Recent Progress of Phytoremediation-Based Technologies for Industrial Wastewater Treatment

Phytoremediation is considered of a cost effective and environmentally friendly technology and has been used successfully for the remediation of soils and water contaminated with various pollutants. Specifically for full scale application to treat industrial wastewater, phytoremediation is used as sole technology for different types of wet - lands. However, phytoremediation of polluted water in wetland type reactor has been mostly studied as black box. The method to measure the performance is only based on pollutant removal efficiency and there is very limited information available about of the pollutant removal mechanisms and process dynamics in these systems. Thus, the aim of this chapter was to briefly review basic processes of phytoremediation, its mechanisms and parameters, and its interaction between rhizo-remediation and microbe-plant. In addition, this chapter also elaborated phytore - mediation challenges and strategies for full-scale application, its techniques to remove both organic and inorganic contaminants by aquatic plants in water, and some examples of applications in industries.


INTRODUCTION
Conventional wastewater treatment is not completely effective method for water contaminants removal. Trace concentrations of toxic contaminants can still be found in wastewater effluent. Thus, an alternative technology to reduce the contaminant concentration to the safe level is necessary. Different types of wastewater treatment technology are introduced. However, most of these technologies are considered to have high energy requirement, high carbon emission, excess sludge discharge and high maintenance cost (Mustafa and Hayder, 2020). A sustainable management of aquatic ecosystem needs eco-friendly and low-cost remediation methods. Aquatic plants have the potential to remove inorganic and organic pollutant. Phytoremediation is defined as a bioremediation that utilizes plants for wastewater remediation and utilizes plants roof to adsorb nutrients in the wastewater. Specific species of plants even have the ability to accumulate certain pollutants. Phytoremediation has been proven to be more efficient, cost effective and more environmentally friendly than conventional treatment.
There are the plants that have high phytoremediation ability, such as Brassica juncea, Arundo donax L. Miscanthus sp., Typha latifolia and Thelypteris palustris for heavy metals removal such as Zn and Cu, by using bioaccumulation mechanism (Ullah et al., 2015). Salvinia molesta and Pistia stratiotes also have been widely used for the treatment of agricultural, domestic and industrial wastewater (Mustafa and Hayder, 2020). Type of plants is not only the main factor for successful phytoremediation process, the role of rhizosphere-associated microorganisms is also important. Microorganisms help improving phytoremediation process through biosorption and bio-augmentation. Organisms such as Acidovorax, Alcaligenes, Bacillus 95 mycobacterium, Paenibacillus, Pseudomonas, and Rhodococcus have been reported to enhance the phytoremediation process (Sharma et al., 2021).
However, phytoremediation of polluted water in wetland type reactor has mostly been studied as black box. The method to measure the performance is only based on pollutant removal efficiency and there is very limited information available about of the pollutant removal mechanisms and process dynamics in these systems. This chapter briefly reviews basic processes of phytoremediation, its mechanisms and parameters, and its interaction between rhizo-remediation and microbe-plant. In addition, it also elaborates the phytoremediation challenges and strategies for full-scale application, its techniques to remove both organic and inorganic contaminants by aquatic plants in water, and some examples of applications in industries.

PRINCIPLES OF PHYTOREMEDIATION
The concept of phytoremediation must be differentiated from bioremediation. Bioremediation process is merely assisted by heterotrophic bacteria that are responsible for organic contaminants degradation and mineralization, as well as accumulation of metals and other elements and oxidation of inorganic compounds (McCutcheon and Jørgensen, 2018). In turn, phytoremediation process is based on the role of photoautotroph bacteria to treat contaminants via mechanisms such as: • Release organic matter as their metabolism products (during growth and maintenance), thus improves the number of heterotrophs bacteria. • Pump the oxygen into the plant root zone and also deposit secondary metabolites during root die-back in the rhizosphere to boost the number of aerobic, facultative, or anaerobic organisms to degrade or accumulate contaminants • Transport pollutants into active microbial zones by evapotranspiration, blockage of flows, or other means.
In more detail, phytoremediation mechanisms can be broken down into several types, namely: phytodegradation, phytoextraction, phytovolatilization, phytofiltration and phytostabilization, which is shown in Table 1.

INTERACTION BETWEEN RHIZO-REMEDIATION AND MICROBE-PLANT IN PHYTOREMEDIATION
Rhizosphere is the most important area during phytoremediation . Rhizosphere is the place where pollutants have contact with the treatment agent (plant) (Al-Ajalin et al., 2020a; Ismail et al., 2020). Plants root played an important role in the removal of pollutant from wastewater (Al-Ajalin et al., 2020b). Beside the root, there are also microbes (known as rhizobacteria) that also greatly support the degradation of pollutant in the rhizophore . Plant roots and microbes interact, which leads to the removal of contaminants from the contaminated medium as illustrated in Figure 1.
There are 4 major interactions in rhizosphere occurred during the phytoremediation of pollutants from wastewater: phytostimulation (Hawrot-Paw et al., 2019), rhizofiltration (Rahman and Hasegawa, 2011), rhizodegradation (Imron et al., 2019b;Kadir et al., 2020), and phytostabilization (Bolan et al., 2011). Phytostimulation is a process in which plant releases its exudates in the rhizosphere (Backer et al., 2018). The released exudates nearby the root area provide good environment for rhizobacteria to grow optimally . The release of exudates stimulates the growth of rhizobacteria which the perform symbiotic interactions (Shahid et al., 2020). The phytostimulation cannot be separated from rhizodegradation. Rhizodegradation is  . In addition to the rhizobacterial processing heavy metals, plant exudates contain complex compound that may increase the solubility of metals (to be treated further by rhizobacteria) or to bind directly with heavy metals to produce complex metal-exudates which then stabilized in the rhizosphere (phytostabilization) (Dakora and Phillips, 2002). Plant roots also perform physical treatment of wastewater by performing screening of bulk compounds in their roots. This mechanism mostly occurred in the treatment of pollutants using fi brous root type species (Elias et al., 2014). After performing several mechanisms in rhizosphere, a plant then performs phytoextraction in which it absorbs pollutants via transfer mechanism to bioconcentrate it into its cell . Phytoextraction can occur directly to pollutant and also its intermediate compounds (after degraded by rhizobacteria). There are no signifi cant diff erence mechanisms between the treatments of wastewater using sub-surface or freesurface constructed wetland. The major diff erences are the species used and the contaminated medium that need to be treated (Kadir et

RHIZO-REMEDIATION AND MICROBE-PLANT INTERACTION IN PHYTOREMEDIATION
Despite many advantages of phytoremediation application for industrial wastewater treatment, this method still has some challenges to be faced during application. Some challenges of phytoremediation application and strategies that may cover the challenges are summarized in Figure 2.
Phytoremediation needs certain conditions to work well, including the requirement of sunlight These requirements need to be fulfi lled during application to obtain the best removal performance. Phytoremediation is considered to be very suitable for use in tropical countries (Ahmad et al., 2017) due to the availability of sunlight throughout the year and optimum temperature and humidity for plant growth, while in sub-tropical countries, controlled environment is highly needed . Greenhouse treatment is suggested to be applied to maintain the optimum environmental conditions for plants to treat pollutants. Under controlled environment, plants will be able to maintain their performance throughout the year that may lead to the desired removal effi ciencies.
Rhizosphere is the most important area in phytoremediation since the contact of pollutants and treatment agents occurs there (Kamaruzzaman et al., 2019). This may become a challenge when plants root do not have a good contact with pollutants. To overcome this issue, the design of appropriate constructed wetland needs to be conducted prior to the application Application of phytoremediation to treat industrial wastewater requires large area and is also considered to be time consuming . These issues are highly related with the rate of pollutants degradation by plants during treatment. Biological treatment has different reaction as compared to chemical treatment . In chemical treatment, stoichiometry of reaction controls the degradation of pollutant based on the equilibrium of reactants and products (Kis et al., 2017). In biological treatment, the capability of plants cannot be simply calculated as reactants and products equilibrium due to the complex mechanisms that involve many factors occurring during treatment (Karpowicz et al., 2020;Nottingham et al., 2018). To overcome these issues, most researchers suggest the utilization of phytoremediation technique as secondary or tertiary treatment to purify wastewater before discharge into water bodies. Chemical treatment is suggested as primary treatment, which may reduce the pollutant load in phytoremediation stage that may produce better removal rate, reducing the required time and surface area for treatment.
As plant grows during the treatment, plant biomass is produced, and its amount can be considered as abundant. If phytoremediation was applied to treat toxic substances (commonly heavy metals), the produced plant biomass needs to be handled following the standard procedure of handling toxic substances (Kwoczynski and Čmelík, 2021). If phytoremediation was applied to treat organic-rich or nutrient-rich wastewater, several conversion possibilities can be selected. Several biomass utilization studies had been successfully applied to convert biomass into animal feed . With these conversion options, the wastewater treatment using phytoremediation may lead to the cleaner production strategy from utilization of treatment by-product.

Aquatic plants selection
Aquatic plants are required in phytoremediation for degrading and removing contaminants within aquatic environments. These plants include ferns, pteridophytes, and freshwater adapted angiosperms. Aquatic plants are preferable to terrestrial plants for wastewater treatment because of their faster growth rate, larger biomass production, and better contaminant removal ability due to direct contact with the wastewater. The effectiveness of these plants in phytoremediation can be assessed by estimating the contaminants removed from the target area. Not only for remediation purposes, many of such aquatic plants also serve as bioindicators and biomonitors (Rai, 2009).
In addition, some keys principles that need to be considered in operating a phytoremediation system are as follows: a) identifying the suitable and efficient aquatic plants for the phytoremediation system; b) uptake of dissolved nutrients (e.g. N, P, and metals) by the aquatic plants; and c) harvesting process and utilization of the plant biomass generated from the phytoremediation system (Lu et al., 2010). Regular harvest of the aquatic plant biomass from a remediation site is necessary. Otherwise, the plants' biomass will be decomposed and subsequently release the stored contaminants back to the aquatic environment (Kumwimba et al., 2020).
Selection of aquatic plants that can grow well while degrading targeted contaminants is critical. Some plants commonly used for phytoremediation could experience disrupted growth if exposed to a high level of contaminants. The toxicity effects of the contaminants against aquatic plants are varied. Some negative responses of aquatic plants toward aquatic contaminants are growth reduction, wilting, chlorosis, reduction of roots and shoots length or volume, chlorophyll reduction, reduction in photosynthetic activity, and plant mortality (Ansari et al., 2020). For instance, in the case of water hyacinth, the exposure to high levels of cadmium and zinc to (Eichornia crassipes) resulted in reduced growth, as determined from biomass production, survival rate, and crown root number (Sricoth et al., 2018b). Another study by de Campos et al. (2019) that exposed water lettuce (Pistia stratiotes) with a high level of arsenite showed that although P. stratiotes was able to maintain its biomass, there had been a significant reduction in the root volume, chlorosis in the leaves, and damage in the cell membranes.
The ability of aquatic plants to reduce contaminants varies between plants. Therefore, to reduce the unfavourable effects on the plants' growth in a phytoremediation system, it is necessary to pay attention to the characteristics of the selected plants. The ideal characteristics of aquatic plants used for phytoremediators are as follows: high growth rate, production of more above-ground biomass, widely distributed and highly branched root system, high bioaccumulation potential, ability to transform or degrade contaminants, ability to regulate chemical speciation, capacity to treat both organic and inorganic contaminants, high accumulation of the target heavy metals from soil (bioconcentration factor > 1), translocation of the accumulated heavy metals from roots to shoots (translocation factor > 1), tolerance to the toxic effects of the target heavy metals, good adaptation to prevailing environmental and climatic condition, resistance to pathogens and pests, easy cultivation and harvest, and repulsion to herbivores to avoid food chain contamination (Dhir, 2013 and Thampatti et al., 2020).
Another primary factor that needs to be considered in the utilization of aquatic plants in a phytoremediation system corresponds to understanding the characteristics of the wastewater to be treated. Wastewater is a mixture of pure water with a large number of chemicals (including organic and inorganic chemicals) and heavy metals produced from domestic, agriculture, industrial and commercial activities. Organic contaminants can be categorized into persistent organic pollutants (POP)/xenobiotics (i.e., dioxins, polycyclic aromatic hydrocarbons, and polychlorinated biphenyls), pesticides (i.e., glyphosate, hexachlorocyclohexane, fenhexamid, and deltamethrin), and pharmaceutical and personal care products (PPCPs) (i.e., antibiotics, hormones, and pain relief medication) (Al Falahi et al., 2022). Meanwhile, primary inorganic contaminants are nutrients (i.e., N, P, and K) and metalloid elements (i.e., Fe, Al, Pb, Ni, Cd, and Cu). The existence of these various pollutants in the environment needs serious attention, since they can cause various harmful effects (Rahim et al., 2022). Potential adverse effects of those contaminants on the surrounding environment and living things are as follows: eutrophication, chronic toxicity, endocrine disruption, and antibiotic resistance (Fletcher et al., 2020).

Types of aquatic plants
Aquatic plants have earned an immense reputation due to its capacity to clean up contaminated water bodies. With their extensive roots system, these plants become the best option for degrading contaminants in a phytoremediation system. On the basis of their growth form, aquatic plants can be classified into free-floating, submerged, and emergent plants (Al Falahi et al., 2022).
Free-floating aquatic plants are the plants with floating leaves and submerged roots. Several free-floating aquatic plants have been studied extensively and approved to be applied in different phytoremediation systems. Some recognized free-floating aquatic plants are duckweeds (Lemna, Spirodela, and Wolffia), water hyacinth (Eichhornia), water ferns (Salvinia, Azolla), and water lettuce (Pistia). Those plants are known for having the capability to remove a wide variety of inorganic and organic contaminants, heavy metals, pesticides, and nutrients from various sources, such as industrial and domestic wastewater, sewage, and agricultural runoff. Moreover, those plants can grow in polluted sites with tremendous variation in temperature, pH, and nutrient level   (Fletcher et al., 2020). Furthermore, another type of plants that becomes a new interest in phytoremediation corresponds to transgenic plants. Transgenic plants were engineered so that specific genes in the plants can increase its metabolism and enhance detoxification process of organic pollutants for more effective phytoremediation. In this approach, incorporated genes secrete enzymes which degrade organic pollutants in the rhizosphere zone. This might solve the problem in plant harvesting and handling loaded with toxic metals, as all the metal detoxification and removal process occur in the rhizosphere by roots. Different species of aquatic plants have been long studied for its potential in phytoremediation with notable successes. Table 2  However, competition between plants should be understood as this may impact the effectiveness of contaminants removal. Moreover, a study by Geng et al. (2017) also suggests that the composition of appropriate plants species might be more important than increasing species richness. Therefore, further studies to find optimal plant combinations for removal of particular contaminants are required, as this would help optimize phytoremediation efficiency. Constructed wetland can remove high number of organic pollutants, especially nutrients, such as nitrogen and phosphorus. In integrated system of wastewater treatment plant, constructed wetland  could be placed after biological secondary treatment (i.e., activated sludge system) to enhance the quality of the effluent. Table 3 reviews full scale constructed wetland application in industries.

CONCLUSIONS
Phytoremediation is one of the oldest techniques to remove pollutants from the environment, particularly in water and soil. The basic principle of phytoremediation is using the interaction between plant roots and root microorganisms. Deep knowledge about microbe-root plant interaction mechanisms is required to develop a more robust, effective and efficient model. Constructed wetland is the most used phytoremediation model. This model has a great potential in the future due to its robustness and flexibility. Nowadays, many advance technologies, such as Microbial Fuel Cell (MFC), could be integrated in the constructed wetland system. The possibility of system integration between phytoremediation and another advance technology should be explored extensively to enhance the effluent quality and reduce the cost.