Water treatment agents，including coagulants / flocculants，bactericidal agents，and scale inhibitors，play important roles in the field of water treatment． However，traditional water treatment agents usually bear only single function，which result in a wide variety of agents with high doses， complicated devices，and fussy operations in real applications． Thus，it is of great significance in scientific research and practical applications to develop multifunctional water treatment agents containing coagulation / flocculation， bacteriostasis / sterilization， and scale inhibition． Moreover， natural polymer-based water agents have been recently paid much more attention due to their vital characteristics of high efficiency，low cost，and environmental friendliness． It is greatly significant to develop natural polymer-based multi-functional water treatment agents． However，field operation data in natural polymer-based multi-functional water treatment agents are very limited，and their widespread use awaits field operation experience． This paper introduces coagulants / flocculants， bactericidal agents，and scale inhibitors as well as their application mechanisms． On the basis of structure-activity relationship，the realization of their multiple functions and the future development have been discussed in detail．
The world is currently facing a serious problem of freshwater shortage. At the same time, the increase in the number of various types of sewage discharges has exacerbated the water stress situation. Water treatment agent is an important class of fine chemical products, in the purification and treatment of industrial wastewater and domestic wastewater has an important role. The large-scale use of water treatment agents not only provides a large amount of clean drinking water and industrial and agricultural water for human beings greatly alleviates the environmental hazards of the sewage, but also effectively maintains the balance of the ecosystem.
Specifically, water treatment agents can be used to remove suspended solids, deodorize and decolorize water bodies, inhibit/sterilize, soften water quality, inhibit the formation of scale, and reduce raw water damage to equipment, etc. Therefore, according to its functional characteristics, water treatment agents can be divided into coagulants/flocculants, biocides, scale inhibitors, corrosion inhibitors, and adsorbents, etc. In the years of research practice, in the face of the complex situation of various types of water bodies, people often need to add a variety of water treatment agents to meet the purification requirements. However, the simultaneous addition of a variety of agents not only increases the difficulty of operation, increasing production costs. And a variety of agents are also easy to form interference between each other, affecting the treatment effect. In addition, the traditional chemical agents in the use of the process will inevitably occur in the water residue, causing secondary pollution, increasing the risk of water safety.
Therefore, the future of water treatment agents should have both green, efficient, and multi-functional, and other important features. The so-called multi-functional water treatment agent refers to the coagulation/flocculation, inhibition/sterilization, scale inhibition, etc. in two or more functions of the water treatment agent. This coagulation / flocculation function is fundamental. China has carried out a lot of useful work in the development of multifunctional water treatment agents. Natural polymers as the base material for the development of new efficient multifunctional water treatment agent materials are undoubtedly of great practical significance.
In addition to coagulants/flocculants and biocides, scale inhibitors are also an important class of water treatment agents. Its main role is to disperse the insoluble inorganic salts in water, to prevent or interfere with its precipitation and scaling on the metal surface so that metal equipment maintains a good heat transfer effect. There are many kinds of scale inhibitors, which can be divided into small molecule scale inhibitors and high molecule scale inhibitors.
The mechanism of scale inhibition generally includes complexation and solubilization, dispersion, lattice distortion, and solubility-limiting effect. These mechanisms can better explain some of the phenomena in the scale inhibition process, but they are not comprehensive enough. In general, multiple effects are synergistic, which jointly affect the final scale inhibition effect.
Although traditional water treatment agents generally have the advantages of good performance, low cost, and stable operation, but most of them have many safety hazards, inevitably after the use of water residues in the body of secondary pollution. This is undoubtedly an urgent need for more green and efficient materials to replace. Natural polymers are macromolecules from natural animal, plant, and microbial resources, such as cellulose, starch, chitosan, and so on. They can be easily decomposed into the water, carbon dioxide, etc. after being discarded, and are environmentally friendly materials because they are widely available and non-toxic. In addition, it is worth mentioning that natural polymer materials are renewable resources, which can be inexhaustible. The modification of natural polymers by chemical or physical methods to produce efficient and environmentally friendly water treatment agents is undoubtedly of great economic and social value.
Since natural polymer chains are rich in functional groups suitable for chemical modification, from the above introduction of various natural polymer water treatment agents and their mechanisms, we can see that different functional groups can be introduced chemically to achieve and expand the water treatment properties of natural polymer materials according to their application purposes. Next, some common chemical modification methods suitable for natural polymer materials are briefly introduced, such as etherification, amination, esterification, acylation, graft copolymerization, oxidation, and Mannich reaction, etc.
Generally speaking, coagulation/flocculation, sterilization, and scale inhibition are in the mechanism of action or there are major differences. For example, in terms of molecular weight, flocculants in addition to electrical neutralization, but also emphasize the role of bridging, net capture, which can generally be achieved by increasing the molecular weight of the agent; and for the sterilization effect, a small molecular weight of the bactericide seems to have stronger activity, easier to destroy the cell membrane into the cell effectively kill cells; and for scale inhibition
For scale inhibition, small molecular weight scale inhibitors also tend to show better performance than large molecular weight scale inhibitors.
However, long-term studies have found that coagulants/flocculants, biocides, and scale inhibitors still have many common features, especially the characteristic functional groups. Quaternary ammonium biocides and strong cationic flocculants contain a large number of quaternary ammonium groups, which have good bactericidal and electro-neutralizing coagulation effects due to the negative charge of most of the bacteria and colloidal particles in the water body. It is found that a variety of flocculants have a good flocculation effect on algae-bacterial symbionts in culture wastewater, and the removal rate of Chlorella can reach 90%-98%. The best flocculation rate of Chlorella can reach 90%-98%, and the best flocculation rate of Chlorella can reach more than 80%. Organic polymeric scale inhibitors and anionic flocculants Organic polymer scale inhibitors and anionic flocculants often contain anionic functional groups, such as carboxylic acid groups, sulfonic acid groups, etc., so that they have the possibility of both scale inhibition and flocculation.
In summary, the similarity of active functional groups provides an important basis for the development of multifunctional water treatment agents, and these active functional groups, such as These active functional groups, such as quaternary ammonium groups, carboxylic acid groups, sulfonic acid groups, etc., can be easily introduced into the polymer chain, and the preparation of multifunctional natural polymeric water treatment agents is entirely possible. On the other hand, although the molecular weight distributions required to achieve different functions are different, they overlap and are not completely independent. With the development of research, some large molecular weight scale inhibitors and biocides are gradually found, which also have flocculation functions. In addition, through the same agent with different molecular weight composition, the use of the same agent with different molecular weight composition (that is, the molecular weight of each component of the agent is different) can make up for the defects of a single molecular weight water treatment agent, while Realize multi-functionalization
At present, the research on the multi-functionalization of natural polymer water treatment agents is still in its initial stage, far from systematic and comprehensive. This is both certain objective factors, such as the complexity of the actual water conditions, water treatment agent molecular structure (such as molecular weight, functional groups, etc.) on flocculation, inhibition/sterilization, and inhibition of There are some objective factors such as the complexity of the actual water conditions, the molecular structure of the water treatment agent (e.g., molecular weight, characteristic functional groups, etc.) on flocculation, inhibition/sterilization, and scale inhibition, etc., and there are also reasons that the mechanism of water treatment is not deep enough. Figure 6 shows the effect of flocculation - inhibition/sterilization - scale inhibition relationship. Given the urgent need for green and efficient water treatment agent research and development in the future, and the natural polymers themselves have a wide range of sources, low toxicity, are easy to degrade, rich in active functional groups.
If we can make full use of these advantages and develop multifunctional natural polymers, we can effectively develop natural polymers. If this can be fully utilized, the development of multifunctional natural polymers water treatment agents can effectively overcome and avoid the limitations of traditional water treatment agent function and potential secondary pollution of water bodies. Therefore, it can be said that multi-functional natural polymer water treatment agent has a broad space for development.