Optimization studies for water defluoridation with two-stage coagulation processes using new industrial-based chemical coagulants

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Abstract

The fluoride removal capability of a new industrial-based chemical coagulant (Formula A) in one- and two-stage coagulation treatments was investigated in present study. Three main controlling factors, the coagulant concentration, reaction time and the initial pH of Formula A were studied on the fluoride removal accordingly, with the aid of other coagulants: Ecogent F-Loc, potassium ferrate (K2FeO4), and sodium aluminate (NaAlO2). Response surface methodology (RSM) was adopted to analyse the controlling factors on the fluoride removal efficiency (R%) using Box-Behnken designs (BBD). The coagulant concentration of Formula A shows the most significant effect on R% and better fluoride removal capability at an initial pH 12. Nevertheless, the R% has been shown to be independent of reaction time. Response optimisation revealed the highest R% (70.12%) can be achieved at 1000 ppm, 10 min of reaction time and initial pH of 2. Furthermore, K2FeO4 has a better R% in low pH regions of 2 to 4. Additionally, K2FeO4 achieved the highest R% (84%) at 1190 ppm and pH 9 with 8.5 min of reaction time. In a single coagulation stage, the use of Formula A or K2FeO4 are not able to meet Environmental Quality (Industrial Effluent) Regulations 2009 with discharge Standard B as specified by Department of Environment Malaysia, while a two-stage coagulation process can meet Standard A. Specifically, the combined formulation of using Ecogent F-Loc(1)- Formula A(2) and NaAlO2(1)- Formula A(2) were able to achieve treated water with less than 2 ppm fluoride. Moreover, recycling treated water also plays a major role in helping industries meet the strict treatment standards.

Introduction

About 30% of the industrial waste from semiconductor plants is fluorine-containing waste [1]. Kim et al. [2] studied the flow analysis of hydrofluoric acid which showed that in 2014, 220 tons of hydrofluoric acid were exported to Malaysia; 59% of the exported hydrofluoric acid were for the purpose of semiconductor manufacturing. The main applications of fluorine compounds in semiconductor manufacturing are to etch or remove silicon and clean reaction chambers. Other industrial activities such as the production of aluminium, steel and oil, phosphate fertilizer productions, and the burning of fluoride-rich coal to the environment involve fluorine-containing waste. In 2009, Department of Environment (DOE) Malaysia enforced a new regulation Environmental Quality (Industrial Effluent) Regulations 2009 by providing standards to control the concentrations of chemicals discharged from industries. There are two standards for maximum fluoride discharge concentration which are 2.0 ppm for Standard A and 5.0 ppm for Standard B. The industries in Malaysia must at least meet the requirement of Standard B before discharging effluent to the environment.

In the current state of industries, there are many types of fluoride removal technologies being used such as Coagulation-Flocculation (CF), electrocoagulation, adsorption, reverse osmosis, and nanofiltration membrane. A comparison table between different types of fluoride removal technologies is shown in Table 1. There are very scarce research findings on using electrocoagulation, adsorption, reverse osmosis or nanofiltration for the application of treating high fluoride content wastewater from the industries. These methods are more suitable in treating low-fluoride content wastewater for drinking water. Further to that, it is not economical to use expensive methods, resulting many industries would prefer to use CF method which is more economical and easier to operate. However, CF methods also pose a slight disadvantage of sludge production as a secondary pollution. Therefore, recent research aims to explore green chemicals that produce less sludge which can reduce the cost of sludge treatment.

High or low fluoride content may be present in wastewater. In general, wastewater with low fluoride content is usually found in the household area while high fluoride content is usually found in the effluent discharge from the industries especially in the semiconductor industries. Wastewater discharges from the industries usually have very high content of fluoride ranging from 250 to 11,000 ppm [16]. By far, coagulation is the most conventional method used in the industries to reduce high fluoride content due to cost effectiveness as compared to other technologies. Thus, a coagulation method is selected in this research to help industries which use one- or two-stages coagulation methods with the aid of new industry-based chemicals to meet the Malaysia DOE standards. In fact, the modern use of coagulants for water treatment started approximately 100 years ago [17]. In the coagulation process, chemical coagulants with positive charge are added to neutralize the negative charges (fluoride ions) in wastewater to form precipitate. The precipitate will enter a second stage where flocculants are added which causes the precipitate to agglomerate, settle down and collect as sludge.

Despite the fluoride standard enforced by DOE, the existing conventional wastewater treatment coagulation method practice by the industries in Malaysia was not able to meet the DOE standards. Many studies have been previously investigated on chemical coagulants used in the industry such as calcium salts, aluminium salts, and ferrous salts. Chemical coagulants such as aluminium sulphate [6,18], calcium chloride [19], calcium hydroxide [19], ferric sulphate [18], ferric chloride [20] and magnesium chloride [21] were studied in the reduction of fluoride concentration (see Table 2).

As elucidated in Table 2, most of the chemical coagulants used in a single stage of coagulation process cannot meet at least 5 ppm of Standard B. Aoudj et al. (2012) studied the effect of initial fluoride concentration on aluminium sulphate coagulant and shown that at initial fluoride concentration of 60 ppm, the final fluoride concentration is about 5.5 ppm which still exceeded Standard B [18]. Additionally, high fluoride concentration is usually found in wastewater produced by the industry, such as semiconductor, and the fluoride can only be reduced to a range between 25 and 60 ppm [23] after a single stage coagulation process. In fact, aluminium, calcium and ferric-based coagulants have been studied for many years, but there are no new metal coagulants that have been explored and studied for the removal of fluoride which lead to slow advancement.

In recent years, potassium ferrate (K2FeO4) has been investigated due to its unique properties and green chemical. It can be used as a coagulant, disinfectant, and oxidant [24]. Rahman et al. [25] showed that K2FeO4 can achieve a removal rate of ammonia, turbidity and total coliform by 92.2%, 82.1%, 99.99% respectively which showed higher efficiency than using both alum coagulant and chlorine. Furthermore, K2FeO4 is able to reduce 40% of the amount of sludge produced by alum/chlorine. These potentials have proved that K2FeO4 can be selected for both coagulation and disinfection processes in a wastewater treatment plant. Furthermore, it become as our intention to further investigate the fluoride removal capability of K2FeO4. Aside from K2FeO4, a new industrial-based coagulant Formula A, was also investigated as Formula A ions has higher charge of 4+ as compare to calcium and aluminium which have charge of 2+ and 3+, respectively. It is expected that this new industrial-based coagulant will show a better fluoride removal capability. In addition, Formula A is rather well known in the enhancement of photocatalytic water treatment studies. More importantly, Formula A is non-toxic and rather safe to human compare to conventional coagulants such as aluminium salts which is harmful to human health.

In this study, Formula A was studied on the fluoride removal accordingly with the aid of other coagulants: Ecogent F-Loc, K2FeO4 and NaAlO2. Response Surface Methodology was adopted to analyse the three main controlling factors on fluoride removal efficiency (R%) and final pH using Box-Behnken designs (BBD). Different chemical formulations are study in two-stages coagulation process: Ecogent F-Loc(1)- Formula A(2), NaAlO2(1)- Formula A(2) and NaAlO2(1)- K2FeO4(2). In addition, recycling treated water in two-stages coagulation process is studied as well to observe if there is any improvement/increase in R%. This study would greatly help the industries to explore new chemicals with better R%, environmentally friendly and consider upgrading their coagulation treatment process into two-stage coagulation process and might add recycling stream in the future revamp. Previous studies have reported on various coagulant recovery methods from sludge such as acidification, alkalization and ion exchange & membranes [[26], [27], [28]]. However, it is costly to recover coagulant from sludge. Studies have reported that water works sludge contains 39% aluminium by weight after coagulation [29]. Instead of recovering the coagulant from the sludge stage, this paper studied the recycling of treated water which may still contain excess coagulant ions species. Therefore, this study hypothesise that excess coagulant ions species remain in treated water after the coagulation stage. Thus, recycling the treated water into BK1 or BK2 might help improve the overall R% of the two-stage coagulation stage.

Section snippets

Materials

Formula A, Ecogent F-Loc (10–30% Aluminium hydroxide, 1–10% Polyelectrolyte), 55% Hydrofluoric acid solution, Poly-Aluminium Chloride (PAC) and Sodium Aluminate (consist of >92% AlNaO2) were provided as industry grade by Platinum Strike Sdn. Bhd., Malaysia.

Potassium Ferrate (Industry grade, Baoji Guokang Bio-Technology Co., Ltd., China), 97–98% Sulphuric acid (Fulltime Asia Sdn. Bhd., Malaysia) and SPADNS2 Reagent for Fluoride, Arsenic-Free (Arachem (M) Sdn Bhd, Malaysia) were purchased from

Effects of concentration, reaction time & initial pH on R%

Coagulant Formula A concentration has the most significant effect (p < 0.05) on R%. As the concentration of Formula A increases, the R% decreases as shown in Fig. 3. This could be due to excess dosage of coagulants resulting in charge reversal and restabilisaiton of colloids [34,35]. As the Formula A concentration increases, the negative charge of colloidal particles decreases, the positive charge becomes more dominant, which leads to electrostatic repulsion between positively charged particles

Conclusion

The controlling factors are coagulant concentration, reaction time and initial pH of Formula A and K2FeO4 used in determining fluoride removal efficiency. Formula A and K2FeO4 are able to reduce fluoride concentration with efficiency of 70% and 84% respectively. Formula A requires further research at lower coagulant concentration as it has potential in achieving higher fluoride removal efficiency. K2FeO4 shows better coagulation activity under acidic conditions and can act as a preliminary pH

Declaration of competing interest

The authors report no declarations of interest.

Acknowledgements

The authors wish to acknowledge the support given by Ir. Ts. Lee Teck Lii from the Giti Eco Sdn. Bhd. for sponsoring chemicals for this research project.

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