Sorption studies for the removal of methylene blue showed that SBAC produced with carbide lime had a capacity of 255 mg/g. The maximum sorption capacity of SBAC produced using lime carbide is comparable to SBAC produced using KOH and comparable to commercial activated carbon. Additional studies could investigate the use of carbide-lime activated SBAC for the removal of emerging organic pollutants as well as inorganic pollutants.
Introduction
- Background
- Research Questions
- Objectives
- Impact of the Study
- Scope of Work
- General Approach
Carbide lime (CaC2) is a by-product of acetylene production that can be used as an alternative to KOH to produce activated carbon, as it has similar properties to conventional hydrated lime Ca(OH)2 (Al-Khaja et al., 1992). No studies have been conducted on the production of SBAC using carbide lime as an activator. This study aims to evaluate the efficiency of adsorption of methylene blue on activated carbon produced from sewage sludge using carbide lime as an activator.
Literature Review
Wastewater Treatment in the UAE
In the UAE, the majority of treated wastewater is used for landscaping and industrial activities (Shawish et al., 2019). Dubai, Abu Dhabi and Sharjah are the largest producers of sludge in the UAE (Figure 1) (Bhattacharjee et al., 2020). About 60% of the treated water is used for irrigation, and 34% is discharged into the Arabian Gulf.
Composition of Sewage Sludge
The sludge produced in the secondary step is also known as activated sludge and contains biological material, inorganic material and microorganisms (mainly bacteria). Activated sludge is further processed by aeration, thickening, and dewatering, resulting in a semi-solid material called a sludge cake (Wang et al., 2008). Advanced sewage treatment plants also produce secondary, mostly dewatered sludge to obtain a solid sludge with a low water content (Smith et al., 2009).
Activated Carbon as an Adsorbent Material
Sewage sludge that is rich in phosphorus, nitrogen and zinc can be used as fertilizer or soil conditioner. Research has suggested that sludge can also be used as an energy source or converted into activated carbon or syngas through processes such as combustion, pyrolysis and gasification (Raheem et al., 2018). Activated carbon can be produced from various sources such as coal, wood, shells, waste material, sludge, etc.
Production of SBAC
- Physical activation for production of SBAC
- Chemical activation for production of SBAC
SSL= Sewage Sludge; HR= Heating rate; SBET= area determined according to the method of Brunauer, Emmett and Teller, T = Temperature, t = time. Activated carbon produced by chemical activation using sewage sludge is superior to physically activated carbon in terms of surface area and adsorbent properties (Lu et al., 2022). The table shows that chemical activation produces activated carbon with a larger BET surface area than physical activation.
Carbide Lime as Sludge Activator
Ferrentino et al., (2020) also investigated the influence of alkali treatment on the adsorption capacity of activated carbon produced from sewage sludge. But none of these studies used lime carbide as an exclusive chemical activator and compared its production with SBAC activated by KOH or ZnCl2. However, the studies mentioned above indicate that lime carbide can potentially be used as an activating agent to produce SBAC and enhance the adsorption of the cationic dye methylene blue.
Characteristics of SBAC as an Adsorbent
Comparison between SBAC and Commercial Activated Carbon
Removal of Methylene Blue using SBAC
Pore volume, surface area and functional groups present on the surface significantly affect the adsorption of methylene blue on activated carbon. Li et al., 2021; Streit et al., 2021), but this section focuses on the use of SBAC for the removal of methylene blue. 2007) calculated the sorption of methylene blue using the Freundlich and Langmuir adsorption isotherm models. They found that ZnCl2-activated sewage sludge had a methylene blue uptake of 137 mg/g. 2020) used carbon briquettes made from pyrolysis of sewage sludge to remove methylene blue.
Adsorption capacity was significantly linked to activation temperature and binder content. 2017) turned municipal sludge into biochar by air-drying sludge from the dewatering stage. Methylene blue removal was also positively related to temperature and contact time. The highest removal efficiency of methylene blue was 99% at pH 7. 2020) used sludge from a wastewater treatment plant to produce SBAC using physical and chemical activation.
Methylene blue was better absorbed by SBAC prepared by chemical activation (1.74 mg/g corresponding to an adsorption efficiency of 99%) than SBAC prepared by physical activation (1.69 mg/g and 96% removal efficiency). 2020) investigated the impact of alkaline modification of hydrochars obtained from sewage sludge on the adsorption of methylene blue. The results indicated that, although the surface area of raw and modified hydrochars was very similar, the adsorption of methylene blue on modified hydrochars was significantly higher, with a lower reaction time. Methylene blue (MB) with heteropolyaromatic structure, which has a strong inhibitory function for biodegradation and is very difficult to degrade into small inorganic molecules by common methods, is mostly used as a model dye to study the adsorption properties of activated carbon (AC ) materials (Ahmed and Dhedan, 2012; Hameed et al., 2007b; Li et al., 2013).
The literature has shown that SBAC typically produces mesopores, which is optimal for methylene blue removal, and the research data also confirmed this.
Removal of other Organic Compounds using SBAC
- Materials
- Raw sludge collection and storage
- Carbide lime collection and storage
- Experimental Procedure
- Physical Characterizations of Sludge
- Production of SBAC
- Chemical activation of sludge
- Thermal activation
- Acid treatment of CS and SBAC
- Characterization of SBAC
- Percent yield
- X-ray fluorescence analysis
- FTIR analysis
- Scanning electron microscopy
- BET specific surface area
- Sorption Studies for Methylene Blue
- Sorption kinetics analysis
- Sorption kinetics and equilibrium
- Sorption isotherm
The boat was inserted into the tube furnace from the entrance, reaching the center of the heat source. Where Wbefore is the weight of dry sludge, Wafter is the weight of CS or SBAC after conversion. Semi-quantitative XRF analysis was also performed to ascertain the approximate percentage of mineral compounds in CS and SBAC.
The XRF analysis was performed in the laboratories of the Department of Civil and Environmental Engineering (UAE University) using Shimadzu 7000EDX. The results are used to compare the effect of the preparation method on the mineral composition. Scanning electron microscopy was used to study the surface morphology of the prepared SBAC and CS materials.
The absorbance of the filtrate was checked at 664 nm using HACH model DR nm) spectrophotometer. The term qe is also referred to as the absorption capacity of the sorbent (mg/g). Judgment of the best model is based on the values of the coefficient of determination (R2) associated with the model fit.
Sorption equilibrium data of target compounds were fitted using Langmuir, Freundlich and Sips isotherm sorption models.
Results and Discussion
Characterization of Raw Sludge
- X-ray fluorescence analysis
- FTIR analysis of prepared SBACs
- Scanning electron microscopy
- BET surface area analysis
The results also show that CL-SBACs have a higher calcium concentration, while KOH-SBACs have a higher potassium concentration compared to CS. 2015) mention that the adsorption mechanism for SBAC is not one-dimensional and that many characteristics of SBAC play a role in the successful adsorption of pollutants. It involves migration and diffusion of the dye to the surface through the boundary layer and adsorption to the active site on the carbon surface (Zhang & Xu, 2014). Regarding the hydroxyl group, Godlewska et al. 2019), in his comprehensive review of the relationship between functional group and carbon surface sorption capacity, showed that the presence of hydroxyl groups can block the access of the molecule to the hydrophobic centers of the adsorbent surface.
We used scanning electron microscopy (SEM) to study the morphology of the prepared SBAC samples. SBAC activated with KOH gave better results in terms of surface area and pore volume than those produced with carbide lime. Increasing the activation temperature from 700 to 800°C decreased the surface area as well as the pore volume for both KOH and SBAC activated with carbide lime.
2015) argued that surface area is not the ultimate indicator of the extent of sorption capacity. To investigate the surface characteristics of the generated SBAC materials, SBAC-CL and SBAC-KOH were considered here, while the rest of the results are presented in Appendix C. The N2 adsorption–desorption isotherms and pore size distribution of SBAC are shown in Figures 13A and 13B.
Examples of the pore size distributions of some of the fabricated SBAC materials are shown in Figure 14.
Adsorption Studies
- Sorption rate and equilibrium
- Effect of chemical activation
- Effect of initial concentration of methylene blue
- Effect of impregnation ratio
- Effect of activation time
- Effect of activation temperature
- Effect of acid wash
- Comparison with commercial activated carbon
- Modelling sorption kinetics
- Modelling sorption equilibrium
A comparative study was conducted to investigate the effect of the type of alkali activator on the adsorption of methylene blue using KOH and carbide lime. The effect of the initial concentration of methylene blue on the adsorption capacity of the prepared SBAC was investigated. The graph shows that KOH-activated SBAC with an impregnation ratio of 1:1 and activated at 700 ℃ for 60 minutes reached a sorption capacity of 345 mg/g at an initial methylene blue concentration of 600 mg/l.
Under the same activation conditions, SBAC activated with carbide lime achieved the highest sorption capacity of 267 mg/g at a concentration of 500 mg/L methylene blue. We studied the influence of the chemical impregnation ratio for the SBAC preparation on the adsorption of methylene blue. As shown in Figure 17, increasing the impregnation ratio improves the sorption capacity of the prepared SBAC towards methylene blue.
An opposite trend was found for KOH-activated SBAC (Figure 20), where higher activation temperature showed better sorptivity for methylene blue. Furthermore, it appears that carbide lime based SBAC and CAC have similar behavior at similar initial methylene blue concentrations. The Sips exponent (n) shows the non-linear sorption behavior of methylene blue on the SBAC material and ranges from 0.3-1.5.
The results show that a larger surface area does not always correlate with a higher sorption capacity of methylene blue.
Conclusion and Recommendations
Conclusion
SBAC can be used in industrial wastewater treatment for the removal of cationic dyes, especially for the removal of methylene blue from the wastewater of textile, paint and pharmaceutical industries (Santoso et al., 2020). Kinetic sorption modeling indicated that methylene blue sorption by SBAC could be described by pseudo first order or pseudo second order models, but the latter is more superior. Equilibrium sorption modeling suggests that sorption of methylene blue onto the produced SBAC material is non-linear and could be adequately described by the Langmuir and Sips models.
The results of this study showed that lime carbide waste can be used without any further treatment for the production of SBAC, which is very applicable for the removal of dyes from wastewater, thus addressing three issues simultaneously: sludge management, of waste lime carbide and water pollution.
Recommendations
Preparation and characterization of activated carbon derived from sewage sludge for the removal of pollutants from wastewater. Structural and functional relationships of pyrolysis activated carbon briquettes of sewage sludge for methylene blue removal. Removal of methylene blue from aqueous solution by sewage sludge-derived biofuel: Adsorption kinetics, equilibrium, thermodynamics and mechanism.
Preparation of activated carbon briquettes without binder from sludge pyrolysis of sewage treatment plants for liquid-phase adsorption of methylene blue. Adsorption properties of dyes in columns of activated carbon prepared from the sludge of sewage treatment plants in a paper mill. Preparation and characterization of carbon adsorbents from sewage sludge using pilot-scale microwave heating equipment.
Removal of methylene blue from aqueous solutions by sewage sludge-based activated carbon: adsorption equilibrium, kinetics and thermodynamics. Preparation of activated carbon from sewage sludge using a green activator and its effectiveness in dyeing wastewater treatment. Activated carbon developed from excess sewage sludge to remove dyes from dilute aqueous solution.
Preparation of biochar from sewage sludge and its effectiveness in removing Cr (VI) from aqueous solutions. Study of an adsorbent obtained from sewage sludge for the removal of Cd2+, Ni2+ in aqueous solutions. Structure and adsorption properties of carbon obtained from sewage sludge with removal of inorganic impurities and high porosity.
