Phycocyanin is a blue colored pigment, synthesized by several species of cyanobacteria and red algae. Besides the application as a food-colorant, the pigmented protein is of high interest as a pharmaceutically and nutritionally valuable compound. Since cyanobacteria-derived phycocyanin is thermolabile, red algae that are adapted to high temperatures are an interesting source for phycocyanin extraction. Still, the extraction of high quality phycocyanin from red algae is challenging due to the strong and rigid cell wall. Since standard techniques show low yields, alternative methods are needed. Recently, spark discharges have been shown to gently disintegrate microalgae and thereby enable the efficient extraction of susceptible proteins. In this study, the applicability of spark discharges for phycocyanin extraction from the red alga Cyanidium caldarium was investigated. The efficiency of 30 min spark discharges was compared with standard treatment protocols, such as three times repeated freeze-thaw cycles, sonication, and pulsed electric fields. Input energy for all physical methods were kept constant at 11,880 J to ensure comparability. The obtained extracts were evaluated by photometric and fluorescent spectroscopy. Highest extraction yields were achieved with sonication (53 mg/g dry weight (dw)) and disintegration by spark discharges (4 mg/g dw) while neither freeze-thawing nor pulsed electric field disintegration proved effective. The protein analysis via LC-MS of the former two extracts revealed a comparable composition of phycobiliproteins. Despite the lower total concentration of phycocyanin after application of spark discharges, the purity in the raw extract was higher in comparison to the extract attained by sonication.

Cylindrospermopsin (CYN) is an important cyanobacterial toxin posing a major threat to surface waters during cyanobacterial blooms. Hence, methods for cyanotoxin removal are required to confront seasonal or local incidences to sustain the safety of potable water reservoirs. Non-thermal plasmas provide the possibility for an environmentally benign treatment which can be adapted to specific concentrations and environmental conditions without the need of additional chemicals. We therefore investigated the potential of two different non-thermal plasma approaches for CYN degradation, operated either in a water mist, i.e. in air, or submerged in water. A degradation efficacy of 0.03 ± 0.00 g kWh^-1 L^-1 was found for a dielectric barrier discharge (DBD) operated in air, while a submerged pulsed corona-like discharge resulted in an efficacy of 0.24 ± 0.02 g kWh^-1 L^-1. CYN degradation followed a pseudo zeroth order or pseudo first order reaction kinetic, respectively. Treatment efficacy of the corona-like discharge submerged in water increased with pH values of the initial solution changing from 5.0 to 7.5. Notably, a pH-depending residual oxidative effect was observed for the submerged discharge, resulting in ongoing CYN degradation, even without further plasma treatment. In this case hydroxyl radicals were identified as the dominant oxidants of CYN at acidic pH values. In comparison, degradation by the DBD could be related primarily to the generation of ozone.

Microalgae possess a cell wall with remarkable physical and chemical strength, which has proved to be a bottleneck for extraction techniques. With the application of spark discharges instigated by 100-ns high voltage pulses, a novel method was established that was found to be effective and yet gentle for the extraction of sensitive, especially heat-sensitive, compounds. The spark, which was ignited directly in the submerged algae suspension, is characterised by several physical and chemical processes that might promote a successful disintegration of the cells. Among other characteristics, the conceivably most destructive mechanism, i.e. strong shockwaves, was examined with schlieren diagnostics to evaluate associated pressures. The results were compared with the tensile strength, determined by atomic force microscopy, of Chlorella vulgaris , which was chosen as model organism. A shockwave pressure of 500 MPa in the close vicinity of the discharge by far exceeded the elasticity modulus of the algae with 13.95 MPa, confirming the potential of mechanically breaking the cell wall. At the same time, bulk temperatures could be maintained close to room temperature by adjusting the operating parameters for the spark-application, hence, especially encouraging the extraction of thermally unstable intracellular substances.

Pulsed streamer discharges submerged in water have demonstrated potential in a number of applications. Especially the generation of discharges by short high-voltage pulses in the nanosecond range has been found to offer advantages with respect to efficacies and efficiencies. The exploited plasma chemistry generally relies on the initial production of short-lived species, e.g. hydroxyl radicals. Since the diagnostic of these transient species is not readily possible, a quantification of hydrogen peroxide provides an adequate assessment of underlying reactions. These conceivably depend on the characteristics of the high-voltage pulses, such as pulse duration, pulse amplitude, as well as pulse steepness. A novel electrochemical flow-injection system was used to relate these parameters to hydrogen peroxide concentrations. Accordingly, the accumulated hydrogen peroxide production for streamer discharges ignited in deionized water was investigated for pulse durations of 100 ns and 300 ns, pulse amplitudes between 54 kV and 64 kV, and pulse rise times from 16 ns to 31 ns. An independent control of the individual pulse parameters was enabled by providing the high-voltage pulses with a Blumlein line. Applied voltage, discharge current, optical light emission and time-integrated images were recorded for each individual discharge to determine dissipated energy, inception statistic, discharge expansion and the lifetime of a discharge. Pulse steepness did not affect the hydrogen peroxide production rate, but an increase in amplitude of 10 kV for 100 ns pulses nearly doubled the rate to (0.19 ± 0.01) mol l⁻¹ s⁻¹, which was overall the highest determined rate. The energy efficiency did not change with pulse amplitude, but was sensitive to pulse duration. Notably, production rate and efficiency doubled when the pulse duration decreased from 300 ns to 100 ns, resulting in the best peroxide production efficiency of (9.2 ± 0.9) g kWh⁻¹. The detailed analysis revealed that the hydrogen peroxide production rate could be described by the energy dissipation in a representative single streamer. The production efficiency was affected by the corresponding discharge volume, which was comprised by the collective volume of all filaments. Hence, dissipating more energy in a filament resulted in an increased production rate, while increasing the relative volume of the discharge compared to its propagation time increased the energy efficiency.

Pronounced reilluminations can be observed for discharges that are generated in water by submicrosecond high-voltage pulses. This phenomenon can appear during the decrease of the applied voltage, depending on the pulse characteristics, especially fall times. The respective processes are expected to be crucial for optical investigations of discharge characteristics and also for applications. Accordingly, the development of individual corona-like discharges in water was investigated in a needle-to-plate geometry for their ignition by single rectangular high-voltage pulses with a duration of 100 ns. The pulse amplitudes of about 50 kV and adjusted pulse fall times of 20, 26, and 43 ns were used. Voltages, currents and light emissions were synchronized with subsequent images of individual discharge events. Two distinct stages, a propagation phase and a reillumination phase, were identified. The individual discharges expanded for all investigated fall times with the same velocity of (29 ± 2) km s ⁻¹ . No significant differences of streamer morphologies and electrical characteristics were observed for the propagation phase. As the voltage began to decrease, light emissions first went through a minimum before increasing again. The following maxima were strongly dependent on fall times. Corresponding images showed that discharge channels were reilluminated within 6 ns. However, not all of the previously observed filaments reignited at once for longer fall times and instead single channels reilluminated subsequently. The development of reillumination can be explained from the external electric field in the electrode gap during the streamer propagation, which determines the head charge distributions at the discharge channel ends. This space charges generate an internal electrical field in the filaments resulting in the reillumination during the falling edge of the applied high-voltage pulse.