**4. Ecological-chemical approach to the reduction of the microplastic burden in seawater-based processes**

At this time, there is no economical possibility yet to remove microplastics simply and cost-effectively from seawater. A promising research approach based on the adaptation of a concept by Herbort and Schuhen for freshwater systems and the simultaneous development of add-on technology for static (e.g., waterside plants) and mobile (e.g., ships) seawater utilization processes [32].

 In the process developed by Herbort and Schuhen, silane-based microplastic agglomerates are formed according to the cloud point principle through the application of special organosilane-based precursors, which, via Van der Waals forces, have a high affinity to unreactive microplastics (IOCS, inert organicchemical macromolecules) and, at the same time, a high reactivity in water. [32–35]. A Video shows the fixation process in a batch reactor for use in wastewater treatment [36].

 Organosilanes are hybrid compounds of inorganic silanes and organic hydrocarbons [37]. Through the selection of the functional groups in the organic unit (functional design), it is possible to exploit an adaptable system for the respective application (e.g., removal of reactive and/or inert organic-chemical compounds). By means of the substituent pattern within the organic unit and also directly on the silicon atom, the affinity of the organosilanes can be adapted to various polymer types and, simultaneously, the reactivity respectively the stability can be controlled.

Organosilanes with corresponding reactivity can react to organic-inorganic hybrid silica gels in the sol-gel process [38, 39]. In the first step, disposal groups are split off and reactive silanol groups generated through hydrolysis. The silanol groups subsequently form silanone bonds in a condensation reaction and link the organosilanes via a bridging unit (**Figure 5**).

A three-dimensional networked hybrid silica gel forms, which is stabilized via the respective bridging unit. Through the selection of the disposal groups and the organic groups, the properties and reactivity of the organosilanes can be specifically controlled. The selection of the disposal groups is decisive for the hydrolyzability of the organosilanes [40, 41].

The organic groups influence the water solubility, the stability of the resulting silanols, and the affinity of the organosilanes respectively the ability of the resulting silanols to fix microplastic particles. By choosing specific organic groups, the affinity to different polymer types can be controlled. Groups with low polarity can be used to attach to polymers with lower polarity like polyethylene or polypropylene. More polar polymers containing, e.g., heteroatoms, like polyester or polyamide, can be fixed by using organic groups with a similar chemical structure and polarity.

*Technological Approaches for the Reduction of Microplastic Pollution in Seawater… DOI: http://dx.doi.org/10.5772/intechopen.81180* 

**Figure 5.**  *Localization and agglomeration of microplastic particles [42].* 

 The interaction of the disposal groups and organic basis units must be so coordinated that the highest possible affinity to microplastics and optimal reaction kinetics are achieved [33]. The reaction kinetics takes on a decisive function within the research approach. It is influenced, among other things, by the water matrix and the temperature. Knowing that an increase in the water temperature accelerates the speed of the sol-gel process and that the temperature of seawater is subject to seasonal fluctuations and spatial variations, challenges are presented for the implementation of the concept in the seawater environment [39]. In addition, the factors of salinity, temperature, UV radiation, and pH value play a determining role and are also to be considered (**Figure 6**) [43].

Dissolved salts influence reaction speed and the reaction mechanism, for example, through the electrolyte effect [44, 45]. It results in the stabilization of the intermediate stages with higher ionic strength as well as catalytic effects or competitive influences of dissolved ions. The pH value influences reaction kinetics through the catalytic effect of hydroxide and oxonium ions [39]. Temperature differences directly influence the reaction speed [46, 47]. Thus, a temperature increase of 10°C is accompanied, as a rule, by approximately double the reaction speed. UV light can also facilitate the formation of reactive intermediate stages via a photocatalytic effect and thus accelerate the reaction [46]. However, it can also break down the precursors used or already linked molecules.

Initial experiments at a laboratory scale show that the salinity has a slowing influence on the fixation process and the entire process of agglomeration formation also works in an artificial salt water matrix. To produce the salt water (3.5% salt by mass), 27.5 g NaCl, 5 g MgCl2, 2 g MgSO4, 1 g KCl, and 0.5 g CaCl2 were dissolved in distilled water. This results in a mass concentration of 58.8% chloride (Cl<sup>−</sup>), 29.6% sodium (Na<sup>+</sup> ), 4.7% sulfate (SO4 <sup>2</sup>−), 4.9% magnesium (Mg2+), 1.5% calcium (Ca2+), and 0.5% potassium (K<sup>+</sup> ).

Subsequently, 0.1 g polyethylene powder (PE) (average particle size 350 μm), 0.1 g polypropylene powder (PP) (average particle size 350 μm), and 0.1 g of a 50:50 mixture of PE and PP were stirred in a beaker with 1 l of salt water respectively distilled water at room temperature. After 24 h, 0.15 ml agglomeration reagent was added and the mixture was stirred for an additional 24 h. The formed

#### **Figure 6.**

*Influence on the reaction kinetics of the fixation process in the marine environment (percentage by mass) [39, 43–47].* 

aggregates were removed and dried at 60°C for 24 h. ESEM images were taken using a FEI Quanta 250 ESEM (FEI Company, Hillsboro, USA) equipped with a large field detector (LFD). The chamber pressures were between 60 and 80 Pa and the acceleration voltage between 7 and 20 kV. The remaining water was filtered using a paper filter (Rotilabo 111A, 12–15 μm pore size). The tare weight of the filter was noted before filtration using a AX105DR (Mettler Toledo, Switzerland). Afterward, the filter was dried at 105° C for 24 h and weighted again, to check if there is remaining microplastic in the water (accuracy ±0.2 mg).

 In distilled water, an aggregation of the microplastic particles begins 15 s after the addition of the agglomeration reagent. After 2–3 min, the agglomeration is completed and an aggregate is present, which contains all of the microplastics. In the artificially produced salt water samples, the agglomeration process begins after 10 min and is concluded after 15 min. It is, therefore, significantly slower, but nevertheless fixes all the microplastics. This shows that salt water has a stabilizing effect on the reactive intermediate stages and thus slows down the sol-gel process, whereby the agglomeration starts later and also takes longer. **Figure 7** shows ESEM images of the mircoplastic particles used and their aggregates formed during the fixation process respectively fragments of the aggregate prepared for the images. How the microplastic particles are linked and embedded by the agglomeration reagent can be observed, where a considerable increase in size results. As the agglomeration reagent reacts to a solid hybrid silica, which will be removed within the aggregates from the water, residues in the water will be avoided. To ensure a complete removal of the agglomeration reagent even in the trace substance range, further TOC analysis and particle analysis will be proceeded [48, 49].

*Technological Approaches for the Reduction of Microplastic Pollution in Seawater… DOI: http://dx.doi.org/10.5772/intechopen.81180* 

#### **Figure 7.**

*ESEM images of the microplastic blanks (a, b = PE; c, d = PP) and of the agglomerates formed during the fixation process (e = PE; f = PP; g = PE/PP (50:50)).* 

## **5. Procedural implementation of microplastic elimination in seawater utilization processes**

The procedural implementation pursues the goal of increasing the service life of the existing desalination plants by already holding back the microplastic particles (0.1–5 μm) initially in pretreatment, which cause blockages of the membranes (pore size 0.002–0.1 μm). On the one hand, the service life of the microporous membranes can be thus extended and the operational expenses reduced (without the addition of suspect additives, such as, e.g., antiscalants) as well as, on the other hand, sustainably eliminating the much-criticized microplastic particles from the water cycle. As the removal is based on a physicochemical agglomeration process, it is not limited by particles size or shape like a filtration process. The agglomeration reagent can bind to fibers, films, and fragments in all size classes and fix them in big agglomerates. As a consequence, the quality of the water on the removal side near the coast/surface as well as on the output side will be improved through the reduction of the microplastic particle load.

In the application in sea salt extraction, contamination of the resulting sea salt and thus the transmission to people will be effectively avoided through the removal of microplastics from the seawater flowing into the evaporation basins.

This is possible through the combined development of a pretreatment stage of a series of stirred tanks and the inorganic-organic functional material. In addition, a high-performance cascade process is developed as add-on technology in order to

facilitate the material reaction and to make a throughput of >600 m3 a day possible. Continuous operation is sought by connecting several cascades. The adapted concept as well as the related technological implementation strategy provides for the first time the opportunity to remove plastic particles with a particle size of <5 μm effectively and sustainably from salt water in an upstream, modular pretreatment step. The diagram of the process is presented in **Figure 8**.

The process is divided in the following stages:


The feed stream, that is the extracted seawater with unwanted microplastic load, is piped to the first partial reactor via an existing suction pipe. This should take place by means of upstream, abrasion, and corrosion-resistant pumps and use the suction pipes present in the existing seawater desalination plants. Depending on the load of the feed stream, a defined amount of the organosilanes is simultaneously added by means of the dosing device and blended with the salt water already in the first partial reactor through a mechanical mixing concept.

In accordance with the concept, the addition of the material takes place in several steps within the process. In this way, the required initialization period of the material and the reaction time can be responded to through the variable design, dimensioning, and number of dosing stages, mixing installations as well as partial reactors. For this purpose, können reactors of different sizes respectively different volumes can be used so that, for example, the reaction starts in the first partial reactor and continues to react optimally in the subsequent reactors. The organosilane is mixed with the salt water according to the required concentration in the respective partial reactors, at which time it is successively bonded to the microplastics to be removed. Through injection or chemical interaction, the concentration of free,

**Figure 8.**  *Process flow chart.* 

*Technological Approaches for the Reduction of Microplastic Pollution in Seawater… DOI: http://dx.doi.org/10.5772/intechopen.81180* 


#### **Table 1.**

*Technical target criteria.* 

nonbonded microplastic particles should continuously decrease along the length of the reactor respectively the residence time. A consistent residence time of the material in the reactor is needed so that a complete conversion can be surmised. This is essential for the process, since an incomplete conversion would mean additional burdens through further foreign substances/particles.

In order to prevent the disintegration of the agglomerates, the reactor has a strict and clearly defined residence time distribution (RTD) so that ideally all particles can pass through the reactor in the same ideal time frame and grow and that the disintegration of the agglomerates can be avoided. The implementation of microplastic elimination in seawater utilization plants pursues the technical target criteria presented in **Table 1**.

## **6. Conclusions**

The innovative add-on technology for the removal of microplastics from industrial seawater utilization plants pursues the first problem solution regarding the risk of blocking from the immense microplastic particle load in the sea, among other things, in industrial, membrane-based seawater desalination plants (especially RO plants). Due to the significant reduction of the microplastic load in the pretreatment stage, alleviation results for the downstream RO membranes. The service life of the RO membranes will be significantly increased and the membranes can, thanks to the improved performance, be operated on a smaller scale and more cost-effectively.

Through the application of new add-on technology in sea salt extraction, the entry of potentially harmful microplastics in sea salt is reduced and thus, at the same time, also the contamination of everyday food. Based on the future increasing contamination of the oceans with microplastics, this technology helps to ensure the sustainable use of seawater.
