Efficient Nitrous Oxide Capture from Dam Lake Treatment by Malt Dust-Derived Biochar ^†

Abstract

Owing to the natural texture of dam lakes, they emit nitrous oxide (N₂O) emissions. The main aim of this study was to reduce N₂O emissions resulting from dam lake treatment using malt dust-derived biochar. On average, a 21.1% reduction in N₂O emissions from dam lake treatment was reported using malt dust-derived biochar. The maximum nitrous oxide capture capacity corresponded to the malt dust-derived biochar fabricated at the minimum pyrolysis temperature (MD1). The statistical analysis results revealed that the optimum parameters were 4 mg/L of dissolved oxygen (DO) and 11 mg/L of nitrate (NO₃⁻) for the minimum N₂O emission. The highest correlation was calculated between N₂O emission and NO₃⁻ with the value of 97.99%. This study evidenced that agro-industrial biochar can adsorb N₂O from dam lakes.

1. Introduction

Dam lake (reservoir) treatment is regarded as one of the most significant greenhouse gas sources by the Intergovernmental Panel on Climate Change (IPCC) [1,2,3,4,5]. Nitrogen (N) is a critical constituent of the global N cycle which is earning recognition for its significant contribution to nitrous oxide (N₂O) emissions through nitrification and denitrification in water supplies [6,7]. N₂O is a major greenhouse gas (GHG) which can be released from dam lake treatment due to the nitrification and denitrification processes. From this perspective, this study investigated the capture of N₂O emissions resulting from dam lake treatment using malt dust-derived biochar.

Biochar is a significant substance as it can dispose of pollutants and serve as a carbon-negative technology [6,8,9,10]. This study mainly aimed to reduce the N₂O emissions released from dam lake treatment using malt dust-derived biochar. In the literature, several studies have focused on the determination of N₂O emissions from dam lakes [11,12,13,14,15]. Separate from these studies, this study concentrated on minimizing N₂O emissions using biochar.

This study’s originality stems from its use of malt dust-derived biochar as a N₂O adsorbent for potable water treatment. The limitations and assumptions of this study were that the biochar adsorption process was performed as a lab-scale study, and that gas sampling was performed under optimum and ideal conditions. Also, fugitive N₂O emissions were considered. According to the calibration test, the gas leakage ratio was 0.05 (0.5%). This ratio was added to the measurement values.

2. Materials and Methods

Experimental Procedure/Methodology/System Description

The experimental procedure was based on the N₂O measurements before and after the biochar adsorption process and water analyses [16]. Also, N₂O adsorption by biochar was used to validate the uptake capacity of biochar. The malt dust was obtained from a brewery industry in Turkey as the feedstock of the biochar. Slow pyrolysis was performed under the temperatures of 250 (MD1), 300 (MD2), and 500 °C (MD3) in a fluidized bed reactor. The water samples were taken from Ataturk Dam Lake (Sanliurfa Irrigation Tunnels). The raw water had a pH of 7.39, 3.29 mg/L DO, and 18.39 mg/L NO₃⁻.

The N₂O measurement was performed using a gas chromatographer (GC) equipped with an electron capture detector (GC-ECD (GC-2010, Shimadzu, Kyoto, Japan)). The concentration of N₂O was determined by the GC-ECD. The N₂O was sampled before and after the biochar adsorption process. In this study, the N₂O emission calculation methodology was derived based on the IPCC approach (Equation (1)) [2]. In Equation (1), GHG represents the N₂O emissions (kg CO₂ e/d), G represents the N₂O concentration, and GWP is the Global Warming Potential of N₂O. The GWP of N₂O is 273 [2], and K is the Henry’s Law constant. The Langmuir isotherm model (Equation (2)) [17] was used to analyze the equilibrium of N₂O adsorption by biochar. In Equation (2), qe and qm (mmol/g) are the equilibrium and maximum monolayer adsorption capacity, respectively. K_L (1/atm) is the Langmuir co-efficient, and P is the partial pressure of N₂O (atm).

$$GHG =G\times GWP\times K$$

(1)

$$qe={\displaystyle \frac{qm KL P}{(1+KL P)}}$$

(2)

The correlation between the N₂O emission and water quality parameters was investigated by Monte Carlo simulation. One simulation and 500 iterations were used. The simulation tool is shown in Equation (3). The correlation co-efficient (C) was determined according to this tool. The inputs were the N₂O emissions (GHG) and the water quality parameters which were DO, pH, and NO₃⁻. In Equation (3), C is the correspondence between GHG emissions and the water quality parameters.

C = Riskoutput (Lognormal) + RiskLognorm (GHG; DO, pH, NO₃⁻)

(3)

3. Results and Discussion

3.1. GHG Reduction Amounts

An average reduction of 21.1% was reported for N₂O emissions based on the N₂O measurements.

Table 1. N₂O removal amounts (%) by biochar.

Biochar	Average N2O Removal Amount (%)
MD1	23.3
MD2	21.0
MD3	19.0

shows the removal amounts of N₂O emissions for each biochar in detail. The highest N₂O removal amount corresponded to MD1 (23.3%) which was fabricated at the lowest pyrolysis temperature.

3.2. N₂O Adsorption by Malt Dust-Derived Biochar

According to the adsorption results, the highest adsorption capacity was corresponded to MD1 (7.21 mmol/g). The Brunauer, Emmet and Teller (BET) analyses results validated this result. The highest total volume and the highest Langmuir and BET surface areas related to the malt dust-derived biochar fabricated at the lowest temperature (MD1). The experimental gas adsorption results are presented in

Table 2. Results of experimental N₂O adsorption (Langmuir isotherm model).

N2O Adsorption	MD1	MD2	MD3
qe (mmol/g)	7.21	7.00	6.97
qm (mmol/g)	7.02	6.98	6.95
KL (1/atm)	12.21	11.74	10.95
R2	0.991	0.988	0.981

.

The previous studies in this research area focused on the determination of N₂O emissions [11,12,13,14,15]. The main advantage of the biochar adsorption process is that there is no N₂O accumulation in the system. The other main advantages of biochar application are its low-cost system and regeneration and reuse capability. The Monte Carlo simulation results showed that the highest correlation was observed (C = 0.9799) between N₂O emissions and NO₃⁻. The correlation co-efficient of N₂O emission and DO was 0.9689. The minimum correlation was between N₂O emissions and pH, with a value of 0.7456.

4. Conclusions

This study was evidenced that malt dust-derived biochar could not only adsorb the pollutants from potable water but also remove N₂O, a major air pollutant. On average, a 21.1% reduction in N₂O emissions was reported using malt dust-derived biochar. The highest correlation was reported between N₂O emission and NO₃⁻ at 97.99%. The regeneration capacity of biochar should be investigated after the biochar adsorption process to contribute to circular economy principles in future research.