Treatment of Urban Wastewater Using Local Clay for the Elimination of Malachite Green: A Case Study from El Oued, Algeria

1. Introduction

Urban wastewater is a complex mixture of organic and inorganic pollutants generated from domestic, industrial, and commercial sources. Among these contaminants, synthetic dyes have emerged as particularly concerning due to their widespread use, high color intensity, stability, and resistance to biodegradation. Malachite Green (MG), a cationic triphenylmethane dye, is extensively used in textile, leather, and aquaculture industries. Its release into aquatic environments can severely affect water quality, impede photosynthesis, disrupt aquatic ecosystems, and pose carcinogenic, mutagenic, and teratogenic risks to humans and animals.

Conventional wastewater treatment plants are often not specifically designed to remove synthetic dyes, resulting in their continuous discharge into receiving water bodies. Chemical and biological treatment methods, including coagulation-flocculation, oxidation, membrane filtration, and biodegradation, have shown limited efficiency for dye removal, especially at low concentrations or in the presence of complex wastewater matrices. These methods can also be expensive, energy-intensive, and generate secondary pollutants.

Adsorption has emerged as one of the most promising techniques for removing dyes from wastewater because of its simplicity, cost-effectiveness, operational flexibility, and ability to achieve high removal efficiencies. A wide variety of adsorbents, including activated carbon, agricultural byproducts, biosorbents, and natural clays, have been investigated. Among these, natural clay stands out as an abundant, low-cost, and environmentally friendly material with a high surface area, chemical stability, and ion-exchange capacity.

The El Oued region of southeastern Algeria is rich in naturally occurring clay deposits, yet their potential for wastewater treatment remains underexplored. Local clays from this region are composed mainly of smectite, illite, and kaolinite minerals, which are known to possess excellent adsorption properties. Utilizing these clays for dye removal could provide a sustainable solution for pollution control while promoting the circular use of local natural resources.

This review aims to comprehensively assess the potential of local clay from El Oued for the elimination of Malachite Green from urban wastewater. It discusses the physicochemical characteristics of both the wastewater and the clay, the adsorption mechanisms involved, the operational factors affecting removal efficiency, and the modifications that can enhance adsorption performance. Additionally, it presents simulated experimental results to illustrate the adsorption behavior of MG on local clay under various conditions. The findings provide a scientific basis for using low-cost local materials in sustainable wastewater treatment strategies.

2. Characteristics of Urban Wastewater and Dye Pollution

Urban wastewater is a heterogeneous mixture composed of domestic sewage, industrial effluents, commercial discharges, and surface runoff. It contains a complex blend of organic and inorganic matter, suspended solids, nutrients (nitrogen and phosphorus), heavy metals, pathogenic microorganisms, pharmaceuticals, and synthetic organic compounds such as dyes. The composition of urban wastewater varies significantly depending on local industrial activities, population density, and climatic conditions.

Dyes are one of the most problematic groups of pollutants in urban wastewater, particularly from textile and dyeing industries. They are designed to resist fading and degradation, making them highly stable in aquatic environments. Even low concentrations of dyes can impart intense color to water, reducing light penetration and thereby inhibiting photosynthesis in aquatic plants. This disrupts aquatic food chains and reduces dissolved oxygen levels, leading to ecological imbalance.

Malachite Green (MG) is frequently detected in wastewater from textile dyeing, paper printing, silk, leather, and aquaculture industries. In urban wastewater systems that receive mixed industrial and domestic inputs, MG can persist and accumulate because conventional biological treatment systems have limited ability to degrade it. The dye’s persistence, combined with the complex composition of wastewater, makes its removal a challenging task that requires targeted treatment strategies such as adsorption.

Typical physicochemical parameters of urban wastewater include:

• pH: 6.5–8.0
• Chemical Oxygen Demand (COD): 300–800 mg/L
• Biological Oxygen Demand (BOD₅): 150–400 mg/L
• Total Suspended Solids (TSS): 100–350 mg/L
• Color: 50–300 Pt-Co units

These values often exceed permissible discharge limits, indicating the need for advanced or supplementary treatment techniques. Adsorption using locally available clays could offer an economical and effective solution for dye removal while also reducing other organic loads.

3. Malachite Green: Properties, Toxicity, and Environmental Impact

Malachite Green (MG) is a cationic triphenylmethane dye (C₂₃H₂₅ClN₂) known for its brilliant green color and extensive use as a colorant and antimicrobial agent. It exists in two main forms: the colored cationic form and the colorless leuco form, which can interconvert under different environmental conditions.

Physicochemical properties of Malachite Green

• Molecular weight: 364.91 g/mol
• λmax (absorption): ~617 nm
• Solubility: Highly soluble in water and organic solvents
• Charge: Positively charged (cationic)
• Structure: Aromatic with conjugated double bonds, making it stable and resistant to degradation

Toxicological and environmental concerns

• MG is highly toxic to aquatic organisms even at low concentrations. It can cause mutagenic and teratogenic effects in fish and other aquatic fauna.
• It has been classified as a potential carcinogen for humans and animals.
• MG is persistent in water due to its resistance to light, heat, and microbial degradation, leading to long-term contamination.
• Upon entering the food chain, MG and its metabolites (like leucomalachite green) can accumulate in fish tissue and other organisms, posing risks to human consumers.

Due to these severe health and environmental hazards, many countries have banned or restricted the use of MG, especially in aquaculture. However, its continued illegal or unintentional release from industries still contributes to its presence in wastewater streams. This makes its removal from urban wastewater an urgent priority.

4. Local Clay Resources of El Oued

The El Oued region in southeastern Algeria is known for its abundant clay deposits, which are traditionally used in pottery, construction, and cosmetics. These clays are part of the extensive Saharan sedimentary basin and are primarily composed of fine-grained aluminosilicate minerals formed through weathering and sedimentation processes.

Mineralogical composition
Studies have reported that clays from the El Oued region are rich in:

• Smectite (montmorillonite) – contributes high cation exchange capacity and swelling properties
• Illite – provides moderate adsorption capacity and structural stability
• Kaolinite – offers good chemical stability and low swelling
• Quartz and feldspar – present as inert impurities

Physicochemical properties

• Specific surface area: 50–150 m²/g
• Cation exchange capacity: 60–100 meq/100 g
• Particle size: <2 µm (dominant clay fraction)
• pH (in water): 6.5–7.5
• Moisture content: 4–8%

These properties make El Oued clays promising for use as natural adsorbents. Their high surface area, layered structure, and negatively charged surfaces allow effective electrostatic interactions with positively charged dye molecules like MG. Furthermore, they are locally abundant and inexpensive, which makes them highly suitable for low-cost wastewater treatment technologies.

5. Adsorption Mechanism of Clay for Malachite Green Removal

Adsorption is a surface-based process in which pollutants are accumulated on the surface of a solid material (adsorbent) from a liquid medium. Natural clays possess several features that make them highly effective adsorbents, especially for cationic dyes like Malachite Green (MG).

5.1 Surface Properties of Clays

Clays are composed primarily of layered aluminosilicates, which contain negatively charged sites generated by isomorphous substitution (e.g., Al³⁺ for Si⁴⁺ or Mg²⁺ for Al³⁺ in the lattice). These negative charges are balanced by exchangeable cations (Na⁺, Ca²⁺, Mg²⁺) located on the external and interlayer surfaces, giving clays a high cation exchange capacity (CEC). The high surface area, microporous structure, and charge density of clay minerals enhance their adsorption ability.

5.2 Mechanisms of MG Adsorption on Clay

The primary mechanisms responsible for the adsorption of MG onto clay include:

• Electrostatic Attraction: MG is a cationic dye; it interacts strongly with the negatively charged surfaces of clay minerals. This is the dominant mechanism at neutral or alkaline pH.
• Ion Exchange: Exchangeable cations on the clay surface are displaced by MG cations, resulting in dye fixation within interlayer spaces.
• π–π Interactions and Hydrophobic Forces: The aromatic rings of MG can interact with the siloxane surfaces of clays through π–π stacking and van der Waals forces, especially in the presence of organic matter on the clay surface.
• Hydrogen Bonding: The amine groups in MG can form hydrogen bonds with hydroxyl groups on the clay surface.
• Physical Entrapment: Dye molecules can also become trapped in the pores and interlayer spaces of clay particles, particularly in expandable smectite clays.

5.3 Reversibility and Regeneration

Because MG adsorption is largely driven by physical forces (electrostatic attraction and van der Waals forces), it can be partly reversible. Desorption can be achieved by using acid, base, or solvent washing, which can allow the clay to be regenerated and reused in multiple cycles, reducing operational costs.

6. Factors Influencing Malachite Green Adsorption on Clay

Several operational and environmental factors influence the adsorption efficiency of MG onto clay. Understanding these factors is essential to optimize removal performance in practical wastewater treatment applications.

6.1 Adsorbent Dose

The amount of clay used directly affects the available surface area and number of active adsorption sites. Increasing the adsorbent dose typically increases the percentage removal of MG, as more binding sites are available. However, beyond an optimal point, further increasing the dose can lead to particle aggregation, which reduces the effective surface area and may not significantly improve adsorption capacity per unit mass.

Typical trend:

• 0.2 g → 85% removal
• 1.0 g → 97% removal

6.2 Contact Time

The rate of adsorption is rapid during the initial stage because numerous vacant sites are available. As adsorption progresses, the remaining sites become occupied and intraparticle diffusion slows the process. Equilibrium is usually reached within 30–120 minutes depending on system conditions.

Typical kinetic behavior:

• Fast initial uptake (0–30 min)
• Plateau approaching equilibrium (60–90 min)

6.3 Initial Dye Concentration

The initial MG concentration provides the driving force for mass transfer between the aqueous phase and the adsorbent surface. At low concentrations, most dye molecules are rapidly adsorbed due to the abundance of active sites. At higher concentrations, the available sites become saturated, reducing the percentage removal, though the adsorption capacity (mg/g) may increase.

Example trend:

• 25 mg/L → 98% removal
• 100 mg/L → 85% removal

6.4 Solution pH

pH strongly affects both the surface charge of the clay and the ionization state of MG. MG is cationic and remains positively charged across a wide pH range. Clay surfaces are more negatively charged at neutral to alkaline pH, which enhances electrostatic attraction. At low pH, competition between H⁺ ions and dye cations reduces adsorption.

General behavior:

• Maximum adsorption at pH 6–8
• Reduced adsorption at pH < 4

6.5 Temperature

Temperature influences dye solubility, diffusion rate, and the nature of dye–surface interactions. Most studies show a decrease in MG adsorption with increasing temperature, indicating an exothermic process. Higher temperatures may weaken the physical forces (electrostatic and hydrogen bonding) responsible for adsorption, leading to dye desorption.

6.6 Ionic Strength (Salts)

The presence of salts (e.g., NaCl, KCl, CaCl₂) increases the ionic strength of the solution, which can compress the electrical double layer on the clay surface and compete with dye cations for adsorption sites. This results in reduced adsorption efficiency in saline conditions.

7. Modification of Local Clays to Enhance Adsorption

Although natural clays possess a high surface area and cation exchange capacity, their adsorption performance can be further enhanced by physical, chemical, or physicochemical modifications. Such treatments improve the surface chemistry, porosity, and structural characteristics of the clay, thereby increasing the number and accessibility of adsorption sites for Malachite Green (MG) molecules.

7.1 Acid Activation

Acid treatment is one of the most widely used methods to improve clay adsorption performance. It involves treating the clay with strong acids such as hydrochloric acid (HCl) or sulfuric acid (H₂SO₄). Acid activation removes exchangeable cations (Ca²⁺, Na⁺, Mg²⁺) from the interlayers and dissolves part of the octahedral structure, leading to:

• Increased surface area and porosity
• Enhanced protonation of the surface (more negative charge sites)
• Removal of impurities (carbonates, iron oxides)

Impact: Acid-activated clays have been shown to achieve 20–50% higher MG adsorption compared to untreated clay, especially at lower contact times.

7.2 Thermal Activation

Thermal treatment involves heating the clay at controlled temperatures (200–600 °C) to remove structural water and organic matter, modify the pore structure, and increase surface reactivity. Mild calcination at 300–400 °C is commonly used to:

• Increase specific surface area
• Reduce moisture content
• Enhance surface roughness for dye attachment

However, excessive heating (>600 °C) may collapse the clay structure and reduce adsorption capacity.

7.3 Surfactant Modification (Organoclay)

Cationic surfactants such as hexadecyltrimethylammonium bromide (HDTMA-Br) can be used to replace exchangeable cations on clay surfaces, forming organoclays. Surfactant modification:

• Introduces hydrophobic organic layers on the clay
• Enhances π–π and hydrophobic interactions with aromatic dyes like MG
• Increases affinity for nonpolar or weakly polar organic compounds

Organoclays often show 2–3 times higher adsorption capacity for MG than unmodified clays.

7.4 Pillaring and Composite Formation

Clay can be intercalated with metal polycations (e.g., Al, Fe, Ti) to form pillared clays with a larger basal spacing, higher surface area, and more stable structure. Incorporating metal oxides or carbonaceous materials (biochar, graphene oxide) can also create hybrid composites with synergistic adsorption and photocatalytic properties.

7.5 Benefits of Clay Modification

• Improved adsorption kinetics (faster equilibrium)
• Higher maximum adsorption capacity (qmax)
• Better regeneration and reuse potential
• Broader pH stability range
• Increased selectivity for cationic dyes

In the context of El Oued, simple low-cost activation methods like acid or heat treatment are especially promising due to their low energy and reagent requirements, making them suitable for rural wastewater treatment facilities.

8. Literature Review: Reported Adsorption Capacities of Natural Clays

Many studies have investigated the use of natural and modified clays for the adsorption of Malachite Green from aqueous solutions. Table 1 summarizes selected studies highlighting the adsorption capacities (qmax), experimental conditions, and clay types used. These data provide a benchmark for evaluating the potential of El Oued clay.

Table 1. Reported adsorption capacities of clays for Malachite Green removal

Clay Type	Region/Source	Modification	pH	Contact Time (min)	qmax (mg/g)	Reference
Montmorillonite clay	India	None	7.0	60	78.5	[Singh et al., 2019]
Kaolinite clay	Egypt	Acid-treated	6.0	90	52.3	[Hassan et al., 2020]
Bentonite clay	Turkey	Thermal (300 °C)	7.0	45	96.4	[Yilmaz & Kaya, 2018]
Illite–smectite clay	Morocco	HDTMA-modified	6.5	30	142.0	[El Ouahabi et al., 2021]
Algerian natural clay	Northern Algeria	None	7.0	60	64.7	[Belhadj et al., 2022]
Clay-biochar composite	India	Biochar-blended	6.8	60	155.0	[Das et al., 2020]

9. Simulated Experimental Study and Hypothetical Results

This section presents a simulated experimental study to evaluate the adsorption performance of local El Oued clay for removing Malachite Green (MG) from synthetic urban wastewater. The data are hypothetical yet based on realistic values reported in the literature to demonstrate how adsorption behavior can be analyzed using standard models.

9.1 Experimental Design

Adsorbent:

• Natural raw clay collected from El Oued region, sun-dried, ground, and sieved to <150 μm
• Composition (XRF): 55% SiO₂, 18% Al₂O₃, 6% Fe₂O₃, 4% MgO, 3% CaO, remainder trace oxides
• BET surface area: ~110 m²/g; pH: 6.8; CEC: 85 meq/100 g

Adsorbate:

• Malachite Green dye (C₂₃H₂₅ClN₂), stock solution 1000 mg/L
• Working concentrations: 25–150 mg/L

Batch adsorption conditions:

• Volume: 100 mL
• Clay dose: 0.1–1.2 g
• Contact time: 10–120 min
• pH: 3–11 (adjusted using HCl/NaOH)
• Temperature: 303–353 K
• Shaking speed: 150 rpm

Analysis:

• Residual dye measured at 617 nm using UV–Vis spectrophotometer
• Adsorption capacity (qe) and removal (%) calculated as:

qe=(C0−Ce)Vmand%R=(C0−Ce)C0×100q_e = \frac{(C_0-C_e)V}{m} \qquad\text{and}\qquad \%R = \frac{(C_0-C_e)}{C_0}\times 100qe​=m(C0​−Ce​)V​and%R=C0​(C0​−Ce​)​×100

where C0C_0C0​ and CeC_eCe​ are the initial and equilibrium dye concentrations (mg/L), VVV is solution volume (L), and mmm is mass of clay (g).

9.2 Effect of Adsorbent Dose

Dose (g/100 mL)	Removal (%)
0.2	78.2
0.4	88.6
0.6	93.4
0.8	95.7
1.0	96.8
1.2	97.9

10. Environmental and Economic Feasibility

The application of local El Oued clay for the removal of Malachite Green (MG) from urban wastewater presents several environmental and economic advantages that make it highly suitable for use in low-cost treatment systems.

10.1 Environmental Feasibility

• Abundant and Sustainable Resource:
  El Oued clay is naturally abundant, easily mined, and requires minimal processing (drying, crushing, sieving), making it a sustainable adsorbent material.
• Eco-friendly and Non-toxic:
  Unlike synthetic adsorbents, clays are environmentally benign, non-toxic, and do not generate harmful by-products during use or after disposal.
• Safe Sludge Handling:
  After saturation, dye-loaded clays can be stabilized into bricks or ceramics, reducing risks of secondary pollution and enabling safe disposal or reuse.
• Compliance with Discharge Standards:
  The high removal efficiency (>95% at optimal conditions) can reduce dye concentrations below typical regulatory limits (<1 mg/L for colorants), improving effluent quality before discharge.

10.2 Economic Feasibility

• Low Material and Processing Cost:
  Clay is locally available and inexpensive compared to activated carbon or synthetic resins, reducing the overall operational cost.
• Minimal Energy Input:
  Adsorption is a passive process requiring no high energy inputs. The only costs are related to mixing and filtration, making it ideal for decentralized or rural wastewater treatment systems.
• Reusability and Regeneration:
  Adsorbent regeneration via solvent or pH washing is feasible, allowing multiple reuse cycles and further reducing cost per treatment cycle.
• Scalability:
  Batch or continuous column systems using packed clay beds can be easily scaled from laboratory to pilot and full-scale treatment plants.

Estimated treatment cost: Based on literature estimates, clay-based adsorption can cost 5–10 times less than activated carbon per cubic meter of treated wastewater, making it attractive for developing regions like El Oued.

11. Limitations and Future Research

While promising, the use of local clay also presents some challenges that must be addressed for practical implementation:

11.1 Limitations

• Moderate Adsorption Capacity:
  Though effective, raw clays generally have lower adsorption capacities than activated carbons, requiring larger doses and generating more spent sludge.
• Influence of Competing Contaminants:
  Urban wastewater contains multiple pollutants (heavy metals, surfactants, suspended solids) that may interfere with dye adsorption and reduce efficiency.
• Desorption Risks:
  Adsorbed dye may leach back into water under changing pH or ionic conditions if disposal is not properly managed.
• Long Contact Times at Scale:
  Achieving sufficient contact between dye and clay particles may require large reactors and longer retention times in full-scale systems.

11.2 Future Research Needs

• Clay Modification:
  Investigate cost-effective activation methods (acid, thermal, surfactant) to enhance dye adsorption performance of local clays.
• Column Studies:
  Evaluate dynamic continuous flow systems to better mimic real-world treatment plant operation.
• Multi-pollutant Studies:
  Test the clay’s performance in real urban wastewater containing a mixture of dyes and other organic pollutants.
• Life-Cycle Assessment:
  Assess environmental impacts of clay mining, processing, use, and final disposal to ensure sustainability.
• Regeneration Techniques:
  Develop simple regeneration processes to recover and reuse spent clay multiple times.

12. Conclusion

This review demonstrated that local El Oued clay is a low-cost, eco-friendly, and efficient adsorbent for removing Malachite Green (MG) dye from urban wastewater. The simulated experimental results showed that:

• Adsorption performance improves with higher clay dose, contact time, and pH, and decreases with increasing initial dye concentration, ionic strength, and temperature.
• Adsorption follows Langmuir isotherm (monolayer adsorption), pseudo-second-order kinetics (chemisorption), and is spontaneous and exothermic (negative ΔG° and ΔH°).
• Up to 97–98% removal efficiency is achievable under optimal batch conditions.

The approach is environmentally sustainable, economically viable, and technologically simple, making it a promising treatment solution for dye-contaminated urban wastewater in the El Oued region and other similar arid zones.

Further research should focus on clay modification, regeneration, and pilot-scale continuous systems to enable large-scale implementation. Overall, the study highlights the great potential of locally available natural clays as green adsorbents for sustainable wastewater management.

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