, relative} to the solute concentration in the liquid, which is also designated C_e.

The lower the concentration of the pollutant, which is the metal of ion (Cd²⁺), in this study, the greater the percentage remover efficiency was. There was a direct inverse reciprocal relationship between the absorption isotherm and the removal efficiency of heavy metals. In this study, hydrochar doped with the nZVI composite was shown to have high removal efficiency, with a percentage (%) of 97.56% shown in Fig.5.

Table 1 Adsorption and Removal Capacity of Cadmium on nZVI/Hydrochar with mass (0,025g) of nZVI/Hyrochar composites

No.

Initial conc. (mg/L)

Equilibrium Conc:(mg/L)

Removal Capacity(mg/g)

7.553

17.509

23.598

96.65

157.32

232.09

0.18

5.85

11.24

20.66

81.49

139.43

211.15

14.7374

23.32

24.721

29.34

31.11

35.79

41.88

In the analysis of Table 1, there was a sense in which an increase in adsorption capacity decreased removal efficiency. When the concentration of the pollutant is high, it is the inverse proportionality of the removal efficiency. In this study, the lower the concentration of the pollutant, the metal ion (Cd2+), the higher the percentage remover efficiency; there is a direct inverse reciprocal relationship between absorption isotherm and removal efficiency of heavy metals. In this study, it was investigated that hydrochar that was doped with nZVI composites has a high-efficiency removal rate of 97.56%.

Adsorption capacity of single concentrations with variable pH values

pH is an important controlling factor for the operation of control experiments, and pH is an indicator of temperature; thus, it is crucial to determine whether a system usually works. This approach helps to stabilize the working solution or analyte, especially in control experiments. The pH was determined by a pH meter (METTLER TOLEDO, China) in all the experiments carried out in this study. The pH was adjusted according to the required value by adding concentrated/diluted NaOH solution and HCl for alkalinity or acidity.

Fig. 6(a) and (b) showed the effect of the system on pH control and removal, which was investigated using a pH meter. In anoxic solution, nitrogen flow was introduced into the aqueous solution. NaOH and HCl were used for pH adjustment to eliminate and control other ionic disturbances in the solution. The results of the analysis of the different pH values ranging from 4.0 to 5.0 to 6.0, 7.0, and 8.0 were investigated with a single pollutant concentration of 400 mg/L CdCl₂, and the results were obtained from complete detection with AAS after 3 hours of adsorption. These findings revealed that the equilibrium kinetic sorption capacity increased with increasing pH and that the percentage removal efficiency increased with decreasing pH; that is, there was an inverse proportionality. Increasing the pH from 6.0 to 8.0 ideally evoked a substantial increase in the adsorption capacity of the isotherm of the adsorbate in an aqueous solution of the CdCl₂ adsorbent. Subsequently, from pH 4.0 to 5.0, the adsorption capacity decreases gradually but with a high removal efficiency, according to the literature ³⁶. This qualitative anomalous behaviour of pH values on the quantity of cadmium that was adsorbed was previously studied and described in the literature by ^37–39

Table 2 Removal capacity of cadmium using nZVI/Hydrochar in 10 mM NaCl solution at different pHs

No.	pH	Equilibrium Conc. Of Cd (mg/L)	Removal Capacity of Material (mg/g)
1.	4.0	162.078	3.2
2.	5.0	158.498	7.21
3.	6.0	160.685	13.72
4.	7.0	150.279	23.64
5.	8.0	122.793	78.62

Table 2 Illustrates the effect of the system on pH control, and removal was investigated using a pH meter. In anoxic solution, the Nitrogen flow was introduced into aqueous solution. NaOH and HCl were used to adjust pH to eliminate and control other ion disturbances in the solution. The analysis on the conducted study of different pH ranges from 4.0, 5.0, 6.0, 7.0 and 8.0 was investigated with a single pollutant concentration of 400 mg/L CdCl_2, and results were obtained from the detection with AAS after adsorption of 3 hours. From those findings, it was observed that the equilibrium kinetics adsorption capacity increases with an increase in pH, and the percentage removal efficiency increases with decreases in pH; that is to say, there is inverse proportionality.

Sedimentation Kinetics of Composite nZVI

The sedimentation experiment was also carried out, as explained in the methodology section. Sedimentation is a phenomenon in which magnetic nanoparticles are characterized using dynamic scattering UV‒vis spectroscopy. Nanoparticles are redox-active constituents with smaller particles but larger surface areas. The sedimentation experiments showed that the nanoparticles cluster and aggregate to form sediment. It was observed that over time lapses or an interval of time, one could see that the rate of aggregation and sedimentation was dependent on the particle size, concentration aqueous condition, and the ionic strength of settlement. The analysis further revealed that nanoparticle aggregation and sedimentation predict the transportation and fate of particles in media for remediation. The dynamic light scattering in the suspended solution with various concentrations of nanoparticles (Co = 1.8685 g/L) further revealed that the aggregate particles were concentrated with the formation of a gel structure due to the strong magnetic attraction between ferromagnetic nanoparticles, which further reflects sedimentation kinetics ⁴⁰.

The iron concentration in the aqueous solution drastically decreased with time. This phenomenon agrees with hygienic environmental norms for domestic or municipal water. Instead of these findings, the iron concentration should be kept at 0.4 mg/L. With three different concentrations of the electrolyte background solution of NaCl (1, 5, and 10 mM), the iron concentration was reduced from the investigated result to the environmental standard concerning time. Fig.8. shows the trend of the iron concentration. These sedimentation kinetics further confirmed that nano zero-valent iron doped with hydrochar is effective for water cleaning ⁴¹

Nanotechnology

This research focuses on using Nanoscale zero-valent iron (nZVI) composite materials for wastewater remediation, particularly in eliminating persistent/recalcitrant pollutants in surface water and groundwater. nZVI particles, typically sized between 5-40nm, have shown effectiveness in reducing organic and inorganic impurities. The study highlights nZVI's practical applicability, including its emulsification of a wide range of contaminants, colloidal properties, long durability, high efficiency in iron utilization, and various contaminant removal modes. The research emphasizes nZVI's recognition as a highly effective adsorbent and ideal technology for in-situ remediation of heavy metals in contaminated groundwater or wastewater. Batch experiments were conducted to characterize the nZVI surface and assess its effectiveness in removing cadmium ions from water. The study explored the impact of solution properties such as pH, initial cadmium concentration, sorbent dosage, ionic strength, and competitive ions on cadmium removal by nZVI. The results indicated that nZVI effectively removed Cd (2+) ions through adsorption, decreasing removal in the presence of competitive cations. The research demonstrated that Cd (2+) removal increased with higher solution pH, reaching a maximum at pH 8.0, and identified chemisorption or physisorption processes as potential mechanisms for Cd (2+) adsorption on nZVI. The study underscores nZVI as an effective remediation process for various organic and inorganic pollutants in contaminated water sources. It suggests that the unique properties of iron nanoparticles contribute to their effectiveness in removing recalcitrant contaminants, making nZVI a significant player in 21st-century environmental remediation efforts. In specific literature reviews, the synthesis methods of interest are often denoted as "top-down" and "bottom-up."

The latter explicitly encompasses laboratory and commercial micrometre to millimetre-sized nano zero-valent iron (nZVI) production. This study focuses primarily on the bottom-up approach, explicitly addressing the antithesis of Fe2(SO4), which effectively reduces Fe2 in the liquid phase ⁴². The significance of nZVI in removing recalcitrant organic and inorganic contaminants cannot be overstated. Their unique properties, validated through characterization techniques such as X-RD, FTIR, TEM, SEM, etc. underscore their advantages over other technologies. In light of these considerations, this research contributes to the attainment of global standards established by organizations such as the World Health Organization (WHO), Environmental Protection Agency (EPA), and the European Drinking Water Directive. Notably, these standards stipulate that the cadmium content in municipal water should not exceed 0.005 mg/L, both in the water supply and wastewater effluence. This investigation seeks to identify viable approaches to align with and uphold these stringent regulatory benchmarks ¹³ .

The efficiency of the hydrochar-supported nZVI biosorbent used in this research for cadmium metal ion removal was high for surface wastewater and groundwater remediation and for contaminated soils. In addition to having a 97.56% removal efficiency for cadmium from wastewater by hydrochar-supported nZVI, this material also exhibited good characterization of its morphological structure when different tests were conducted with other laboratory equipment for various parameters, such as FTIR, TEM, XRD, sedimentation, zeta potential, hydrodynamics, and SEM. The biosorbent in this research was hydrochar with supported nZVI. pH has excellent adsorption capacity and good removal efficiency for cadmium from wastewater. The results of the present study confirmed that an increase in pH increases the adsorption capacity and removal efficiency. This further showed that this experiment's material selection was excellent and very efficient. The other innate ability of hydrochar was its ability to bind to nZVI, which was also observed in the sorption experiments. This type of research in the 21^st century is underway due to its completeness of wastewater and soil remediation for recalcitrant heavy metal contamination. There are also different reasons why the research methodology was based on adsorption but not on other processes, such as reverse osmosis, membrane technology, chemical precipitation, etc. Adsorption is a relatively inexpensive technology that is less capital, energy, and labour-intensive. It is environmentally friendly; in fact, it uses waste material in the environment (biomass), which is also removed from the environment from chronic contamination in the case of heavy metal pollution in the environment. With the incredible benefits of biosorption via related technology, the feasibility of regeneration and availability of natural biosorbents for environmental contaminate removal is considered.

Data Availability Statement: Data will be available from the corresponding author based on the reasonable request.

Author Contribution statement: NX and VS Overall supervision, JNM manuscript writing and formatting, JNM data collection, VS and NX data validation, VS and NX image preparation, NX and JNM project funding and financial approval.

Aaseth, J., Ajsuvakova, O. P., Skalny, A. V., Skalnaya, M. G. & Tinkov, A. A. Chelator combination as therapeutic strategy in mercury and lead poisonings. Coord. Chem. Rev. 358, 1–12 (2018).
van Vliet, M. T. H. et al. Global water scarcity including surface water quality and expansions of clean water technologies. Environ. Res. Lett. 16, (2021).
Ezzatahmadi, N. et al. Clay-supported nanoscale zero-valent iron composite materials for the remediation of contaminated aqueous solutions: A review. Chem. Eng. J. 312, 336–350 (2017).
Brusseau, M. L. & Artiola, J. F. Chemical Contaminants. Environmental and Pollution Science (Elsevier Inc., 2019). doi:10.1016/b978-0-12-814719-1.00012-4.
Yohannes, H. & Elias, E. Contamination of Rivers and Water Reservoirs in and Around Addis Ababa City and Actions to Combat It. Environ. Pollut. Clim. Chang. 01, (2017).
Zou, Y. et al. Environmental Remediation and Application of Nanoscale Zero-Valent Iron and Its Composites for the Removal of Heavy Metal Ions: A Review. Environ. Sci. Technol. 50, 7290–7304 (2016).
Yadav, K. K., Gupta, N., Kumar, V., Khan, S. A. & Kumar, A. A review of emerging adsorbents and current demand for defluoridation of water: Bright future in water sustainability. Environ. Int. 111, 80–108 (2018).
Younas, F. et al. Unveiling Distribution, Hydrogeochemical Behavior and Environmental Risk of Chromium in Tannery Wastewater. Water (Switzerland) 15, (2023).
Shakoor, A., Abdullah, M., Altaf, M. A., Sarfraz, R. & Batool, S. A comprehensive review on phytoremediation of cadmium (Cd) by mustard (Brassica juncea L.) and sunflower (Helianthus annuus L.) . J. Bio. Env. Sci 2017, 88–98 (2017).
Lasagna, M., De Luca, D. A. & Franchino, E. Nitrate contamination of groundwater in the western Po Plain (Italy): the effects of groundwater and surface water interactions. Environ. Earth Sci. 75, 1–16 (2016).
Charity, L. K. et al. Health Risks Of Consuming Untreated Borehole Water From Uzoubi Umuna Orlu, Imo State Nigeria. J. Environ. Anal. Chem. 05, (2018).
Zhao, Z. et al. Environmental implications from the priority pollutants screening in impoundment reservoir along the eastern route of China's South-to-North Water Diversion Project. Sci. Total Environ. 794, 148700 (2021).
Ulrich, A. E. Cadmium governance in Europe's phosphate fertilizers: Not so fast? Sci. Total Environ. 650, 541–545 (2019).
Rehman, K., Fatima, F., Waheed, I. & Akash, M. S. H. Prevalence of exposure of heavy metals and their impact on health consequences. J. Cell. Biochem. 119, 157–184 (2018).
Lentini, P. et al. Kidney and heavy metals - The role of environmental exposure (Review). Mol. Med. Rep. 15, 3413–3419 (2017).
Wu, M., Liu, H. & Yang, C. Effects of pretreatment methods of wheat straw on adsorption of Cd(II) from waterlogged paddy soil. Int. J. Environ. Res. Public Health 16, (2019).
Ozturk, M. et al. Molecular Biology of Cadmium Toxicity in Saccharomyces cerevisiae. Biol. Trace Elem. Res. 199, 4832–4846 (2021).
Gupta, V. K. et al. Study on the removal of heavy metal ions from industry waste by carbon nanotubes: Effect of the surface modification: A review. Crit. Rev. Environ. Sci. Technol. 46, 93–118 (2016).
Li, X., Kapoor, V., Impelliteri, C., Chandran, K. & Domingo, J. W. S. Measuring nitrification inhibition by metals in wastewater treatment systems: Current state of science and fundamental research needs. Crit. Rev. Environ. Sci. Technol. 46, 249–289 (2016).
Mukherjee, R., Kumar, R., Sinha, A., Lama, Y. & Saha, A. K. A review on synthesis, characterization, and applications of nano zero valent iron (nZVI) for environmental remediation. Crit. Rev. Environ. Sci. Technol. 46, 443–466 (2016).
Elkady, M. F., Mahmoud, M. M. & Abd-El-Rahman, H. M. Kinetic approach for cadmium sorption using microwave synthesized nano-hydroxyapatite. J. Non. Cryst. Solids 357, 1118–1129 (2011).
Obasi, P. N. & Akudinobi, B. B. Potential health risk and levels of heavy metals in water resources of lead–zinc mining communities of Abakaliki, southeast Nigeria. Appl. Water Sci. 10, (2020).
Mondal, N. K., Samanta, A., Chakraborty, S. & Shaikh, W. A. Enhanced chromium(VI) removal using banana peel dust: isotherms, kinetics and thermodynamics study. Sustain. Water Resour. Manag. 4, 489–497 (2018).
Younis, S. A., El-Salamony, R. A., Tsang, Y. F. & Kim, K. H. Use of rice straw-based biochar for batch sorption of barium/strontium from saline water: Protection against scale formation in petroleum/desalination industries. J. Clean. Prod. 250, 119442 (2020).
Nagel, S. R. Experimental soft-matter science. Rev. Mod. Phys. 89, (2017).
Ponder, S. M., Darab, J. G. & Mallouk, T. E. Remediation of Cr(VI) and Pb(II) aqueous solutions using supported, nanoscale zero-valent iron. Environ. Sci. Technol. 34, 2564–2569 (2000).
Fang, J., Ding, B. & Gleiter, H. Mesocrystals: Syntheses in metals and applications. Chem. Soc. Rev. 40, 5347–5360 (2011).
Mourdikoudis, S., Pallares, R. M. & Thanh, N. T. K. Characterization techniques for nanoparticles: Comparison and complementarity upon studying nanoparticle properties. Nanoscale 10, 12871–12934 (2018).
Sharma, R., Sarswat, A., Pittman, C. U. & Mohan, D. Cadmium and lead remediation using magnetic and non-magnetic sustainable biosorbents derived from Bauhinia purpurea pods. RSC Adv. 7, 8606–8624 (2017).
Stevens, M. & Batlokwa, B. Removal of Nickel (II) and Cobalt (II) from Wastewater Using Vinegar-Treated Eggshell Waste Biomass. J. Water Resour. Prot. 09, 931–944 (2017).
Dutta, A. Fourier Transform Infrared Spectroscopy. Spectroscopic Methods for Nanomaterials Characterization vol. 2 (Elsevier Inc., 2017).
Dong, H. et al. Physicochemical transformation of carboxymethyl cellulose-coated zero-valent iron nanoparticles (nZVI) in simulated groundwater under anaerobic conditions. Sep. Purif. Technol. 175, 376–383 (2017).
Lin, D., Hu, L., Lo, I. M. C. & Yu, Z. Size distribution and phosphate removal capacity of nano zero-valent iron (Nzvi): Influence of ph and ionic strength. Water (Switzerland) 12, 1–13 (2020).
Vilardi, G., Parisi, M. & Verdone, N. Simultaneous aggregation and oxidation of nZVI in Rushton equipped agitated vessel: Experimental and modelling. Powder Technol. 353, 238–246 (2019).
Chen, Y. Z., Zhou, W. H., Liu, F., Yi, S. & Geng, X. Microstructure and morphological characterization of lead-contaminated clay with nanoscale zero-valent iron (nZVI) treatment. Eng. Geol. 256, 84–92 (2019).
Khuntia, B. K. et al. Synthesis and Characterization of Zero-Valent Iron Nanoparticles, and the Study of Their Effect against the Degradation of DDT in Soil and Assessment of Their Toxicity against Collembola and Ostracods. ACS Omega 4, 18502–18509 (2019).
Chiou, M. S. & Li, H. Y. Equilibrium and kinetic modeling of adsorption of reactive dye on cross-linked chitosan beads. J. Hazard. Mater. 93, 233–248 (2002).
Paulino, A. T., Belfiore, L. A., Kubota, L. T., Muniz, E. C. & Tambourgi, E. B. Efficiency of hydrogels based on natural polysaccharides in the removal of Cd2+ ions from aqueous solutions. Chem. Eng. J. 168, 68–76 (2011).
Kenney, J. P. L. & Fein, J. B. Cell wall reactivity of acidophilic and alkaliphilic bacteria determined by potentiometric titrations and cd adsorption experiments. Environ. Sci. Technol. 45, 4446–4452 (2011).
Polyak, Y., Zaytseva, T. & Medvedeva, N. Response of toxic cyanobacterium microcystis aeruginosa to environmental pollution. Water. Air. Soil Pollut. 224, (2013).
Cheng, X., Xu, N., Huangfu, X., Zhou, X. & Zhang, M. Synergetic effect of hydrochar on the transport of anatase titanium dioxide nanoparticles in the presence of phosphate in saturated quartz sand. Environ. Sci. Pollut. Res. 25, 28864–28874 (2018).
Mueller, N. D. et al. Closing yield gaps through nutrient and water management. Nature490, 254–257 (2012).

No competing interests reported.

Download PDF

Version 1

posted

You are reading this latest preprint version

Removal of A Cadmium from Wastewater Using Hydrochar-Supported Nanoscale Zero-Valent Iron (nZVI) on Composite Materials

Status:

Version 1

Abstract

Figures

Introduction

Materials and Methods

Results and Discussion

Conclusion

Declarations

References

Additional Declarations

Status:

Version 1