Evaluation of a liquid-phase plasma discharge process for ammonia oxidation in wastewater: Process optimization and kinetic modeling.

Removing ammonia-nitrogen (NH3N) from wastewater is of paramount importance for wastewater treatment. In this study, a novel continuous liquid plasma process (CLPD) was evaluated to remove NH3N from synthetic wastewater. The Box-Behnken experimental design was used to optimize the main process parameters, including the initial NH3N concentration (50-200 mg/L), power input (150-300 W), and gas-flow rate (1.5-2.5 L/min), for efficient NH3N removal from wastewater. The gas-flow rate and power input were found to be significant factors affecting the removal efficiency of NH3N, whereas the initial concentration of NH3N played a vital role in determining the energy efficiency of the process. Under the optimal conditions of an initial NH3N concentration of 200 mg/L, applied power of 223 W, and gas-flow rate of 2.4 L/min, 98.91% of NH3N could be removed with a N2 selectivity of 92.91%, and the corresponding energy efficiency was 0.527 g/kWh after 2 hrs of treatment. A small fraction of undesirable NO3--N (7.05 mg/L) and NO2--N (2.83 mg/L) were also produced. Kinetic modeling revealed that NH3N degradation by the CLPD followed a pseudo-first-order reaction model, with a rate constant (k) of 0.03522 min-1. Optical emission spectroscopy (OES) was used to gather information about the active chemical species produced during the plasma discharge. The obtained spectra revealed the presence of several highly oxidative radicals, including ‧OH, ‧O, and ‧O2+. These results demonstrate the potential of liquid phase plasma discharge as a highly efficient technology for removing ammonia from aqueous solutions.

Comprehensive Assessment of Essential and Potentially Toxic Trace Elements in Bovine Milk and Their Feeds in Different Agro-climatic Zones of Sri Lanka.

Fresh milk is an important source of essential mineral supplement for humans. However, the levels of trace elements in milk are an important component of its safety and quality. Trace elements also act as a bio-indicator of agricultural pollution. Samples of raw cow milk (n = 68), animal feed [forage (n = 36) and concentrates (n = 14)], and water (n = 35) were collected from different agro-climatic regions of Sri Lanka. The concentrations of 15 trace elements including toxic heavy metals such as Cd, Pb, and As were quantified using inductively coupled plasma mass spectrometry after microwave-assisted digestion. Among the studied trace elements, the mean elemental concentrations of Se, Cd, As, and Cu in cow milk were 18.1, 1.45, 7.35, and 71.7 µg L-1, respectively. The mean concentrations of these elements in forage were 0.216, 0.066, 0.046, and 9.21 mg kg-1, and in concentrate feed were 0.329, 0.202, 0.229, and 2.28 mg kg-1, respectively. The As content of the studied milk was well below the maximum permissible level while 15% of the samples had Pb exceeding the permissible limit of 20 µg L-1. However, As, Cd, and Pb levels in drinking water provided to animals were well below the WHO permissible limits. The results of this study revealed that the trace elements in cow milk depend mainly on the feed, and hence, levels of these contaminants in feed materials should be monitored. Although due to the consumption of milk, the populations have a minor exposure to trace elements and also minimum health risks, a detailed study with a large number of samples is highly recommended.