Investigations on the applicability of bentonite, dolomite and vermicompost as natural adsorbents for the decolorization of textile wastewater

Abstract

This thesis focused on the applicability of bentonite, dolomite and vermicompost as natural adsorbents for the removal of cationic and anionic textile dyes from colored wastewater. The application of natural granular bentonite in a fixed-bed column for the treatment of wastewater resulting from the acrylic fiber dyeing process pretreated with the coagulation-flocculation process in the wastewater treatment plant of the Iranian textile company of Acryl Tab was studied. The potential reuse of the treated wastewater in the studied company was investigated. Furthermore, the application of the spent bentonite in the production of lightweight aggregates (LWAs) was studied as possibility to minimize the environmental impacts of the disposal of the produced waste material. Natural granular bentonite and other natural materials were studied in this thesis in terms of their color removal efficiency and the possibility to be applied as adsorbent in a fixed-bed column. Granular bentonite was selected for further detailed studies. Adsorption experiments included batch and fixed-bed column experiments. The evaluation of efficiencies of the studied treatment systems was based on decolorization and COD reduction. A series of pilot-scale fixed-bed column (diameter 5 cm/ height 1 m) tests were conducted to examine the breakthrough curves and to evaluate the efficiency of granular bentonite to remove color and COD under different set-up and operating conditions, including bed height and flow rate. The X-ray diffractogram of the natural granular bentonite showed that it was primarily composed of montmorillonite as the main mineral phase. The textile wastewater which beforehand had been subjected to coagulation-flocculation and sedimentation processes had a yellowish-orange color which corresponded to 20.2 m-1 for SAC436 nm, 6.4 m-1 for SAC525 nm, 3.9 m-1 for SAC620 nm and to 456 mg Pt-Co L-1 (The Pt-Co method was used only for the comparison with the Iranian discharge limit standard). These color parameters of the pretreated wastewater neither comply with Iranian nor with German discharge limits into water bodies for textile wastewaters, which are in Iran: 75 mg Pt-Co L-1, in Germany: 7 m-1 for SAC436 nm, 5 m-1 for SAC525 nm and 3 m-1 for SAC620 nm, nor with water reuse requirements for textile processing, i.e. non-visible color (with reference to the European AquaFit4Use project). The pretreated wastewater was characterized by an alkaline pH of 11.6, a COD value of 890 mg L-1 and a conductivity of 2213 μS m-1. In the batch studies, granular bentonite showed an effective removal for color and COD. 5 g granular bentonite/L was found to be the optimized amount for maximum color and COD reduction. In the batch experiments, the color reduction was 88.1% (SAC436 nm), 70.3% (SAC525 nm) and 71.8% (SAC620 nm), respectively. The Langmuir model efficiently described the adsorption process of color on granular bentonite for all three wavelengths (SAC 436, 525 and 620 nm). This suggests that the removal of color using natural granular bentonite occurred via monolayer adsorption. Moreover, the results showed that the adsorption process followed more satisfactorily a pseudo-second-order model than a pseudo-first-order model indicating that the adsorption process is presumably chemical adsorption. Under optimum conditions, results illustrated that the COD could be lowered from initial 890 mg L-1 to 403 mg L-1 (removal efficiency 54.7%). The Freundlich model can effectively describes the adsorption process of COD on granular bentonite, and the COD removal on granular bentonite followed the pseudo-second-order kinetics. In continuous adsorption experiments using a fixed-bed column packed with granular bentonite, the effect of the different flow rates 5, 10 and 20 ml min-1 and bed heights 5, 10 and 20 cm on the removal efficiency of color and COD was studied. The highest bed height and the lowest influent flow rate were found to be the best condition for color adsorption, i.e. color was effectively removed in a fixed-bed column with 20 cm bed height at a flow rate of 5 ml min-1. The color of the treated effluent was lowered from 20.2 to 3.9 for SAC436 nm, 6.4 to 1.5 for SAC525 nm, and 3.9 to 1.4 for SAC620 nm, respectively. Decolorization efficiencies of 93% (Pt-Co method), 80% (SAC436 nm), 67% (SAC525 nm) and 64% (SAC620 nm) were reached. COD of the treated effluent from the fixed-bed column was lowered from 890 mg L-1 to 382 mg L-1, i.e. 57% of the COD was eliminated. These results show that granular bentonite for the decolorization of the textile wastewater is effective. The treated wastewater at a breakthrough point from the fixed-bed column (20 cm adsorbent height, 5 ml min-1 flow rate) not only fulfils the requirement for reuse in the textile wet processing (“non-visible” color) but also reached the German and Iranian discharge limits in terms of the color of the treated effluent. The results show that the treated wastewater from the fixed-bed adsorption column has 32 mg Pt-Co L-1 which is below the determined limited value in Iran for discharging the treated effluent into the environment. The color parameters of the treated wastewater resulted below 7 m-1 for SAC436 nm, 5 m-1 for SAC525 nm and 3 m-1 for SAC620 nm, which fulfilled German minimum discharge limits into water bodies for textile wastewaters. The COD of the treated effluent from the fixed-bed column (382 mg L-1) can meet the COD range limit of “low-quality water” for reuse purposes (with reference to the European AquaFit4Use project). The conductivity and pH of the wastewater after treatment reached 1565 μS m-1 and 8.15, respectively, fulfilled limits for the “general water reuse” for the conductivity, and “medium- to low-quality water” requirement for pH. Therefore, the treated textile wastewater can be proposed to be reused in the production process of the studied textile industry for rinsing/washing applications (washing down the equipment and washing the floors) as well as for washing the fibers for part of the pretreatment before dyeing. In conclusion, the adsorption using granular bentonite can be applied as a treatment step combined with the other conventional treatment methods in the studied industry to remove color and to reduce COD sufficiently from the wastewater of the acrylic fiber dyeing process to be reused for washing purposes. The reuse of the treated wastewater in the studied company can reduce water consumption. Therefore, the solution of water closed-loop recycling is suggested for achieving sustainability in water consumption in this textile finishing industry. Moreover, the treatment with the natural adsorbent can decrease the contamination of surface and ground waters since the studied industry (Acryl Tab CO.) usually discharges the pretreated textile effluent from its wastewater treatment plant into the municipal wastewater sewer and/or on agricultural lands (rice fields). Based on the scale-up method developed by Forntwalt and Hutchins (1966), the parameters to design and implement a fixed-bed column at an industrial scale were calculated as: area A = 0.84 m2, diameter d = 1.04 m and height between 3.12 and 5.12 m. According to the results from 5 cm, 10 cm and 20 cm bentonite bed heights at pilot-scale, the design column can treat 37.5 m3, 56.25 m3 and 100 m3 of wastewater, respectively in 6 h, 9 h and 16 h (at break through points) using 2287 kg, 4569.6 kg and 9150.4 kg of bentonite which is required for the treatment of wastewater with a flow rate of 150 m3 d-1. As the company works mainly in 2 working shifts, 6098 kg bentonite (5 cm bed height), 8123.73 kg bentonite (10 cm bed height) and 9150.4 kg bentonite (20 cm bed height) is required per day to treat the generated wastewater. As the quality of the treated wastewater at breakthrough points of all bentonite bed heights (5, 10 and 20 cm) fulfilled the requirements for water reuse (low-quality water for washing purposes based on the Aquafit4use project), the amount of bentonite scaled up from the pilot-scale column with 5 cm bed height will be reasonable to be considered in practice for the studied industry from the economic point of view. Accordingly, as the company works mainly in 2 working shifts (16 h), 6098 kg of bentonite is needed to treat the produced wastewater per day. Considering the price of the natural bentonite provided from an Iranian company, only 83 euro should be spent to supply this amount of bentonite per day. However, the amount of bentonite can be a considerable quantity to handle and to process. With reference to the life cycle impact assessment (LCIA) and Monte Carlo analysis results, the ratio of the mean price of wastewater treatment with 86.6 kg bentonite per hour to the mean price of wastewater treatment with 3.8 kg activated carbon per hour concluded 0.06. Therefore, natural bentonite was concluded as a much more environmentally friendly and cheaper alternative compared to activated carbon, a common adsorbent in wastewater treatment, for the treatment of the pretreated acrylic fiber dyeing wastewater in Iran. Another aim of this thesis was to investigate the possibility of using the spent adsorbent as a new raw material source in the production of construction materials. In other words, to convert waste (the used adsorbent) into a usable resource. In this thesis, spent bentonite was applied in the production of LWAs such as LECA (lightweight expanded clay aggregate). This solution can help to avoid producing secondary pollution and to increase the value of the produced waste by converting it into an efficient material. The properties of the generated LWAs from the spent waste were compared to the conventional LWAs, i.e. LECA produced at LECA CO. using natural clay. To produce LWAs using spent bentonite, the mixtures were blended with water and an additive (1% mazut), then shaped into pellets and oven-dried, and finally sintered at 1140 °C for 4 min in a muffle furnace. The main technical features of the aggregates (water absorption after 24 h, density and mechanical strength) were determined. As there is a positive correlation between an increase in organic matter and porosity in the produced LWAs, the organic matters in the effluent contributed to a higher porosity in the sintering process, thereby lowering the density as well as increasing the water absorption of the produced LWAs. It was found that the use of spent adsorbent can yield a reasonably high-performance LWA which can reduce the cost of the waste treatment and the cost of LWAs production leading a more circular economy. In preliminary studies, different low-cost natural materials such as vermicompost (organic), dolomite and bentonite (inorganic) were used as adsorbents in single and binary sorbent systems to remove fabric dyes from single and binary dye solutions. The two dyes selected were the cationic dye basic violet 16 (BV16) and the anionic dye reactive red 195 (RR195), two commonly used dyes in the textile industry in Iran. Groundwater samples were spiked with the two dyes and used in this thesis. Firstly, a series of preliminary bench-scale experiments were performed to study the feasibility of removing BV16 and RR195, from single-component solutions using the inorganic materials Persian charred (i.e. treated at 800 °C) dolomite, Persian natural bentonite, organic material vermicompost (all of them adsorbents locally available in Iran) and commercially available 'standard' bentonite. Batch experiments with single sorbent single dye solutions were carried out to examine and optimize the process parameters, including the effect of the initial dye concentration, contact time, adsorbent amount and stirring speed on the adsorption of the dyes. The effect of the above-mentioned parameters was investigated to adjust the process variables and to determine the adsorption isotherms and kinetics curves as well as using the optimized parameters in a single sorbent binary dye solution system and binary sorbent binary dye system. Groundwater samples containing one or both of these dyes ('single' or 'binary dye solutions') were treated with vermicompost and charred dolomite either alone or mixed, or with bentonite and charred dolomite either alone or mixed. In single sorbent single dye experiments, the maximum adsorption capacity of vermicompost, natural bentonite and standard bentonite for BV16 was found to be 16 mg g-1, 434.78 mg g-1 and 500 mg g-1, respectively, and the adsorption capacity of charred dolomite for RR195 was 7.34 mg g-1. Anionic RR195 was not noticeably adsorbed by vermicompost, natural bentonite and standard bentonite (having a negative surface charge) and cationic BV16, not by charred dolomite (having a positive surface charge) but adsorbed by the oppositely charged adsorbents which indicates a selective electrostatic adsorption mechanism. The equilibrium and kinetic data from single dye experiments showed that the adsorption process satisfactorily fitted to the Langmuir isotherm model and pseudo-second-order kinetics which implies that chemisorption dominates the dye-uptake processes. Secondly, analyzing BV16 and RR195 in binary solutions using zero-order derivative spectrophotometry (λmax adsorption of BV16 and RR195 is at 548 nm and 540 nm, respectively), the severe overlapping of the individual dye absorptions impeded the direct determination of the concentrations of each dye from the total absorbance. Therefore, first-order derivative spectrophotometry was used and developed to quantify the concentration of each of the dyes RR195 and BV16 in binary solutions. Various measurements of first-order derivative absorption spectra of binary solutions of BV16 and RR195 led to the result that BV16 can be quantified in the presence of RR195 at 580 nm, at which the absorbance of RR195 is zero; on the other hand, RR195 can be determined in the presence of BV16 at 308 nm, at which the absorbance of BV16 is zero. For adsorption experiments with dye mixtures in principle, there are three possibilities for the interaction of dyes: antagonism, synergism and non-interaction. In binary dye solution, BV16 adsorption onto charred dolomite increased in the presence of RR195 (synergistic effect), yet RR195 adsorption on charred dolomite was not influenced by BV16. An antagonistic effect of RR195 can be concluded for BV16 adsorption onto vermicompost. The amount of RR195 adsorbed on bentonite remarkably increased in the presence of the cationic dye BV16. Moreover, the adsorption capacity for BV16 doubled (833.33 mg g-1) in the presence of the anionic dye demonstrating synergistic effects due to the possible formation of water-insoluble ion pairs of anionic and cationic dyes. Another series of experiments where commercially available 'standard' bentonite was used instead of natural bentonite led to similar results for both dyes. The adsorption equilibrium data for all adsorbents fitted more acceptable to the Langmuir isotherm model than to the Freundlich model in binary solutions, other than the adsorption of BV16 on vermicompost which followed the Freundlich model. The kinetic data from binary dye experiments concluded that the adsorption process suitably fitted to pseudo-second-order kinetics. In the next step, the potential of a mixed sorbent consisting of vermicompost and Persian charred dolomite, and the adsorption potential of natural bentonite and charred dolomite for simultaneous adsorption of basic violet 16 (BV16) and reactive red 195 (RR195) was investigated. More than half of the removal efficiencies determined for both dyes onto the mixed sorbents (vermicompost and charred dolomite (VC + CD); natural bentonite and charred dolomite (NB + CD)) were >70% which highlights that the mixed sorbents investigated are highly efficacious for simultaneous removal of cationic and anionic dyes from contaminated groundwater. In conclusion, a mixture of bentonite and charred dolomite as well as a mixed sorbent consisting of vermicompost and charred dolomite are highly promising and efficacious natural sorbent systems for the simultaneous removal of cationic and anionic dyes from water.