INTRODUCTION
Statement of the problem
Justification
Objectives of this study
Scope of this Study
Thesis Outline
LITERATURE REVIEW
History of Tanning
With the advent of machinery and the Industrial Revolution, scientific studies of tanning began in the nineteenth century and evolved into modern leather technology. Similarly, the tanning business grew around the end of the nineteenth century with the discovery of several chemicals as tanning agents, culminating in the discovery of key tanning agents such as chromium and alum, which signaled the start of industrial tanning.
Steps Involved in Leather Processing
There was no change in skin processing from the Middle Ages to the 17th century. Pickling is a process of correcting the pH suitable for the tanning operation and to prevent swelling of the leather, i.e. dehydration of the leather.
Types of Tanning
The pH is increased after a long time, when the chromium has entered sufficiently to promote the chromium's reaction with the skin. Basification is the process of raising the pH to 3.8-4.0 at the end of the chrome browning process.
Effects of Chrome Tanning
Tannery Industry in Bangladesh
Industrial waste is a major pollutant in all environments and needs to be treated on site before being discharged into the sewage system (Emongor et al., 2005). The Ministry of Environment has identified 900 major polluters that do not have the capacity to treat sewage and waste (Razzaque et al., 2019).
Waste Production in Leather Industries
- Solid and Gaseous Waste in Leather Industry
- Wastewater in Leather Industry
The presence, concentration and forms of chromium in the environment depend on various chemical and physical processes, such as hydrolysis, complexation, redox reactions and adsorption. Cr(III) is the major form at lower redox potentials and can be found as free Cr3+ in the pH range of 0-4 or as the hydroxy-complex Cr(OH)x at higher pH levels as shown in the Pourbaix diagram . (Figure 2.4).
Tannery Wastewater in Bangladesh
The permissible limit for BOD was 100 ppm, which was well maintained in the off season and met the DoE and Leather Working Group (LWG) standards as shown in Table 2.2. Although the permissible limit of BOD has been reached, but the level of COD, chromium and ammonia is lagging behind.
Tannery Effluent Treatment Technology
- Chemical precipitation
- Coagulation and flocculation
- Electrochemical treatment
- Electrocoagulation
- Ion exchange
- Membrane filtration
- Electro flotation
- Electrodialysis
It is a method of destabilizing colloidal particles in waste water by passing an electric current through them. Because it is a cost-effective and convenient technique to operate, it is particularly effective in removing heavy metals from wastewater.
Adsorption Technology in Wastewater Treatment
- Fundamentals of Adsorption
- Types of Adsorptions
- Physical adsorption (physisorption)
- Chemical adsorption (chemisorption)
- Factors Influencing Adsorption
- Adsorption Mechanism
- Adsorption Isotherm
- Langmuir Isotherm
- Freundlich Isotherm
Adsorbent surface area: The rate of adsorption increases with increasing adsorbent surface area. Surface area: Like physisorption, chemisorption also increases with increasing surface area of the adsorbent.
Biosorption
Previous Works Using Various Natural Adsorbents
Prepared Biosorbents
Moringa oleifera Sojne is the most cultivated variety of the genus Moringa and family Moringaceae. Studies have revealed that lignocellulosic plants can be used to remove a wide range of heavy metals from aqueous effluents with high removal efficiency (Sulyman et al., 2016). The groundnut shell attributed to their high amount of lignin, potassium and zinc content (Kothai et al., 2019).
Peanut shell biosorbent has a potential ability to adsorb the hexavalent chromium ions in wastewater due to this binding (Annida et al., 2018).
Carbonization of Bio-sorbents
The ability of these five materials to adsorb heavy metal ions, their ready availability and low cost provide a solid research base for chromium ion removal research, especially for small-scale tanneries.
Physico-Chemical Characterization
- Fourier Transform Infrared Spectroscopy
- Scanning Electron Microscope
Died at room temperature in a shade.. iii. then dried in an air oven at 60 C for 30 hours until it becomes crispy iv. then ground into a fine powder in a mechanical grinder. v. washed again several times with double distilled water until the washing water is free of color and cloudiness. vi. after drying for several hours at room temperature and then sieved and the fraction, 0.3 mm, was chosen as adsorbent. vii. the neem leaf powder was stored in glass bottles to use as an adsorbent. The removal efficiency was found to be highly dependent on the pH of the tannery effluent. Comparison shows that the spectra before and after the adsorption of Cr have differences in the position of the appeared absorption peaks.
From these SEM micrographs it becomes clear that the adsorption of Cr strongly modified the surface of the biosorbents.
METHODOLOGY
Tannery Effluent Collection
A sample of chromium containing wastewater was collected from the chromium waste discharge point of Apex Tannery Limited, Savar, Dhaka. The collected waste water sample was stored in the laboratory in a refrigerator in a pre-washed HDPE container. The sample was taken out of the refrigerator 2-3 hours before the start of the test, so that the sample temperature reached room temperature.
This study was conducted at the Environmental Engineering Laboratory, Department of Civil Engineering, BUET, Dhaka, Bangladesh.
Initial Characterization
Dilution water was prepared by placing the desired volume of water in a bottle and adding phosphate buffer, magnesium sulfate, calcium chloride, and ferric chloride solutions. The bottle was filled with enough dilution water so that inserting the stopper will displace all air without leaving bubbles. The diluted water (blank) was also incubated as a rough control of the quality of diluted water and the purity of incubation bottles.
Bio-sorbent Preparation
Wash three times with distilled water. iii. again, air dry at room temperature until brittle iv. Ground with electric grinder. f. washed until laundry is free of color and turbidity vi. then the powder is dried for 24 hours at 105 C vii. Wash repeatedly with tap water and then with distilled water ii. f. Ground and sieved through sieve size of 0.3mm 5) Groundnut shell.
Batch Experiment
- Adsorption dose
- pH
- Contact time
Adsorption dose determines the capacity of an adsorbent to adsorb. 100 ml of working sample was placed in each conical flask. Then, different adsorbent doses (3–14 g/100 mL for green biosorbents and 0.5–3.5 g/100 mL for ash biosorbents) were added to each conical flask. All of the conical flasks were placed on the mechanical shaker at 200 rpm for 60 min, and then the beakers were withdrawn from the shaker, held steady for 30 min, and filtered.
To find the optimal contact time, 100 ml of working sample was placed in each different Erlenmeyer flask with the best adsorbent dose and pH.
Adsorption Isotherms
The final pH of an adsorbent medium affects the adsorption mechanisms on the adsorbent surface and affects the nature of the physicochemical interactions of the species in the solution and on the adsorption sites of the adsorbents. All conical flasks were placed in the shaker at 200 rpm for a period of time ranging from 30 to 240 minutes for green biosorbents and 30 to 120 minutes for ash-formed biosorbents. Both Freundlich and Langmuir adsorption isotherm models were studied to find the adsorption capacity of the prepared biosorbents.
In the Langmuir isotherm model, it is plotted against ; is the monolayer (maximum) adsorption capacity and KL is the Langmuir constant related to the adsorption energy obtained from the slope and the intercept of the plot.
Physico-chemical Characteristics
- Fourier Transform Infrared Spectroscopy (FTIR)
- Scanning Electron Microscope (SEM)
55 It is found that the maximum removal percentage of Cr from the prepared green biosorbents was found to be 66-65%, for neem leaves (NGB), Sojne leaves (SGB), Chhatim leaves (CGB), and 48- 47% for Banana peels ( BPGB) and Groundnut shells (PSGB) at the experimental condition (Table 4.2). The initial Cr concentration was 3534±18 mg/l and the maximum removal percentage of Cr from the prepared ash biosorbents was 96%, 95% and 98% for Neem leaf ash biosorbents (NAB), Sojne leaf ash biosorbents (SAB) ), leaving Chhatim respectively as-biological agents (CAB) (Figure 4.15). Which in turn indicates that the carbon content of the prepared biosorbents is very high.
Therefore, the higher KF value for NAB confirms that the adsorption capacity was higher than that of the other two fly ash biosorbents.
RESULTS AND DISCUSSION
Characteristics of Collected Raw Tannery Effluent Sample
The sample was filtered to avoid the effect of filtration on test results and to know the actual adsorption capacity of the prepared bio-sorbents. In this study, the pH of the crude sample was 3.47, indicating that the sample was acidic in nature. Therefore, the EC value of this raw sewage sample was much higher than the standard discharge value.
The discharge standard for COD for tannery effluent, laid down in ISW-BDS-ECR 1997, was 400 mg/l for discharge into domestic surface water.
Results and Data Analysis of Batch Adsorption
- Green Bio-sorbents
- Effect of Dosage
- Effect of pH
- Effect of Contact Time
- Adsorption Isotherms
- Ash formed Bio-sorbents
- Variation of pH with Dose of Carbonized Biosorbents
- Effect of dosages
- Effect of pH
- Effect of Contact time
- Adsorption Isotherm
Also from the Pourbaix diagram (figure 2.4) it can be seen that Cr (III) is found as hydroxy complexes Cr(OH)x. The Langmuir and Freundlich isotherm profiles of Cr adsorption for various doses of prepared biosorbents in green form are shown in Figures 4.12 and 4.13 respectively. Variation of residual pH with the increase in dose of biosorbents in ash form is shown in Figure 4.14.
From Figure 4.16-4.18 it can be seen that the sample only became free of color due to the dosage effect. From Figure 4.24 it is seen that the adsorption increased with time and the highest percentage of removals is observed at 180 min for NAB, 120 min for SAB and CAB. The Langmuir and Freundlich isotherm profiles of Cr adsorption for different dosages of prepared ash biosorbents are shown in Figures 4.27 and 4.28, respectively.
Physico-chemical Characterization
- Fourier Transform Infrared Spectroscopy Analysis
- Scanning Electron Microscope Analysis
89 Figure 4.30 reveals the FTIR spectra of SAB before and after adsorption, respectively, where the peaks were slightly shifted. 91 As shown in Figure 4.31, the spectral analysis of CAB before and after adsorption shows a striking difference in the appearance of the peak. The SEM micrographs of ash biosorbents before and after adsorption at 3000x magnification were taken and the micrographs are shown in Figure 4.32(a-f).
It can be seen from Figure 4.32(a, c and e) that the surface of the adsorbents is rough, uneven and has a considerable number of pores that provide suitable sites for Cr binding.
Sludge management
Removal of chromium (VI) from aqueous medium using chemically modified banana peels as efficient low-cost adsorbent. Retrieved from https://www.jocpr.com/articles/equilibrium- studies-on-biosorption-of-chromium-on-psidium-guajava-leaves-powder.pdf. Removal of Chromium (VI) from Aqueous Solution in Continuous Flow Column Using Jackfruit Leaf as Bioadsorbent.
Removal of chromium from an aqueous solution using Azadirachta indica (neem) leaf powder as an adsorbent.
CONCLUSION AND RECOMMENDATION FOR FUTURE STUDY
Recommendation for Future Study
Potential of Mangifera indica activated carbon for removing chromium and iron under a Creative Commons Attribution. Optimization of operational conditions for batch biosorption of chromium (vi) using chemically treated leaves of Alstonia Scholis as biosorbent. Optimization of operational conditions for batch biosorption of chromium (VI) using chemically treated Alstonia Scholis leaves as biosorbent.
Relevant approach to assess the performance of sawdust as adsorbent of chromium (vi) ions from aqueous solutions.
