Removal of Chromium (VI) from Hydrometallurgical Effluents Using Moringa Waste: Isotherm, Kinetic and Thermodynamic Studies ^†

Abstract

The study aims to promote environmental restoration by shedding light on the potential use of moringa waste as an inexpensive, eco-friendly adsorbent for treating wastewater contaminated with Chromium (VI). FTIR was used to characterise the surface functional groups of moringa waste. The one-factor-at-a-time method was used to study the initial concentration in milligrams per litre, contact time in minutes, temperature in degrees Celsius, pH, and adsorbent dosage in milligrams per litre. The output was the removal percentage. Furthermore, adsorption isotherms, kinetics, and thermodynamic models were applied to understand the process behaviour. FTIR examination revealed the moringa waste structure’s stability and aromaticity, confirmed by peaks located around 1596 cm⁻¹ and the stretching of the hydroxyl group around 3321 cm⁻¹, which are important for enhancing Cr (VI) adsorption due to their capability to establish strong bonds with metal ions. Aromatic rings contribute to a large surface area and porosity and are stable; this is important for adsorption applications. At 60 min of contact time with a pH of 6 and 0.5 g of adsorbent dosage at 45 °C for a concentration of 100 mg/L, the highest removal percentage was found to be 77.03%. Adsorption data values indicated a good fit to the Langmuir isotherm model. The thermodynamic study showed that the process is endothermic and spontaneous, hence making the application of moringa waste in wastewater treatment viable.

1. Introduction

Hydrometallurgical effluents containing Chromium (VI) are serious environmental and health concerns on a global scale. According to [1], hydrometallurgy processes commonly used in mining, electroplating, and metallurgical production apply acidic solutions containing chromium compounds to extract metals. However, the discharge of these effluents containing elevated levels of toxic ions poses severe ecological risks, including groundwater contamination, soil degradation, and adverse impacts on aquatic ecosystems.

Removing chromium (VI) from hydrometallurgical industries using the usual methods often comes across challenges such as high cost, secondary pollution, and operational complexity. Thus, conventional techniques like chemical precipitation ion exchange and filtration are highly disadvantaged [2]. There is an increasing curiosity about less costly ways, friendlier means, and other options for eliminating chromium (VI). The employment of industrial natural adsorption agents is becoming popular. They are abundant, cheap, biodegradable and environmentally friendly. Moringa waste–a by-product of farming and processing the Moringa oleifera plant–is another probable adsorbent because of its high amount of protein [3].

In this sense, Moringa wastes are very promising and highly effective in removing Cr (VI) contamination from solutions, as they have low costs, excellent performance, fast adsorption kinetics, and extremely low environmental impact [4]. While most traditional purification methods have extensively utilised expensive chemicals and sophisticated technologies, in most cases forming hazardous by-products, moringa waste is an abundant natural resource often sourced locally, hence being economically feasible and environmentally friendly. In addition, it is quite practical due to its rapid Chromium (VI) adsorption from effluents with minimal energy input, which may be highly useful in resource-poor regions. [5] Researched the biodegradability of Moringa waste, which ensures its environmental safety; thus, it fits into the global ambitions of green technologies and sustainable development. While indeed promising, there is still much to be done to optimise its usage in varying environmental conditions such as pH, temperature, and contaminant concentration. Further application refinement can release even greater efficiencies by tailoring its use to a wide number of industries and geographies.

2. Materials and Methods

The study focuses on the adsorption of chromium (VI) onto moringa waste using a method involving the use of hydrochloric acid HCl, sodium hydroxide NaOH, potassium dichromate K₂Cr₂O₇, and phosphoric acid H₃PO₄. The moringa waste was prepared and characterised through carbonisation, activation, and FTIR analysis. The waste was crushed, dried, prepared in a reactor, placed in a fume hood, and heated to 350 °C. The biochar was activated using 0.1 M phosphoric acid and left to dry overnight. The surface functional groups and bonds of the moringa waste were determined using FTIR spectra. The adsorption batch experiments were conducted to determine the effect of each parameter at a time. The chromium (VI) stock solution was prepared by dissolving 2.8289 g of potassium dichromate K₂Cr₂O₇ in 1000 mL of distilled water. The effect of pH, adsorbent dosage, initial concentration, contact time, and temperature on chromium removal was investigated (

Table 1. Adsorption parameters and their variation levels.

Input	Range	Output
pH	2–10
	0.1–0.5
Dosage	25–65	Removal percentage
Temperature	15–75
Contact time	50–250
Initial concentration

). After the set time lapsed, samples were removed, and Cr content was determined using AAS. The adsorption isotherms, kinetics, and thermodynamics were analysed to understand the adsorption mechanism. The study aims to determine the adsorption capacity for chromium removal onto moringa waste and its potential applications in various industries.

3. Results and Discussion

This section presents and examines the results of this study. The findings shed light on the effectiveness and mechanism of chromium removal and are analysed in terms of adsorption isotherms, kinetics, and thermodynamics. Figure 1 shows FTIR spectra indicating surface-OH groups or water, O-H stretching vibrations, and structural changes during activation. Activated Moringa biochar displays a broader peak, while alkyl groups cause C-H stretching. C=O band stretch is possibly associated with carbonyl or ester content. The aromatic ring around 1596 cm⁻¹ corresponds to C=C stretching. Response in the area is low in raw and activated samples, with an unactivated sample showing a subdued reaction [2].

The Cr (VI) adsorption was investigated by increasing the absorbent dosage, with the maximum removal at 0.5 g for 61.60% in Figure 2a. Adsorption efficiency is low at low sorbent doses due to fewer active sites on the adsorbent surface for binding metal ions. However, more metal ions are adsorbed at higher adsorbent doses, increasing absorption efficiency until saturation [2]. It is demonstrated in Figure 2b, a higher efficiency of 77.30% was observed at 45 °C. From there, the results show a decrease in removal percentage as the temperature continued to rise. Higher temperatures can enhance the mobility of Cr (VI) ions in the solution, leading to a higher rate of diffusion to the adsorbent surface [6]. The results of the removal percentage during the variation in concentration are indicated in Figure 2c. The total amount of Cr (VI) available for adsorption is directly proportional to the increase in Cr (VI) initial concentration. This study’s highest removal percentage reached was 69.79% at 100 mg/L, and the lowest was 50.12% at 250 mg/L. It has been found that the pH of a solution significantly impacts metal adsorption in Figure 2d. Very little metal is removed in acidic conditions due to the competition between H+ ions and metal for adsorption sites. However, metal ion binding increases as pH increases, with pH 6 resulting in the highest removal percentage of 71.91%. High pH also increases the concentration of hydroxide ions, which can reduce the efficiency of Cr (VI) removal [7]. Adsorption was slow, with low removal percentages in the first 15 min compared to 75 min in Figure 2e. The percentage removal of Cr (VI) ion increased with increasing contact time up to 45 min, with minimal removal after 60 min.

Table 2. Isotherms.

Langmuir				Temkin
qmax (mg/g)	L (L/mg)	XL	R2	qT (mol/g)	C (J/mol)	R2
250	0.20	2.5 × 10−2	0.994	56.023	0.62	0.682

displays the correlation coefficient (R²) values. The experimental findings are good and consistent with isotherm investigations. The Langmuir isotherm model fits the data well in Figure 3. The model’s q_max values are consistent with the experimental values (250 mg.g⁻¹). The B value was discovered to be 0.20, and the X_L value indicated that the process is handy when X_L is between 0 and 1, irreversible when X_L equals 0, and linear when X_L = 1 [5].

The rate constant for the pseudo-first order is lower than that of the second order in Figure 4. This could indicate that the process occurs at a slightly faster rate. Chi-squared was used to validate the kinetic models. It is observed in

Table 3. Calculated results for the kinetics model.

Equations	Constant	qe (exp) (mg/g)	R2	X2	MPSD
Pseudo-first order	K1 = 0.01267/min	2.6037	0.926	0.0385	0.30
Pseudo-second order	K2 = 0.036 g/mg. min	2.582	0.917	0.8095	4.92

that the first-order model resulted in a lower chi-squared of 0.0385 as compared to that of the second order. The predicted adsorption capacity in the first model is closer to the experimental. This suggests that the process fits well with the pseudo-first-order model, also confirmed by the coefficient of correlation of 0.926, which is higher than the 0.917 for the second-order model indicated in Table 3. Furthermore, the first order resulted in an MPSD of 0.30% compared to the second order at 4.92%, indicating that the predicted data is closer to the experimental values. In contrast, the second order suggests a more significant deviation [3].

Table 4. Thermodynamic parameters for Cr (VI) ion adsorption.

Temp (K)	KL	$∆\mathit{G}°$(KJ/mol)	$∆\mathit{H}°$(KJ/mol)	$∆\mathit{S}°$	R2
298.15	1.944	−1.648	9.929	39.390	0.768
308.15	2.390	−2.232
318.15	3.115	−3.005
328,15	2.980	−2.979
333.15	2.898	−2.947

and Figure 5 display the levels of the thermodynamic parameters for Cr (VI) adsorption onto moringa waste. The negative values of the Gibbs free energy change confirm the process’s feasibility and spontaneity. The endothermic nature of the Cr (VI) adsorption process is confirmed by the positive value of $∆H°$. When Cr (VI) ions are adsorbed onto moringa waste, irregularity and randomness increase at the solid–aqueous solution interface, as indicated by a positive value of entropy change $∆S°$ [6].

4. Conclusions

FTIR examination revealed the moringa waste structure’s stability and aromaticity, confirmed by peaks located around 1596 cm⁻¹ and the stretching of the hydroxyl group around 3321 cm⁻¹, which are essential for enhancing Cr (VI) adsorption due to their capability to establish strong bonds with metal ions. Aromatic rings contribute to a large surface area and porosity and are stable; this is important for adsorption applications. At 60 min of contact time with a pH of 6 and 0.5 g of adsorbent dosage at 45 °C for a concentration of 100 mg/L, the highest removal percentage was found to be 77.03%. Adsorption data values indicated a good fit to the Langmuir isotherm model. It could be that the adsorption rate is affected by the number of sites available on the bio-sorbent, as the pseudo-first-order model indicated a better fit for this work. The thermodynamic study showed that the process is endothermic and spontaneous, making applying moringa waste in wastewater treatment viable.