Abstract

Adsorption (ADS) and dielectrophoresis (DEP) techniques were combined (ADS/DEP) to efficiently remove As(V) in industrial wastewater. Fly ash, activated carbon, corncob and plant ash were tested to determine the best adsorbent by their adsorption capacity. Plant ash showed the highest adsorption capacity compared with the others. Different parameters such as solution pH and adsorbent dose were explored. The maximum As(V) removal efficiency was 91.4% at the optimized conditions (pH 9.0, adsorbent dose 5 g/L) when the initial concentration of As(V) was 15 mg/L. With the ADS/DEP technique, the plant ash particles with adsorbed As(V) were trapped on the electrodes in a DEP device. The ADS/DEP process could increase the removal efficiency of As(V) to 94.7% at 14 V even when the initial concentration of As(V) was 15 mg/L. And the residual concentration of As(V) decreased to 0.34 mg/L after two series of the ADS/DEP process. The adsorbents before and after DEP were examined by scanning electron microscope (SEM) and energy dispersive X-ray (EDX) analysis. After the DEP process, the weight percentage of As(V) on the adsorbent surface increased to 0.96% from 0.5%. The ADS/DEP process could be a new efficient way to remove arsenic pollutant at high concentrations.

INTRODUCTION

Arsenic is a highly toxic element which has been a serious threat to human health. The sources of arsenic pollution include natural reactions, geochemical reactions, biological activity, volcanic emissions and human activities. However, uncontrolled industrial discharge (>10 mg/L) from mining and metallurgical industries could cause more arsenic pollution to the environment. As reported, long-term consumption of arsenic in drinking water causes various arsenic-related chronic diseases (arsenicosis) in the human body, such as lung, kidney, and prostate cancer. And it also produces adverse cardiovascular, neurological, haematological, renal and respiratory effects, and chronic health problems such as hyperpigmentation and keratosis of the hands (Pandey et al. 2009; Argos et al. 2010). Thus, developing effective wastewater treatment technology for arsenic removal has motivated extensive research.

Over recent decades, a significant effort has been made to evolve effective technologies to remove arsenic from water, including chemical oxidation, coagulation with alum and iron, coupled plasma and electrochemistry (Idris et al. 2016), nano-filtration, reverse osmosis (Vaclavikova et al. 2008; Abejón et al. 2015), electro-dialysis (Ali et al. 2013), ion exchange (Chiavola et al. 2012), foam flotation (Shafique et al. 2012), solvent extraction (Han et al. 2002), electrocoagulation (Thakur & Mondal 2017) and adsorption (Ali & Gupta 2006) using natural and artificial materials.

Among these techniques, adsorption has been widely used to remove arsenic due to its low cost (Ali et al. 2012) and high efficiency (Abejón et al. 2015). Many materials such as biological materials, mineral oxides, activated carbons (Ali 2010), and polymer resins (Taleb et al. 2015), have been used to remove arsenic. However, most sorbents are not suitable for removing some anionic contaminants such as inorganic arsenic (As(V) and As(III)) most likely because of its negative charged surface. But how to remove adsorbent particles is a new problem.

DEP is a powerful tool that can be used to manipulate polarized particles suspended in fluid media in a non-uniform electric field (Martinez-Duarte 2012). It can be used in biology (Bisceglia et al. 2015), for manipulation of nanomaterials (Lungu et al. 2015), and even for a controlled synthesis of nanoparticles (Cui et al. 2015, 2016). It was found that DEP forces can trap particles with heavy metal ions adsorbed on top of them (Batton et al. 2007). Moreover, DEP has been used, in combination with ADS, to remove single types of heavy-metal ions (Hu et al. 2015). It is believed that not only cation (Hu et al. 2015) but also anion ionic pollutants (As(V)) could be efficiently removed by ADS/DEP. In this study, we first selected the best adsorbent. The effect of pH on adsorption removal was investigated since pH affects the removal efficiency greatly. After the adsorption process these adsorbent particles with As(V) were trapped and removed by DEP. The removal efficiencies of As(V) were investigated under the condition of either ADS or ADS/DEP. DEP removed arsenic and adsorbent suspension at the same time.

MATERIALS AND METHODS

Preparation of adsorbent

Fly ash (Xuanen, Hubei), activated carbon (Sinopharm Chemical Reagent Co., Ltd), corncob (Tangshan, Hebei) and plant ash (Xuanen, Hubei) were used as the adsorbents. Corncob was mechanically ground and charred at 800°C for 1 h in the muffle furnace (SX3-4-16, made in Tianjin).

Adsorption experiments

The arsenic stock solution of 750 mg/L was prepared by dissolving Na3AsO4·12H2O (Sinopharm Chemical Reagent Co., Ltd) salt. Then it was diluted by ultra-pure water. Batch experiments were performed with 50 ml or 500 ml of As(V) with the initial concentration 7.5 mg/L or 15 mg/L. The stirring time was 1.5 h. The pH of the solution was adjusted by 0.1 M HCl or 0.1 M NaOH solution. The adsorption capacity q (mg/g) was calculated from the following equation: 
formula
(1)
where C0 and Ce are the initial and equilibrium concentrations of the As solutions, respectively. V is the solution volume (L), m is the adsorbent mass (g).

The concentrations of arsenic in the samples were measured using the water-quality determination of the total arsenic–silver diethyldithiocarbamate spectrophotometric method (GB 7485-87). The absorbance of arsenic was measured at the wavelength of 520 nm by the 721-spectrophotometer (Shanghai Analysis Instrument Co., Ltd, China). The minimum detectable concentration was 0.007 mg/L.

Dielectrophoresis experiments

DEP experiments were conducted with an independently developed device as shown in Figure 1. DC voltage (14 V) was supplied to electrodes made of titanium wire mesh. The voltage was adjusted at 14 V and the constant flow pump speed at 0.074 ml/s in the ADS/DEP. The adsorbent particles with As(V) are trapped on the electrodes. Therefore, the effluent water became cleaner.

Figure 1

Diagrams of the DEP device (left) and adsorbent particles with As(V) trapped on the electrodes (right). (1) Arsenic suspension, (2) pump, (3) capture pool, (4) power, (5) effluent.

Figure 1

Diagrams of the DEP device (left) and adsorbent particles with As(V) trapped on the electrodes (right). (1) Arsenic suspension, (2) pump, (3) capture pool, (4) power, (5) effluent.

As is well known, dielectrophoresis (DEP) is a technique that can be used to manipulate polarized particles suspended in fluid media in a non-uniform electric field (Cui et al. 2015). In the case of a spherical particle, the dielectrophoretic force FDEP is given by: 
formula
(2)
while the Clausius–Mossotti factor, Re[K(ω)], is defined as: 
formula
(3)
where r denotes the radius of the particle, ∇E the magnitude of the electric field gradient, the complex permittivity of the particle and of the media.

A nonuniform electric field is necessary to induce the DEP forces stated in Equation (2) (otherwise ∇E = 0). Positive values of Re[K(ω)], see Equation (2), denote the induction of a positive DEP force that causes a particle to be trapped within regions of high electric field gradient. Negative values of Re[K(ω)] denote negative DEP, which means particles move towards regions of low or no electric field.

Characterization of adsorbents

Morphologies of the absorbent samples were examined by a scanning electron microscope (SEM, Hitachi S-4800, made in Japan). The energy dispersive X-ray (EDX) analysis was used to determine the weight percentage of As on the absorbents.

RESULTS AND DISCUSSION

Screening of the adsorbents

Fly ash, activated carbon, carbonized corncob and plant ash were tested in the adsorption experiments with the adsorbent 10 g/L. The stirring time was 1.5 h. Figure 2(a) shows the adsorption capacity of different adsorbents. It can be observed that the adsorption capacity of As(V) with plant ash (0.58 mg/g) is the highest among the four adsorbents. It is found that the particle size of the plant ash is about ten times smaller than the other adsorbents from Figure 2(b). These plant ash particles look more spherical in shape, and porous in structure, which is believed to help increase the adsorption capacity of As(V). Therefore, plant ash was selected as the adsorbent in the subsequent experiments.

Figure 2

Adsorption capacity and morphology of four adsorbents: (a) adsorption capacity of the different adsorbents, (b) SEM micrograph of the different adsorbents.

Figure 2

Adsorption capacity and morphology of four adsorbents: (a) adsorption capacity of the different adsorbents, (b) SEM micrograph of the different adsorbents.

Effect of pH on As(V) removal

The solution pH was expected to play a role in As(V) removal. Therefore, a series of experiments in the pH range of 3.0, 5.0, 7.0, 9.0 and 11.0 were carried out with 0.5 g plant ash. The solution pH (3, 5, 7) was adjusted using 1 mol/L HCl, pH 11 by 1 mol/L NaOH. The solution at pH 9.0 did not need to be adjusted. The dependence of removal percentage for As(V) upon pH of the aqueous solution is shown in Table 1. It was observed that the adsorption of As(V) on the adsorbent was clearly dependent on the solution pH.

Table 1

Effect of pH on As(V) removal

pHResidual concentration (mg/L)Removal rate
3.00 2.51 66.5% 
5.00 2.60 65.4% 
7.00 5.04 32.8% 
9.00 1.68 77.6% 
11.00 4.22 43.7% 
pHResidual concentration (mg/L)Removal rate
3.00 2.51 66.5% 
5.00 2.60 65.4% 
7.00 5.04 32.8% 
9.00 1.68 77.6% 
11.00 4.22 43.7% 

It can be seen that the maximum removal rate took place at pH 9.0. Therefore, we used pH 9.0 for the subsequent experiments without any adjustment by either base or acid. The result is very significant. We can get a higher removal rate without addition of reagent.

Effect of adsorbent dose on removal of As(V)

Figure 3 shows the effect of plant ash dosage on the adsorption capacity of As(V) by varying the dose from 2.5 to 15 g/L with the initial concentration 7.5 mg/L at pH 9.0. It can be observed that the As(V) adsorption capacity by plant ash decreased as the dose increased. But the higher dosage would increase the removal rate because greater exchangeable sites or bare surface area could be provided (Pandey et al. 2009). Here the removal efficiency reached 91.4% when the dosage was 5 g/L. So we selected 5 g/L as the optimal dosage in the subsequent experiments.

Figure 3

The effect of adsorbent dose on removal of As(V).

Figure 3

The effect of adsorbent dose on removal of As(V).

Effect of DEP process

At the optimized conditions (pH = 9, dosage = 5 g/L), the highest removal efficiency was obtained but how to remove adsorbent particles was a new problem. To resolve these problems, the DEP process was introduced.

Considering that DEP can improve the removal efficiency of As(V), and the concentration of As(V) in the actual wastewater could be more than 10 mg/L, here the initial concentration of the arsenic used in the ADS/DEP experiments was 15 mg/L, which was twice that used in the ADS experiments. After adsorption of 1.5 h, the suspension was pumped into the DEP device. After that, the above steps (the ADS/DEP experiment) were repeated. When each step of the experiment was completed, the processed solution was taken respectively to measure the concentration of the residual arsenic. It can be seen from Figure 4 that after the first adsorption, the removal rate of As(V) was 59.3% and the rate rose to 94.99% after the first ADS/DEP processing. After two treatments of ADS/DEP, it reached 97.73%. That means the residual amount of arsenic was 0.34 mg/L, which is lower than the national industrial standard of China (0.5 mg/L). It can be illustrated by the experiment that the combination of adsorption and DEP can effectively improve the removal rate of As(V). The adsorbent particles could also be trapped on the electrodes by DEP forces, which would make the solution clearer. Therefore, we conclude that ADS/DEP can be an efficient way to remove the arsenic pollutant in water.

Figure 4

The effect of DEP on the removal rate of As(V).

Figure 4

The effect of DEP on the removal rate of As(V).

Analysis of plant ash by SEM and EDX

Figure 5 shows the morphologies of the plant ash after different processes by SEM. Figure 5(a) and 5(b) show that the surface of the plant ash trapped on the electrodes did not significantly change before the ADS/DEP process, with a serious aggregation of the particles. Figure 5(c) and 5(d) show that the surface of the plant ash trapped on the electrodes changed a lot after the ADS/DEP process, with a serious aggregation of the particles. The weight percentage of As(V) on the surface of the plant ash was determined by EDX analysis. The weight percentages of As(V) on the adsorbents increased to 0.96% and 0.72% (after ADS/DEP) from 0.5% (only adsorption) on the anode and cathode electrodes respectively. This means that the DEP process greatly facilitated the removal of As(V) from the aqueous solutions.

Figure 5

SEM images of the plant ash: (a) before treatment, (b) ADS only, (c) after ADS/DEP (anode), (d) after ADS/DEP (cathode).

Figure 5

SEM images of the plant ash: (a) before treatment, (b) ADS only, (c) after ADS/DEP (anode), (d) after ADS/DEP (cathode).

CONCLUSIONS

To efficiently remove As(V), plant ash was screened as the best adsorbent in the adsorption experiments. At the optimal conditions (pH = 9, dosage = 5 g/L), the removal efficiency was 91.4% when the initial arsenic concentration was 7.5 mg/L. The removal rate of As(V) could be further increased to 94.7% after the DEP process, which was 35.4% higher than that achieved by adsorption only, when the initial concentration of As(V) was 15 mg/L. In short, our results indicate that the combined ADS/DEP process can be used to greatly improve the removal efficiency of As(V) in aqueous solution compared with the ADS process only. Not only was inexpensive waste (plant ash) used as adsorbent but no chemical reagents were added throughout the process. Meanwhile, DEP helped to remove suspended adsorbent particles in solution so that any possible second particulate pollution caused by the adsorbent was avoided. The combined approach (ADS/DEP) is hence believed to be able to open up a new fast, low cost and green avenue towards large-scale removal of the anion ionic pollutants (As(V)) in industrial wastewater. As(III) exists in the forms of As(OH)4, AsO2OH2− and AsO33− in solution, which are also anion ions as As(V). Here the removal method for As(V) could be applicable to As(III).

ACKNOWLEDGEMENTS

This work has been supported by the National Major Science and Technology Program for Water Pollution Control and Treatment (No. 2017ZX07101-002), the Fundamental Research Funds for the Central Universities (2016SHXY06) and National Natural Science Foundation of China (No. 51609271).

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