The natural concentration of heavy metal ions in water does not pose a threat to human or ecological health. However, the primary cause of heavy metal pollution in aquatic environments is anthropogenic activities, such as industrial discharges, agricultural fertilizers, construction activities, and vehicular emissions. Among these, the most significant contributor is the unregulated discharge of industrial wastewater. Industries such as chemical manufacturing, leather processing, mining, metallurgy, electroplating, and paper production release excessive amounts of metal ions like chromium (Cr), zinc (Zn), lead (Pb), nickel (Ni), iron (Fe), manganese (Mn), and mercury (Hg) into water bodies. These discharges raise the concentrations of metal ions far beyond the tolerance limits of natural ecosystems, causing irreversible damage and posing serious threats to aquatic organisms and human health.
Heavy metal-contaminated water is highly toxic, and its toxicity can be concentrated and accumulated through environmental cycling. As rivers flow, rainwater recycles, and ocean currents circulate, this pollution gradually spreads to various parts of the ecosystem. Through bioaccumulation and biomagnification in food chains, heavy metals eventually accumulate in living organisms, leading to chronic poisoning and the onset of various difficult-to-treat diseases. Therefore, it is essential to find effective methods to address heavy metal pollution.
Graphene, a two-dimensional carbon nanomaterial composed of carbon atoms arranged in a hexagonal honeycomb lattice via sp² hybridization, offers promising solutions. Due to its excellent optical, electrical, and mechanical properties, graphene has significant potential in materials science, micro/nano-fabrication, energy storage, biomedicine, and drug delivery. It is widely regarded as a revolutionary material of the future.
Adsorption is one of the most effective methods for treating polluted water. Among the key challenges in the adsorption process is determining the optimal dosage ratio between the adsorbent and the wastewater. Orthogonal experiments provide an efficient method for studying multi-factor, multi-level problems. As shown in Table 1, representative experimental points are selected from a comprehensive matrix based on the orthogonality principle. These points are evenly distributed, systematically comparable, and suitable for evaluating the influence of various factors. Orthogonal experimental design is a major strategy in fractional factorial design and is recognized for its efficiency, speed, and cost-effectiveness in experimental planning.
Table 1. Values for Each Level of the Single-Factor Experiment
| Factor | Experimental Conditions | ||
| 1 | 2 | 3 | |
| Magnetic nanoparticles (A)/g | 0.10 | 0.15 | 0.20 |
| Time (B)/min | 45 | 60 | 75 |
| pH value (C) | 1 | 3 | 5 |
| Temperature (D)/°C | 30 | 40 | 50 |
Instruments and Reagents
Instruments: Oscillator, pH meter, spectrophotometer, Erlenmeyer flask, graduated cylinder, pipette, rubber suction bulb, volumetric flask, colorimetric tube.
Reagents: Chromium standard stock solution (0.1 g·L-1), working chromium standard solution (0.001 g·L-1), sulfuric acid (1+1), phosphoric acid (1+1), graphene, diphenylcarbazide solution (color reagent).
Experimental Procedure
1. Preparation of Simulated Chromium-Containing Wastewater
Prepare a solution with a concentration of 0.01 g·L-1.
2. Drawing the Standard Curve
Pipette 0, 0.50 mL, 1.00 mL, 3.00 mL, 5.00 mL, 7.00 mL, and 10.0 mL of the working chromium standard solution into separate 50 mL colorimetric tubes. Dilute with water to the marked line. Add 0.5 mL of sulfuric acid (1+1) and mix well. Then add 0.5 mL of phosphoric acid (1+1) and mix. Finally, add 2 mL of the color reagent (diphenylcarbazide solution) and mix thoroughly. Prepare the standard color series as shown in Table 2.
3. Color Development and Measurement
After allowing the color to develop for 5–10 minutes, measure the absorbance at a wavelength of 540 nm using a 10 mm pathlength cuvette, with water as the reference. Subtract the absorbance of the blank (0 concentration) to obtain the corrected absorbance. Plot a standard curve of Cr⁶⁺ content (mg) versus corrected absorbance.
4. Sample Measurement
Accurately transfer 50 mL of 0.01 g·L-1 simulated chromium-containing wastewater into a 300 mL Erlenmeyer flask. Add a specific amount of graphene and oscillate at a designated temperature for a specified time. Then, using a 1 mL pipette, transfer 0.5 mL of the solution into a 50 mL colorimetric tube. Dilute to the mark with water, add 0.5 mL sulfuric acid (1+1), and mix well. Add 0.5 mL phosphoric acid (1+1) and mix. Finally, add 2 mL of color reagent and mix. Measure the absorbance at 540 nm using a spectrophotometer. Determine the Cr6+ concentration from the standard curve.
Table 2. Orthogonal Experimental Factor-Level Combination Scheme
| Group | A | B | C | D | Removal Rate (%) |
| 1 | 1 | 1 | 1 | 1 | |
| 2 | 1 | 2 | 2 | 2 | |
| 3 | 1 | 3 | 3 | 3 | |
| 4 | 2 | 1 | 2 | 3 | |
| 5 | 2 | 2 | 3 | 1 | |
| 6 | 2 | 3 | 1 | 2 | |
| 7 | 3 | 1 | 3 | 2 | |
| 8 | 3 | 2 | 1 | 3 | |
| 9 | 3 | 3 | 2 | 1 |
Reference
- Li, Leilei, et al. "Adsorbent for chromium removal based on graphene oxide functionalized with magnetic cyclodextrin–chitosan." Colloids and Surfaces B: Biointerfaces 107 (2013): 76-83.
