Acid mine drainage treatment by integrated submerged membrane distillation-sorption system.
TL;DR: The results showed that modified (heat treated) zeolite achieved 26-30% higher removal of heavy metals compared to natural untreatedZeolite, and the integrated system produced high quality fresh water while concentrating sulfuric acid and valuable heavy metals (Cu, Zn and Ni).
Abstract: Acid mine drainage (AMD), an acidic effluent characterized by high concentrations of sulfate and heavy metals, is an environmental and economic concern. The performance of an integrated submerged direct contact membrane distillation (DCMD) – zeolite sorption system for AMD treatment was evaluated. The results showed that modified (heat treated) zeolite achieved 26–30% higher removal of heavy metals compared to natural untreated zeolite. Heavy metal sorption by heat treated zeolite followed the order of Fe > Al > Zn > Cu > Ni and the data fitted well to Langmuir and pseudo second order kinetics model. Slight pH adjustment from 2 to 4 significantly increased Fe and Al removal rate (close to 100%) due to a combination of sorption and partial precipitation. An integrated system of submerged DCMD with zeolite for AMD treatment enabled to achieve 50% water recovery in 30 h. The integrated system provided a favourable condition for zeolite to be used in powder form with full contact time. Likewise, heavy metal removal from AMD by zeolite, specifically Fe and Al, mitigated membrane fouling on the surface of the hollow fiber submerged membrane. The integrated system produced high quality fresh water while concentrating sulfuric acid and valuable heavy metals (Cu, Zn and Ni).
Summary (2 min read)
2.2.2 Heat treated zeolite
- Heat treatment method was used to potentially enhance the performance of natural zeolite (Motsi et al., 2009; Turner et al., 2000) .
- Heat treatment was chosen as it requires no additional chemicals and complex modification process.
- Heat treatment was carried out by placing an appropriate amount of powder form natural zeolite in a ceramic dish.
- The ceramic dish was then placed into preheated air atmosphere muffle furnace (Labec Laboratory Pty Ltd, NSW, Australia).
2.3.1 Surface area and pore width distribution
- Nitrogen adsorption test was used to determine the Brunauer-Emmett-Teller (BET) specific surface area and the Barrett-Joyner-Halenda (BJH) pore width distribution of the natural and heat treated zeolite samples.
- Nitrogen adsorption test was measured with a Micrometrics ASAP 2020 HD analyzer using low temperature, per the procedure of ISO 9277 and ISO 15901-2.
2.3.3 Surface morphology and element contents
- A scanning electron microscopy (SEM) ((Zeiss Supra 55VP Field Emission) was used to analyse the zeolite surface characteristics (before and upon sorption).
- The SEM was integrated with energy dispersive X-ray spectroscopy (EDX) (15kV accelerating voltage) in order to analyse the element contents in zeolite.
2.3.4 Influence of pH and surface charge
- Zeolite surface charge was determined using zeta potential measurement.
- For this purpose, zeolite (1 g/L) placed in beakers with 100ml AMD solution.
- The pH of the initial solutions were varied from 1 -9.
- Zetasizer (nano instrument ZS Zen3600, UK) was used to analyse the zeolite surface charge.
2.5.1 Membrane analysis
- The morphology and element composition on the surface of the used and virgin membranes were analysed using SEM-EDX at a voltage of 15 kV as per the details mentioned in Section 2.3.3.
- The hydrophobicity of the virgin and used membranes were evaluated by measuring the water contact angle of the membrane using a goniometer (Theta Lite, Biolin Scientific, Sweden).
- Measurements were duplicated at different location of the membrane and the average value was used for this study.
3.1 Performance of natural and modified (heat treated) zeolite
- The sorption capacity of natural and modified (heat treated) zeolite was tested for heavy metal removal from AMD.
- Higher heavy metal removal was achieved with heat treated zeolite compared to natural untreated zeolite (Table 3 ).
- Heating may have removed water on the surface as well as internal channels of the natural zeolite, resulting in vacant channels which enhances heavy metal sorption rate, as reported by previous studies (Ohgushi and Nagae, 2003; Turner et al., 2000) .
- Heavy metal removal by zeolite minimally improved beyond 500 °C of heating.
- This trend could be attributed to characteristics change of zeolite upon heat treatment.
3.2.1 Permeate flux and quality
- Meanwhile, the concentration of permeate solution remained low (TDS less than 0.01 g/L).
- The sulfate concentration in the permeate solution increased significantly from 0.13 mg/L to 50 mg/L.
3.2.2. Membrane analysis
- Visible brown deposition (resembling iron oxides) was observed on the used membrane (Fig. 8b ) compared to the virgin membrane (Fig. 8a ).
- SEM-EDX analysis revealed Fe, S and Al deposition on the membrane.
- The precipitated metals predominantly deposited on the membrane surface and was loosely attached to the surface.
- It is likely that the deposition only partially blocked the membrane pores, and therefore, a stable permeate flux was maintained throughout the operation duration.
- Nevertheless, the contact angle of the used membrane (68.6 ± 0.8°) reduced by 38 -40% compared to the virgin membrane (109.5 ± 0.5°), suggesting that the Fe deposition resulted in the reduction of membrane hydrophobicity and partial wetting of sulfate ions.
3.3 Performance of integrated submerged DCMD-sorption
- An integration of zeolite with submerged DCMD (Fig. 1 ) offers the potential for improving the performance of both processes in a single system.
- The integrated system enable zeolite to be used in fine powder form with long contact time (more than 24 h) when kept suspended in a storage tank.
- In return, the heavy metal removal by 500 °C heat treated zeolite (dose = 10.0 ± 0.2 g/L) at pH 4 will ensure minimal Fe and Al deposition onto the membrane during the submerged DCMD process.
3.3.1 Permeate flux and quality
- The integrated submerged DCMD-sorption system showed similar flux pattern as the submerged DCMD (Fig. 7 ), indicating that the DCMD performance was not affected by the presence of sorbent in the storage tank.
- The integrated system enabled to achieve high rejection of all ions, maintaining a permeate TDS of less than 0.01 g/L.
- The sulfate concentration in the feed solution was increased from 4.2 g/L to around 8.2 g/L, while the sulfate concentration in the permeate solution remained low (less than 0.13-0.15 mg/L).
3.3.2. Membrane analysis
- A simple heat treatment was effective to increase the performance of natural zeolite for heavy metal removal from AMD solution.
- Heat treatment of natural zeolite at 500 °C enhanced heavy metal removal by 26-30%.
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