Monitoring dissolved active trace elements in biogas plants using total reflection X‐ray fluorescence spectrometry

Abstract

Generally, full‐scale biogas plants require trace elements supplementation to sustain a stable process. The aim was to apply total reflection X‐ray fluorescence (TXRF) to monitor the dissolved, active elements, for the first time, at μg/L levels in biogas plants. The digester liquid is comparable to sludge with high solids content and high salt concentrations. The ions are usually measured in the dried fermenter slurry after acid digestion in its inactive bound form, although microorganisms can take up only dissolved ions as active agent. In this study, a procedure was developed to monitor dissolved ions in such environmental processes to prevent an overdose or underdose. The analytical figures of merit considered were the lowest possible limit of detection, the recovery of spiked ions and linearity. Samples were spiked with nickel, selenium and tungsten in the range of 10–200 μg/L. Highest recovery rates of >93% for concentrations of 10 to <100 μg/L were obtained after the samples were centrifugated twice, followed by application of microwave‐assisted acid digestion. The calculated lowest limit of detection was 2–4 μg/L. The developed method was first tested in laboratory‐scale biogas digesters with wheat straw as substrate. Successful application of TXRF was also achieved in a full‐scale biogas plant to estimate the dynamics of “active” trace element ions in the range of 2–1,000 μg/L for several months. Therefore, cost‐effective biogas plant management is possible through the application of TXRF spectrometry to monitor trace elements.

Mesophilic and Thermophilic Anaerobic Digestion of Wheat Straw in a CSTR System with ‘Synthetic Manure’: Impact of Nickel and Tungsten on Methane Yields, Cell Count, and Microbiome

“…Low dissolved ions concentration of around 0.05 mg/L was found during the fermentation process. However, dissolved nickel and tungsten concentrations were below the detection limit (less than 0.002 mg/L [ 9 ]) under both temperature conditions. There was only a small difference between the iron concentration of all the six fermenters, as the average concentration was about 9.1 for mesophilic and 7.8 mg/L for thermophilic fermenters, respectively ( Table 3 ).…”

“…Potassium, calcium, manganese, iron, cobalt, nickel copper, zinc, selenium, and tungsten were determined every fortnight (in some periods weekly) using total reflection X-ray fluorescence spectrometry (TXRF), which is mainly suitable for heavy elements analysis (mol weight > 50), according to recent studies [ 8 , 9 ]. The methods [ 8 , 9 ] were developed to enable easy quantification of trace elements in their dissolved, active forms, as only ions can be taken up through the microbial cell membrane. TXRF is applicable for liquid and dry samples.…”

“…Several fermentation experiments with straw were already performed in a continuously stirred tank reactor system (CSTR), but not with a defined medium and automatic intermittent feeding (8 h cycle), similar to a full-scale plant. Furthermore, a high number of publications about trace element supplementation of anaerobic digesters already exists [ 4 , 5 , 6 , 7 ], but not with simultaneous monitoring and quantification of the dissolved active, bioavailable trace elements [ 8 , 9 ]. By the presented fermentations, as well as the necessary duration to completely wash the trace elements from the inoculum, should be determined.…”

“…However, this method is time consuming and, therefore, not used frequently. A new method to analyse dissolved, bioavailable trace elements in biogas plants by total reflection X-ray fluorescence spectrometry (TXRF) was developed to overcome this basic problem in a practical manner [ 8 , 9 ].…”

Atomic spectrometry update: review of advances in the analysis of metals, chemicals and materials

“…The sole paper, by Arthur et al 94 investigated the monitoring of dissolved active trace elements in biogas plants using TXRF . Natural plant biomass or biowaste plants in many cases show deficiencies in Fe, Co and Ni and they are supplemented with trace elements to guarantee stable gas and energy production.…”