Adsorption des matières organiques des eaux usées urbaines sur la bentonite modifiée par Fe(III), Al(III) et Cu(II) Adsorption of organic matter, from urban wastewater, onto bentonite modified by Fe(III), Al(III and Cu(II)

Résumé

Une argile naturelle, la bentonite de Maghnia (ouest d’Algérie), a été purifiée et caractérisée par calcination, pH‑métrie, IR, XRD et SEM. Les résultats obtenus ont montré que l’argile a une conductance de 66,4 µS, une capacité d’échange cationique (CEC) de 0,91 méq/g et contient 12 % de matières organiques. Les caractérisations par IR, XRD et SEM ont montré que la bentonite de Maghnia (B) est composée de quartz comme impureté majeure, d’illite (7 %) et principalement de montmorillonite.

Cette argile a été saturée au sodium (Na⁺) et associée au fer (III), à l’aluminium (III) et au cuivre (II). Ces derniers sont insérés, par couple (Al-Cu, Fe-Cu et Fe-Al, 50 ‑ 50 % en atomes), dans l’espace inter-feuillets de la montmorillonite sodée. Le rapport massique métaux/B est égal à 0,0625.

Les échantillons obtenus sont appliqués pour fixer les matières organiques (MO) des eaux usées, très chargées, de la ville de Sidi Bel-Abbès (ouest d’Algérie). Ces matières organiques représentent 60 % des matières en suspension (MES).

L’adsorption des MO a été suivie par photométrie à 470 nm et à température ambiante (20 °C). Cette adsorption a été ajustée au modèle de Freundlich dans le cas de la bentonite et au modèle d’Elovich dans le cas de la bentonite modifiée. Sur l’argile seule l’adsorption des MO est en monocouche, alors qu’elle est en multicouche sur tous les trois autres systèmes. Les taux de fixation des matières organiques sont égaux à 67,1; 76,0; 82,6 et 87,7 % respectivement sur B, Al‑Cu/B, Fe‑Cu/B et Fe‑Al/B. Ces systèmes ont des capacités maximales d’adsorption (q_m), respectives, de 570; 860; 890 et 1 010 mg MO/g. Les systèmes au fer sont les plus performants, en particulier Fe‑Al/B.

Mots clés:

• Eaux usées,
• Bentonite,
• Fer,
• Aluminium,
• Cuivre,
• Matières organiques,
• Adsorption

Abstract

Clay functions as a recyclable surfactant support for the adsorption and subsequent combustion of organic pollutants. The fact that this technology is coupled with general water treatment processes is in itself advantageous. The adsorption of organic matter (OM) from the urban wastewater of Sidi Bel-Abbes City (western Algeria) onto modified clays was investigated in this study. Bentonite from Maghnia (western Algeria) was used in a purified form. The natural clays were washed several times with distilled and deionised water and were completely dispersed in water. After 17 hours at rest, the dispersion was centrifuged for one hour at 2400 rpm. The size of the clay particles obtained was < 2 µm.

These clay particles were dispersed in water and heated at 75°C in the presence of a solution composed of the sodium salts of bicarbonate (1 M), citrate (0.3 M), and chloride (2 M). The purpose of this operation was to eliminate inorganic and organic compounds, aluminium found in the inter-layer spaces and various free cations. Carbonates were removed by treatment with HCl (0.5 M) and chloride was eliminated after several washings. The organic matter was eliminated completely by treatment with H₂O₂ (30% v/v) at 70°C. The purified clay was dried at 110 °C, and then saturated with sodium (Na⁺). To ensure complete transformation into the sodium form, all samples were washed several times with a NaOH solution (1 M).

The exchanged clay was examined by X-Ray Diffraction (XRD), infrared (IR) and scanning electron microscopy (SEM). The results obtained demonstrated that the clay had a conductance of 66.4 µS, a cation exchange capacity of 0.9 meq/g (CEC) and contained 12.2 % organic matter. These techniques also show that the purified bentonite of Maghnia (B) was composed essentially of Na-montmorillonite and some illite (~ 7%).

The clay was associated with iron (III), aluminium (III) and copper (II). These metals were inserted in couples (Al‑Cu, Fe‑Cu and Fe‑Al; 50/50% on an atomic basis) in the interlayer space of the montmorillonite. The mass ratio of metals/bentonite was 0.0625. The obtained structures were based on the intercalation of metals into the clay, thus ensuring uniform porosity with a high interlayer pore volume.

These systems (metals/bentonite) were applied to fix the organic matter (OM) present in urban wastewater from the city of Sidi Bel-Abbes (western Algeria). The samples of wastewater were taken from the effluent of the wastewater treatment station located on the Mekerra River at the exit of the city. Wastewater was sampled at 1:00 pm on four dates (June 1998, September 1998, April 1999 and December 1999) and preserved according to standard protocols (AFNOR). Water quality parameters of this effluent were indicative of pollution: suspended solids (675 ‑ 940 mg/L), Chemical Oxygen Demand (COD 340 ‑ 1000 mg O₂/L), Biological Oxygen Demand (DBO₅ 190 ‑ 520 mg O₂/L), Total Kjeldahl Nitrogen (79.8 ‑ 82.6 mg N/L) and total phosphates (20 ‑ 47 mg P /L). The treatments were carried out on diluted wastewater; the dilution factors were 3.16, 1.92, 3.72 and 4.53 respectively for the four dates cited above. These dilutions were used so as to obtain an initial organic matter concentration 126 mg OM/L. For the four sampling periods the average dilution factor was 3.33. At ambient temperature, a mass (10 ‑ 30 mg) of bentonite alone (B) or modified (Al-Cu/B, Fe-Cu/B and Fe‑Al/B) was added to 50 mL of the polluted and diluted water.

The adsorption of organic matter was followed by spectrophotometry at 470 nm, using batch equilibration, under various conditions established by controlling the following parameters: adsorbate/adsorbent ratio, contact time, pH and temperature. This adsorption was adjusted to the Freundlich model in the case of bentonite alone and to the Elovich model in the case of the modified bentonite. Indeed, the isotherm studies showed that the adsorption of organic matter was best described by the Freundlich isotherm equation (with a correlation coefficient R = 0.98 and an adsorption coefficient K_F = 70.91 L•g^‑1) for bentonite alone and the Elovich isotherm equation for the modified clay (with R > 0.96 and K_E = 2•10^‑3, 15•10^‑3 and 11•10^‑3 L•g^‑1 respectively for Al-Cu/B, Fe-Cu/B and Fe-Al/B). The adsorption was in monolayers on bentonite and in multilayer form on the other systems. The degree of OM removal was 67.1, 76.0, 82.6 and 87.7% respectively for bentonite (B), Al-Cu/B, Fe-Cu/B and Fe-Al/B. The maximal adsorption capacities (q_m) for these systems were: 570, 860, 890 and 1010 mg OM•g^‑1, respectively for the four sorbents. The q_m values indicate the degree of surface covering of the solid phase (θ); this coverage was 0.80, 0.72 and 0.86 respectively for Al-Cu/B, Fe- Cu/B and Fe-Al/B.

The systems containing iron were powerful, particularly Fe-Al/B; this system was able to completely remove the organic matter found in the wastewater. The result was confirmed by the abatement of the COD. This parameter decreased from 537 to 55 mg/L in a trial where one litre of wastewater was treated with 0.67 g of bentonite and 42 mg of Fe-Al/B (50/50 atomic ratio).

Key words:

• Wastewater,
• Bentonite,
• Iron,
• Aluminium,
• Copper,
• Organic Matter,
• Adsorption