Comparison of homogeneous and heterogeneous electrochemical advanced oxidation processes for treatment of textile industry wastewater.

This study aimed at understanding the influence of the generation of oxidants in a heterogeneous way at boron-doped diamond (BDD) anode (anodic oxidation (AO)) or homogeneously in the bulk (electro-Fenton (EF)) during treatment of a textile industry wastewater. Both processes achieved high TOC removal. A yield of 95 % was obtained by combining EF with BDD anode during 6 h of treatment. The EF process was found to be faster and more efficient for discoloration of the effluent, whereas AO was more effective to limit the formation of degradation by-products in the bulk. An advantage of AO was to treat this alkaline effluent without any pH adjustment. Operating these processes under current limitation allowed optimizing energy consumption in both cases. However, using BDD anode led to the formation of very high concentration of ClO3-/ClO4- from Cl- oxidation (even at low current density), which appears as a key challenge for treatment of such effluent by AO. By comparison, EF with Pt anode strongly reduced the formation of ClO3-/ClO4-. Operating EF at low current density even maintained these concentrations below 0.5 % of the initial Cl- concentration. A trade-off should be considered between TOC removal and formation of toxic chlorinated by-products.

Efficiency of the heterogeneous catalyst from electrocoagulation sludge for the removal of methylene blue in the presence of persulfate.

Persulfate activation by heterogeneous catalysts based on transition metals is of interest in textile effluent treatment processes. Thus, iron-rich electrocoagulation sludge has been thermally treated to obtain new catalysts. The characterization of this catalyst by X-ray diffraction revealed the presence of FeAl2O4 nanoparticles active in the decomposition of persulfate into sulfate radicals (SO4•-). The efficiency of catalyst/persulfate was monitored during the methylene blue (MB) solution discoloration. The effects of temperature, pH, initial MB concentration, catalyst dose and persulfate dose were also studied. MB removal catalytic activity showed around 94% discoloration and 45.7% TOC reduction after 180 minutes batch reaction at pH = 4.0 (catalyst dose: 0.5 g/L, persulfate dose: 1 g/L; initial MB concentration: 20 mg/L). This catalyst reuse further confirmed its catalytic potential as a discoloration rate of about 82.45% was obtained after five cycles. The biodegradability monitoring measured by the carbon oxidation state (COS) has revealed a remarkable and continuous degradation of organic compounds. The EPR tests revealed that this catalytic reaction generates the radical species responsible for the degradation of MB. Finally, these results show that this catalyst from the thermal activation of electrocoagulation sludge is capable of decomposing persulfate to degrade bioresistant compounds such as textile dyes.

Optimization of the phytoremediation conditions of wastewater in post-treatment by Eichhornia crassipes and Pistia stratiotes: kinetic model for pollutants removal.

This study aims at determining the optimal conditions for pollutants removal in wastewater using Eichhornia crassipes (E. crassipes) and Pistia stratiotes (P. stratiotes) as appropriate aquatic plants for a post-treatment by phytoremediation. From factors such as residence time, plant density and initial PO43- concentration, four responses, i.e. the removal efficiency of PO43-, NO3-, NH4+ and the chemical oxygen demand (COD) were followed, using complete factorial design. After validation of the regression models by the statistical analyses, optimal conditions were obtained by using the global desirability function. Global desirabilities of 0.96 and 0.97 were respectively obtained for E. crassipes and P. stratiotes, for a residence time of 30 days, a plant density of 60 feet/m2 and an initial PO43- concentration of 10 mg/L. Using E. crassipes, this corresponds to the elimination of 94.2% of PO43-; 93.3% of NO3-; 95.0% of NH4+ and 63.6% of COD. In the case of P. stratiotes, 93.9% of PO43-; 83.4% of NO3-, 99.5% of NH4+ and 84.4% of COD were removed. Finally, under the used conditions, E. crassipes are able to better eliminate phosphorus and nitrates, while P. stratiotes are very effective in removing NH4+ and COD.

Bivalve shells (Corbula trigona) as a new adsorbent for the defluoridation of groundwater by adsorption-precipitation.

Defluoridation of groundwater was performed in a batch reactor using bivalve shell powder (BSP) as adsorbent. The physicochemical characteristics of BSP, studied by Fourier Transform Infrared, X-ray Diffraction and Inductively Coupled Plasma-Optical Emission Spectrometry after dissolution, have shown that BSP was mainly composed of crystalline CaCO3 (∼97.8%). The effects of pH, initial fluoride concentration, adsorbent dose and contact time on the adsorption capacity of BSP were investigated. For an initial fluoride concentration of 2.2 mg/L and with 16 g/L of BSP, after 8 hours of treatment, 27.3% were eliminated at pH 7.5 versus 68% at pH 3, highlighting the efficiency of the adsorption process. The difference in adsorption capacity as a function of pH was correlated to the pHpzc of the BSP, which was equal to 8.2. Thus, at pH below pHpzc, electrostatic attraction between the fluoride anions and the positively charged adsorbent could justify the adsorption mechanism. Fittings of experimental data have evidenced that the adsorption kinetics were of pseudo-second order whereas the adsorption isotherms were of Langmuir type. The chemical precipitation of calcium fluoride was also revealed to occur upon release of Ca2+ from partial dissolution of CaCO3 in acidic conditions.

Optimization process of organic matter removal from wastewater by using Eichhornia crassipes.

This study aimed to determine the optimal conditions for organic matter removal from wastewater by Eichhornia crassipes (E.C). As a matter of fact, a complete factorial design was used to determine the effect of residence time (X1), plant density (X2) and initial chemical oxygen demand (COD) concentration (X3) on the phytoremediation process. The process's performance was measured on COD (Y1), NH4+ (Y2) and PO43- (Y3), with the results indicating a reduction of 8.59-81.71% of COD (Y1); 22.53-95.81% of NH4+ (Y2) and 0.54-99.35% of PO43- (Y3). Then, the first-order models obtained for COD, NH4+ and PO43- removal were validated using different statistical approaches such as statistical and experimental validation. Moreover, multi-response optimization was carried out through different scenarios. On the whole, the results obtained indicated that two serial ponds are required for an optimum organic matter removal by Eichhornia crassipes. Indeed, for the first pond, a residence time of 15 days is needed with a plant density of 60 ft/m2 and an initial concentration of about 944 mg/L. The second was the same residence time as the first with similar plant density of 60 ft/m2 and an initial load 192 mg/L (> 200 mg/L). Optimal organic matter removal from wastewater using Eichhornia crassipes requires two ponds arranged in chain.