Technological breakthroughs

A research team at Assiut University, Egypt, has recently published experimental findings on the use of waste rock from the Abu Tartur phosphate mine to adsorb toxic metal ions in water, including lead (Pb²⁺), copper (Cu²⁺), and cadmium (Cd²⁺).

Background

The Abu Tartur Plateau, located in Egypt’s Western Desert, is home to the largest phosphate mine in the Middle East. According to internal data from Misr Phosphate Company, mining activities at the site generated approximately 30 million tons of waste rock between 2008 and 2021, equivalent to around 3.3 million tons annually. Most of this waste consists of dolomite, black shale, glauconite, sandstone, and mudstone.

These waste rocks contain numerous hazardous elements such as cadmium, lead, mercury, chromium, and uranium. When exposed to weathering, they can leach into nearby water sources and contaminate agricultural land. This represents a long-term environmental challenge commonly associated with large phosphate mining operations.

This raises an important question: instead of simply burying the waste or leaving it exposed to natural weathering, could these materials be transformed into useful products?

Materials and Methods

The researchers collected phosphate dolomite samples (denoted as PD) and black shale from the waste dumps of the Abu Tartur mine. The PD samples were calcined at 1,100°C to produce calcined phosphate dolomite (CPD). CPD was then combined with black shale and aluminum in an alkaline environment to synthesize a new material called sodalite-based phosphate dolomite (SBPD).

Sodalite is a mineral belonging to the zeolite group, characterized by a microporous structure with a large surface area and well-known adsorption capabilities.

The materials were analyzed using X-ray diffraction (XRD), X-ray fluorescence (XRF), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and the BET method to determine surface area and pore structure.
Adsorption experiments were conducted using artificial aqueous solutions containing Pb²⁺, Cu²⁺, and Cd²⁺ at various concentrations. Parameters investigated included adsorbent dosage, pH, contact time, and initial metal concentration.

Main Findings

Surface area increased significantly after synthesis. The raw PD material exhibited a BET surface area of 6.8 m²/g, whereas SBPD reached 40.7 m²/g—nearly a sixfold increase. Pore volume also increased from 0.092 cc/g to 0.164 cc/g. These factors explain why SBPD demonstrated superior adsorption performance compared to PD.

Lead removal efficiency. Using 0.2 g of SBPD in a solution containing 1,000 ppm Pb²⁺ at pH 3 with a contact time of 30 minutes resulted in 100% removal efficiency. PD also showed strong performance at lower concentrations: 0.4 g of PD removed 99% of Pb²⁺ from a 350 ppm solution.

Copper removal efficiency. SBPD achieved 92.4% removal from a 300 ppm Cu²⁺ solution at pH 4, whereas PD achieved 69.3% removal from a 100 ppm solution.

Cadmium removal efficiency. SBPD completely removed (100%) Cd²⁺ from a 300 ppm solution at pH 3. PD was not tested for cadmium under optimized conditions.

For both materials, the adsorption affinity followed the order: Pb²⁺ > Cu²⁺ > Cd²⁺. This was attributed to differences in electronegativity and hydration energy among the ions. Pb²⁺ has higher electronegativity and lower hydration energy (1,502 kJ/mol), making it more readily adsorbed than Cu²⁺ and Cd²⁺.

Rapid equilibrium time. Both PD and SBPD reached maximum adsorption efficiency after approximately 30 minutes of contact with all three metal ions.

Mixed-metal experiments. When Pb²⁺, Cu²⁺, and Cd²⁺ were simultaneously present in the same solution, the removal efficiency for each ion decreased compared to single-ion experiments because of competition for adsorption sites. However, the adsorption preference order of Pb²⁺ > Cu²⁺ > Cd²⁺ remained unchanged.

Adsorption Model Analysis

The research team applied the Langmuir and Freundlich isotherm models to describe the adsorption mechanisms.

The adsorption of Pb²⁺ and Cu²⁺ onto both PD and SBPD fit the Langmuir model well (R² > 0.87), suggesting monolayer adsorption on a homogeneous surface. The maximum adsorption capacity (q_max) of SBPD for Pb²⁺ reached 123.5 mg/g, significantly higher than that of PD (19.7 mg/g).

For Cd²⁺ adsorption onto SBPD, the Freundlich model provided a better fit (R² = 0.98), indicating multilayer adsorption on a heterogeneous surface. This may be related to weaker interactions between Cd²⁺ and the material surface compared with Pb²⁺ and Cu²⁺.

In terms of kinetics, the pseudo-second-order model showed the best agreement with the experimental data (R² > 0.96), indicating that the adsorption process was primarily chemical in nature (chemisorption). The intraparticle diffusion model also fit the data, suggesting that adsorption occurred through multiple sequential stages rather than a single-step process.

Evaluation

The study demonstrates that both raw PD and synthesized SBPD possess the ability to adsorb heavy metals under laboratory conditions, with SBPD showing markedly superior performance due to its larger surface area and zeolite-like structure.

A major advantage of this approach is the utilization of locally available mining waste without requiring expensive commercial raw materials. Consequently, treatment costs may be considerably lower than those associated with many other adsorption methods currently under investigation.

However, these findings were obtained using artificial laboratory solutions. Real wastewater typically contains a wide range of impurities that may affect adsorption efficiency. Furthermore, the synthesis of SBPD requires high-temperature calcination and hydrothermal treatment, both of which must be evaluated in terms of cost and feasibility on a larger scale before practical application can be considered.

The article was published in Scientific Reports (Springer Nature) in June 2026.