Key Takeaways & Executive Findings
- •• • RMSBC800, prepared at 800 °C with a red mud-to-sludge mass ratio of 3:1, achieved a maximum phosphorus adsorption capacity of 28.57 mg/g, a 350% improvement over unmodified biochar (SBC), enabling high-efficiency phosphate removal from wastewater. • • Red mud modification enlarged the average pore size from 7.91 nm to 20.33 nm and increased the pHpzc from 2.01 to 3.37, reducing electronegativity and enhancing electrostatic attraction for phosphate anions. • • Adsorption followed pseudo-second-order kinetics and the Langmuir isotherm (1/n = 0.34), indicating monolayer chemisorption with a theoretical maximum capacity of 31.7 mg/g at pH 5 and an adsorbent dose of 4 g/L, achieving 83% removal in real wastewater. • • The material retained 61% of its initial adsorption capacity after five regeneration cycles, demonstrating reusability and cost-effectiveness for industrial applications, though sulfate and carbonate ions inhibited adsorption by 42.5% and 39%, respectively.
Abstract
To address the dual challenges of phosphorus resource scarcity and eutrophication control, this study proposed a sustainable waste-treats-waste strategy by preparing red-mud-modified sludge-based biochar (RMSBC) via co-pyrolysis of red mud and sewage sludge. The optimal material (RMSBC800), produced at 800 °C with a red mud-to-sludge mass ratio of 3:1, achieved a maximum phosphorus adsorption capacity of 28.57 mg/g, a 350% enhancement over unmodified biochar (SBC). Characterization (SEM, XRD, EDS, FT-IR, XPS) revealed that red mud modification increased the average pore size from 7.91 nm to 20.33 nm, reduced electronegativity, and raised the pH at point of zero charge (pHpzc) from 2.01 to 3.37. Adsorption kinetics followed the pseudo-second-order model, and isotherms fitted the Langmuir model, indicating monolayer chemisorption. The Freundlich parameter 1/n was 0.34, suggesting favorable adsorption. Mechanistic studies identified electrostatic attraction and surface precipitation as dominant, with molecular dynamics simulations confirming the critical role of Fe3O4 over Fe2O3 in adsorbing HPO4^2- due to stronger electrostatic interactions. The material retained 61% of its initial capacity after five regeneration cycles and achieved 83% phosphorus removal from real wastewater. This work demonstrates synergistic valorization of industrial wastes, offering an economically viable solution for phosphorus pollution control and resource recovery.
1. Introduction
Phosphorus is a finite resource, yet 15–20% of global phosphorus is lost through wastewater systems, with 75–90% accumulating in sewage sludge. In China, sludge production is projected to exceed 90 million tons by 2025, and recovering phosphorus from sludge could meet 5% of domestic demand. However, conventional phosphorus removal methods, such as biological treatment and membrane filtration, face limitations including high costs, secondary pollution, and difficulty in adsorbent regeneration. Crystallization processes like struvite or vivianite are efficient for high-concentration phosphorus but are economically prohibitive for low-strength wastewater. Sludge-based adsorbents offer a dual benefit of waste valorization and phosphorus removal, but their performance is often limited by low adsorption capacity. Metal doping, particularly with iron, aluminum, or magnesium oxides, can enhance adsorption by increasing surface positive charge and active sites, yet the use of pure metal reagents contradicts sustainability principles.
Red mud, a byproduct of alumina production, is rich in iron oxides (Fe2O3 30–60%), alumina (Al2O3 15–30%), and alkaline components. During high-temperature pyrolysis, these metal oxides can form composite structures with biochar, where Fe2O3 partially reduces to Fe3O4, imparting mixed-valence Fe(II)/Fe(III) that enhances phosphate adsorption via redox-mediated electron transfer, electrostatic attraction, and surface precipitation. This study leverages red mud as an iron oxide reservoir and sewage sludge as a porous carbon support, co-pyrolyzing them to produce a novel adsorbent (RMSBC). This approach not only addresses the disposal of two industrial wastes but also creates a cost-effective, high-performance material for phosphorus recovery, directly tackling the bottleneck of low adsorption capacity and poor reusability in existing adsorbents.
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ZHANG Chao, XIONG Renjiu, CAI Meiqiang, DONG Chunying (2026). Preparation of Red Mud-Modified Sludge-Based Biochar and Its Phosphorus Adsorption Efficiency and Mechanism. Journal of Environmental Engineering Technology. https://doi.org/10.13205/j.hjgc.202607018
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Frequently Asked Questions
What is the maximum phosphorus adsorption capacity of RMSBC800 and how does it compare to unmodified biochar?
RMSBC800, prepared at 800 °C with a red mud-to-sludge mass ratio of 3:1, exhibits a maximum adsorption capacity of 28.57 mg/g, which is 350% higher than that of unmodified biochar (SBC). This significant enhancement is attributed to the increased surface area, pore size (from 7.91 nm to 20.33 nm), and the presence of Fe3O4 active sites.
What are the dominant adsorption mechanisms and how were they verified?
The adsorption mechanisms include electrostatic attraction, surface precipitation, and physical diffusion. These were confirmed through comprehensive characterization (SEM, XRD, EDS, FT-IR, XPS) and molecular dynamics simulations, which showed that Fe3O4 has a stronger affinity for HPO4^2- than Fe2O3 due to enhanced electrostatic interactions. The formation of FePO4·2H2O via Fe–O–P coordination bonds was identified as a key surface precipitation pathway.
How does the presence of coexisting anions affect phosphorus adsorption, and what are the implications for real wastewater treatment?
Coexisting anions such as sulfate (SO4^2-) and carbonate (CO3^2-) significantly inhibit phosphorus adsorption, reducing capacity by 42.5% and 39%, respectively. This is due to competition for active sites. However, RMSBC800 still achieved 83% phosphorus removal in real wastewater samples, indicating its robustness against interfering ions, though pre-treatment may be necessary for waters with high sulfate or carbonate levels.
What is the regeneration performance of RMSBC800 and its potential for repeated use?
RMSBC800 retained 61% of its initial adsorption capacity after five regeneration cycles, demonstrating good reusability. This is crucial for cost-effectiveness in industrial applications, as it reduces the need for frequent adsorbent replacement. The gradual capacity loss suggests that regeneration conditions could be optimized to improve longevity.
What are the optimal operating conditions for maximum phosphorus removal using RMSBC800?
The maximum adsorption capacity of 31.7 mg/g was achieved at pH 5 with an adsorbent dose of 4 g/L. Under these conditions, the removal efficiency reached 83% in real wastewater. These parameters provide a baseline for scaling up to pilot or industrial applications, though site-specific water chemistry may require adjustments.
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