Design of Biochar Filter for Arsenic Removal

Groundwater, a vital natural resource, is contaminated with arsenic in many regions, posing significant health risks. Over 2.5 billion people worldwide rely on groundwater for drinking, and arsenic contamination has become a growing global concern. In India, states like Assam, West Bengal, and Bihar, Haryana, Karnataka, and Punjab are severely affected. Arsenic exists in organic and inorganic forms, with the latter being more toxic. Exposure to arsenic can occur through contaminated water, food, and industrial processes, leading to serious health effects like skin cancer, lung cancer, and cardiovascular disorders. In Assam, districts like Jorhat, Golaghat, and Dhemaji have reported high arsenic levels. Various techniques can remove arsenic, including adsorption, coagulation, membrane technologies, and biological methods. Adsorption is often preferred due to its low cost and reliable performance. This study aims to analyze arsenic and iron concentrations in Upper Assam's groundwater and to conduct and design column tests to evaluate arsenic removal using different filter media. Effective arsenic removal techniques are crucial to mitigate health risks in affected regions. The study's findings can help develop sustainable solutions for providing safe drinking water to communities in Upper Assam. By understanding the extent of arsenic contamination and evaluating removal techniques, this study can contribute to improving public health and well-being in the region.

LITERATURE REVIEW

• Zhao (2021) studied the interaction of arsenic with micro sized sulfidated ZVI under both anoxic and oxic conditions. It the absence of oxygen, As (II) was removed from solution primarily through the formation of As      than half of the removal resulting from the adsorption of As(I) and FeAsS precipitation. Under oxic conditions, adsorption onto iron oxy-hydroxides was the dominant mechanism of As (III) removal. Column experiment showed that less than 2% weight of ZVI in sand was able to rapidly reduce As from 300 to 10 ppb or about 96.67%.
• Keerio (2021) conducted an experimental study using modified Nepali bio sand filter (BSFs) to remove arsenic from groundwater with an initial concentration of 150 ppb. They utilized several bioadsorbents, including bio-char, rice husk, and banana peel, alongside iron nails. The study reported that the bio-char modified BSF achieved removal efficiency of 93%, while the iron nails achieved 96% efficiency. The flow rate for the bio-char modified BSF was approximately 68 seconds per 200 cm^3, indicating a practical application for household water treatment. The bio-char modified BSF achieve a 93% arsenic removal efficiency, reducing initial concentration from 150 ppb to around 10.5 ppb or lower.
• Bretzler (2020) conductedan extensive investigation into zero- valent iron (ZV) filters in Burkina Faso, where they assessed low- cost filtration systems utilizing small iron nails positioned between layers of sand. Their results showed that these filters achieved arsenic removal efficiencies ranging from 60% to 80% in field conditions during the initial six months, while laboratory tests indicated efficiencies surpassing 95% under optimized settings. The study emphasized the significance of maintaining a saturated nail bed and regulating flow rates to enhance arsenic removal.
• Huang (2020) introduced an innovation method utilizing a Fenton- like reagent like Hydrogen peroxide with ferrihydrite- loaded bio-char which is a composite material that acts as an adsorbent and catalyst for arsenic removal. The approach generally achieves arsenic (III) to arsenic (V) conversion rates of more than 90% and overall arsenic removal efficiencies in the range of 85% to 99%, depending on the initial arsenic concentration and system parameters. Flow rates for practical applications are generally 0.5 to 2 L/min/m², supporting effective treatment for household and small-scale water systems.
• Diprupa Bhaktiari (2018) from Assam has developed a simple and low-cost filter for both arsenic and iron removal using process based on oxidation- coagulation adsorption by three common chemicals (baking soda, potassium permanganate, ferric chloride) in specific amounts to water to be treated, which takes about three minutes, followed by filtration after one hour of resting. The recurring cost of the chemicals is less than Re. 0.5 per 100 litres of water (Rs. 500 for one lakh litres). The patented technology can bring down arsenic concentration from levels as high as 500 ppb to below 2 ppb which is around 99% removal efficiency and iron concentration is reduced to less than 0.1 ppm.
• Bakshi (2018) investigated the potential of zero-valent iron bio- char complexes for arsenic sorption. Studies on zero-valent iron (ZVI) and bio- char composites have generally showed that these materials can achieve significant arsenic removal efficiencies, often ranging from 70 -95%, depending on conditions such as pH, initial arsenic concentration, and contact time. The synergy between ZvI and bio- char can enhance adsorption due to increased surface area and reactive sites, making the composite effective for arsenic reduction. Typically flow rates for these systems are 0.5 to 3 L/min/m², suitable for household or small-scale water treatment systems.  
• Abedin (2011) investigated the application of mixture of ZVI and sand to remediate the arsenic in contaminated water in a laboratory setup. The flow rate maintained for 180 days. It showed removal of 99% arsenic and below 10 ppb even when the columns were packed with only 25% ZVI by volume.
• Cornejo (2008) conducted an in situ arsenic removal method applicable to highly contaminated water using steel, wool, lemon juice and solar radiation. It was used where arsenic concentration ranged from 1000 to 1300 ppb. Using RSM method of optimization between ZVI (Steel wool) and citrate concentration (lemon juice) and solar radiation, the optimum efficiency came when 1.3 g/l of steel wool was used for 0.04 ml of lemon juice giving percentage removal of higher than 99.5%.          

MATERIALS

1. Iron Nails: It is primarily composed of iron, and can remove arsenic from water through adsorption and precipitation. When exposed to water, iron nails rust, forming ferric and ferrous hydroxides and oxides. Arsenic binds to these corrosion products, forming insoluble particles that can be trapped in sand filter.
2. Bamboo Bio-Char: It is made from bamboo through pyrolysis. It is a porous material effective in removing arsenic from water. Its large surface area and ability to be modified enhance its absorption capacity. Studies show iron-modifies bamboo charcoal can achieve high removal rates of arsenic, making it a low-cost adsorbent.    
3. Sand: Sand in arsenic removal filters acts as a physical filter, supporting the adsorbent media, ensuring proper water flow, and improving contact time for effective arsenic removal in a multi-layered filtration system.
4. River Gravel: Gravel in filtration system supports finer media, improve water flow, and provides pre-filtration of larger particles, enhancing efficiency, stability, and longevity of filters, including those for arsenic removal.
5. Brick Chips: This improves structure, ensure optimal water flow, and provide pre-filtration, supporting low cost, sustainable water treatment solution.
6. Nano Materials: They remove arsenic from water through adsorption, targeted binding, and reactivity. Their high surface area and tailored properties enable efficient capture of arsenic ions, making them an effective solution for water purification and arsenic removal applications. Nanomaterials from bamboo charcoal are used in this study.

METHODOLOGY

Assam’s Public Health Engineering Department reported 6881 arsenic-affected habitations. Our project collected water samples from 8 tube wells of Jorhat and Golaghat district shown in the table below:  

Table: 1

1. Total Arsenic Test: This test used Silver Diethyldithiocarbamate (SDDC) method. The procedure involved taking 35 ml of sample in a conical flask, adding HCL, potassium iodide, and stannous chloride, and waiting for 15 to 20 minutes. Then, 3 to 4 grams of Zinc granules were added, and the setup was connected to an absorber tube with SDDC solution. Arsine gas passed through, forming a reddish colour.The intensity was measured using a spectrophotometer to determine arsenic concentration.
2. Iron Test: Iron in water comes from natural and human sources, existing as dissolved (ferrous) and oxidized (ferric) forms. Iron testing involves adding iron reagent to a 5 ml sample, mixing, and reacting for a few minutes. The treated sample is then measured using a spectrophotometer to determine iron content.
3. Column Tests: They were conducted to evaluate filter materials for removing arsenic and iron from water. A 600 mm long, 42mm diameter glass column was filled with different materials like sand, gravel, bamboo bio-char, brick chips, nanomaterials etc. Seven experiments were done with varying material combinations to find the best for arsenic removal. The best combination was then tested with different filter bed heights to assess arsenic removal efficiency. Similarly, three experiments were conducted for iron removal with varying filter bed heights. Flow rates were measured, and contaminants removal efficiency was evaluated for each combination and height.

RESULTS ANDDISCUSSION

Table 2: Untreated samples test results

Table 3: Column test results

It is found that the filter with brick chips, sand, bamboo biochar and gravel effectively removed arsenic and iron, while nonmaterial filter achieved 97 % arsenic and 100% iron removal, outperforming iron nails.

Table 4: Different Bed height combinations for Arsenic Removal

The filter design at point 4, with specific bed heights of brick chips, sand, bamboo biochar and gravel, achieved exceptional arsenic removal efficiency of 78% and low arsenic value of 0.007 ppm.

Table 4: Different Bed height combinations for Iron Removal

The filter design at point 3 effectively removes iron, achieving 0.00 ppm, making it a reliable and affordable solution.

DESIGN OF FILTER

After conducting all the experiments, it was concluded that using a single filter is sufficient to effectively remove both arsenic and iron from contaminated water. We constructed a filter with layers of Brick chips, Sand, Bamboo biochar and Gravel in an upper plastic bucket, connected to a lower bucket for filtrate storage. Materials were washes, dried, and assembled in specific heights: Brick chips (7 cm), Sand (9cm), Bamboo biochar (10cm) and Gravel (10 cm). Water samples were then filtrated and tested.