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The Effectiveness of Coconut Husk and Sodium hydroxide pretreatment in the removal of Arsenic from model Arsenic solutions

Abstract

Arsenic is highly toxic in its inorganic form and is naturally present in high concentrations in the groundwater of many countries. Contaminated water used for drinking, food preparation, and irrigation of food crops poses a significant threat to public health due to its high levels of arsenic. Among various technologies available for arsenic removal from groundwater, using agricultural wastes as biosorbents offer the most promising solution for developing countries. Biosorption is a physiochemical process that allows various types of biomass to bind contaminants onto its cellular structure. In this study, coconut husk (a biosorbent) and sodium hydroxide pretreatment was used to remove arsenic. The study was conducted with model solutions consisting of 500 parts per billion (ppb) arsenic in distilled water at different pH levels (3 and 7), biosorbent loading (0.5% and 1.0%), sodium hydroxide levels (1% and 2%), and treatment durations (30 and 60 minutes). Arsenic levels were measured with a commercial rapid arsenic test kit. Coconut husk at 1% loading reduced the arsenic levels from 500 ppb to 217 ppb at pH 3 after 60 minutes of treatment. Sodium hydroxide pretreatment further reduced the arsenic levels. Pretreatment with 2% sodium hydroxide, 1% biosorbent loading, pH of 3, and 60 minutes treatment duration resulted in the highest reduction in Arsenic (117 ppb). Additional studies are recommended to further optimize the arsenic treatment process using higher levels of sodium hydroxide concentration, higher loading levels of biosorbent, and longer treatment durations.

Introduction

Arsenic is a major water contaminant in many countries, causing serious health hazards[2,3]. Due to the carcinogenic nature of arsenic, WHO has recommended an allowable concentration of 10 ppb in drinking water [11]. Water with arsenic levels between 10 and 500 ppb can be used for domestic purposes other than drinking [12]. Therefore, it is essential to reduce the arsenic levels in the ground water to safer levels before its domestic use. Most of the technologies used in advanced countries such as ion exchange, reverse osmosis, etc. to remove arsenic, cannot be used in developing countries due to the cost and unavailability of centralised water treatment facilities[8]. In these areas, biosorbents from local agricultural waste offer the most promising results due to their easy availability and low operating cost [1,4,5,6,7,9,10]. Pretreatment with Sodium hydroxide (NaOH) was shown to improve the biosorption abilities of soybean waste [9]. Pretreating with 1% NaOH resulted in 60% improvement in Arsenic removal. Although extensive research has been done on using various types of agricultural waste as biosorbents to remove arsenic, there have not been any additional studies on using a combination of agricultural waste and NaOH pretreatment. In this study, coconut husk was used as a biosorbent in combination with NaOH pretreatment to study the effectiveness of this technology to remove arsenic from model solutions.

Method

The following materials were used in this study: coconut husk, dry grinder, sieves, incubators, weighing scale, NaOH, distilled water, 1000 mg/L Arsenic standard solution (Hach Company, Loveland, Colorado), 250-ml Erlenmeyer flasks, 0.05M Sodium Citrate buffer, Orbital shaker (CO-Z Orbital Shaker on Amazon), rapid Arsenic test kit (Industrial Test Systems Quick 481396-W Arsenic Wood Field Testing Kit), MS Excel software.

Treating arsenic solution with pretreated ground coconut husk (biosorbent) on a horizontal orbital shaker

Measuring arsenic levels in the aqueous solution using the rapid arsenic test kit

The model solution of 500 ppb arsenic was prepared by mixing arsenic standard solution (1000 mg/L) with distilled water. The dried coconut husk was locally sourced, ground using a dry grinder, and sieved to collect uniform solids. The ground coconut husk was treated with 0% (distilled water), 1%, and 2% NaOH solutions (w/v) and dried in an incubator for 24 hours at 30 °C. For the 0.5% biosorbent loading treatment, 0.5 g of pretreated ground coconut husk was mixed with 100 mL of model arsenic solution in a 250-mL Erlenmeyer flask using a horizontal orbital shaker at 100 rpm. For 1.0% biosorbent loading, 1.0 g of pretreated ground coconut husk was mixed with 100 mL of model arsenic solution. The pH of the aqueous solution was adjusted to either 3 or 7 using the 0.05M Citrate buffer. Three replications were used for each combination of NaOH pretreatment, biosorbent loading, and pH (total samples = 36). The arsenic levels were measured in the aqueous solution after 30 and 60 minutes using the rapid arsenic test kit. The data was analyzed using two-way analysis of variance (ANOVA) followed by a t-test to compare means (MS Excel software).

Results

Fig. 1. Effect of biosorbent and NaOH pretreatment on the removal of arsenic from model solution (pH=3; 1% biosorbent loading; 30 min treatment duration)

The coconut husk treatment at 1% loading and pH 3 reduced the arsenic levels from 500 to 283 ppb after 30 minutes without any pretreatment (Fig. 1). Pretreating with 1% and 2% NaOH further reduced the arsenic levels to 200 ppb and 183 ppb, respectively.

Fig. 2. Effect of biosorbent and NaOH pretreatment on the removal of arsenic from model solution (pH=3; 1% biosorbent loading; 60 min treatment duration)

After 60 minutes, arsenic levels were reduced significantly more (217, 150, and 117 ppb from the initial 500 ppb without pretreatment, when pretreated with 1% NaOH, and 2% NaOH, respectively) compared to after 30 minutes. (Figs. 1 and 2).

Fig. 3. Effect of NaOH pretreatment and pH on the removal of arsenic from model solution (60 minute treatment duration; 1% biosorbent loading)

At pH 3, the arsenic levels were consistently lower compared to pH 7 at all the NaOH pretreatment levels at 1% biosorbent loading and after 60 minutes (Fig. 3). Similar trends were observed after 30 minutes.

Fig. 4. Effect of biosorbent loading and NaOH pretreatment on the removal of arsenic from model solution (pH=3; 60 min treatment duration)

Biosorbent loading of 1% resulted in consistently more arsenic removal (217, 150, and 117 ppb with no pretreatment, 1% NaOH, and 2% NaOH, respectively) compared to 0.5% biosorbent loading (367, 283, and 317 ppb) at pH 3 and 60 minutes of treatment.

Discussion

Coconut husk without any pretreatment reduced the arsenic levels from 500 ppb to 283 ppb (43% reduction) at 1% loading and at pH 3 after 30 minutes of treatment. Pretreatment with 2% NaOH further reduced the arsenic levels to 183 ppb (63% reduction) which indicates that NaOH pretreatment is improving the biosorption abilities of coconut husk. This may be attributed to the ability of NaOH pretreatment to increase the porosity and surface area of biomass as well as to expose additional functional groups. These results are consistent with the results from a similar study conducted with soybeans waste with NaOH pretreatment[9]. Based on the trend from Figures 1 to 4, it appears that higher NaOH concentration than 2% may yield further arsenic reduction. Hence, additional studies are necessary to determine the optimum NaOH level at which the highest reduction can be achieved.

Longer treatment durations resulted in reduced arsenic levels (117 ppb after 60 minutes compared to 183 ppb after 30 minutes) at pH 3 and 1% biosorbent loading. Similar trends were observed in other studies with soybean waste [9] and coconut shells [10]. In an earlier study with egg shells and java plum seeds, a 120 minute treatment duration gave the optimum arsenic reduction[1]. Hence, further studies may be necessary with treatment durations longer than 60 minutes to find the optimum time.

Arsenic reduction was better at higher biosorbent loading (Fig. 3 – compare 1% vs 0.5%). Gaur et al studied the biosorbent loading from 1.0% to 4.0% for soybean waste and concluded that 3% resulted in the optimum results[9]. Due to constraints in obtaining coconut husk, biosorbent loading was limited to 0.5% and 1.0% in this study. Additional studies with higher biosorbent loading are recommended if sourcing coconut husk is not a constraint.

In this study, lower pH (3) resulted in better arsenic reduction. This observation is consistent with another study where soybean waste was used as a biosorbent with NaOH pretreatment[9].

Conclusion

Coconut husk is an effective biosorbent to reduce arsenic in water. Pretreating coconut husk with sodium hydroxide resulted in further reduction of arsenic. Higher biosorbent loading (1%), lower pH (3), and longer treatment durations (60 minutes) resulted in a higher reduction in arsenic. Further studies are recommended to optimize the arsenic treatment process by using higher levels of NaOH concentration, higher loading levels of biosorbent, and longer treatment times.

Acknowledgements

My sincere gratitude to my science teacher Dr. Tracy Hughes for providing the guidance on the project, helping to order the chemicals, and allowing me to use the lab facilities at Presentation High School.

Bibliography

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About the author

Manjari Talasila is 16 years old and she is an 11th grader at Presentation High School, a Catholic private school in San Jose, California. Her favorite subjects are biology and calculus. She is interested in both biological sciences and engineering and hopes to pursue a career in biomedical engineering. In her spare time, she loves to bake, play tennis, and read books.