Treatment and resource utilization for a non-biodegradable organophosphorus wastewater using micro-channel reactor

ABSTRACT This study aimed to develop a method to treat and reuse non-biodegradable organophosphorus industrial wastewater using the micro-channel reactor. As an important pharmaceutical and fine chemical intermediate, diethyl hydroxymethylphosphonate (DHP) produces large amounts of industrial wastewater, which poses a serious threat to natural environment and human health. It is of great value to find a treatment method of the DHP industrial wastewater with simple operation, good safety, low cost, and environment friendly. In this study, hydrogen peroxide (H2O2) and DHP industrial wastewater were reacted in the micro-channel reactor. Reaction conditions were optimized by adjusting reaction temperature, reaction time, reaction pressure, and sampling flow rate. The treated wastewater was further neutralized by sodium hydroxide (NaOH). After the oxidation and neutralization reaction of the DHP industrial wastewater, the removal rate of chemical oxygen demand (COD) in treatment wastewater can reach 97.3%. Moreover, after the treated solution being dried, the inorganic phosphate solid was obtained with the phosphorus content as high as 92.1%. It can realize the reuse of phosphorus resources for the DHP industrial wastewater.


INTRODUCTION
Generally, advanced oxidation was often used to treat the high concentration, non-biodegradable organic industrial wastewater.It can decompose or mineralize organics, which cannot be oxidized by ordinary oxidants, based on the high activity hydroxyl radical produced by light, sound, electrical, magnetic, and other physical and chemical processes.The publications (ANDREOZZI et al., 1999;KLAVARIOTI;MANTZAVINOS;KASSINOS, 2009) have summarized the method of advanced oxidation into six types: chemical oxidation, chemical catalytic oxidation, wet oxidation, supercritical water oxidation, photochemical oxidation and catalysis, and electrochemical oxidation.Among these above methods, wet oxidation can decompose organics into CO 2 , H2O, and other inorganics or other small organics with H 2 O 2 , O 3 or air as oxidants under high https://doi.org/10.1590/S1413-415220220235Liu, H. & Wang, Z. temperature and high pressure conditions.Significantly, this method requires much harsh reaction conditions, such as high temperature between 125 to 320 o C and high pressure between 0.5 to 20 MPa.Zeng, Liu and Zhao (2017) and Wu (2022) have described the disadvantages of wet oxidation with high equipment cost and large investment cost.
As for the high concentration phosphorus wastewater, treatment mainly include precipitation, adsorption, and biological methods, as well as the newly developed electrolytic method, calcium method, Sequencing Batch Reactor (SBR) enhanced biological method, ceramic membrane coagulation reaction method, and chemical precipitation-coagulator method-activated carbon adsorption method.Publications (YEOMAN et al., 1988;PRATT;PARSONS;SOARES, 2012;PEISHI;XIAORAN;YUNZHI, 2013) have concluded the following insufficiencies: high requirements for reaction equipment, large investment, and small treatment capacity.Furthermore, none of these methods can achieve the resource utilization of phosphorus.
In the early 1990s, sustainable and high-tech development needs promoted the research of microchemical technology.Shown in Figure 1A, a continuous flow microtubular reactor with characteristic size between 10 and 1,000 μm was manufactured by precision manufacturing technology.Therefore, chemical reactions can be carried out by pumping reagents into the microchannel reactor for mixture and controlling reaction temperature by the hot swap controller and reaction pressure by the back pressure valve.Having a maximum specific surface area (about hundreds of times or even thousands of times fold the specific surface area of the stirred tank), microreactor showed excellent heat and mass transfer ability, which can achieve instant uniform mixing of materials and efficient heat transfer, in addition to process strengthening by adjusting the reaction pressure to change the reaction kinetic process.
Therefore, the microreactor can achieve high yield, high selectivity, and high safety of chemical reaction.Furthermore, the "micro" for microreactor indicated that the fluid channel was at the micron level but did not show that the micro reaction profile was small or that the product yield was small.As shown in Figure 1B, there can be millions of microchannels in the microreactors, so as to achieve high yields.Since the launch of the microchannel reactor, it has rapidly become a new development direction and research hotspot in the field of chemical industry, materials, and medicine, just as described in the publications by Doku et al. (2005), Roberge et al. (2005), Mason et al. (2007), Matsumoto (2015), and Talaiekhozani et al. (2020).At present, there is no literature reporting on the application of microchannel reactors in the field of environmental protection, especially in the study of enhancing the degradation reaction of organic matters.
Diethyl hydroxymethylphosphonate (DHP) is an important pharmaceutical and fine chemical intermediate, which is widely used in the synthesis of anti-HIV and anti-HBV drug tenofovir disoproxil fumarate (TDF), anti-HBV drug adefovir dipivoxil, and anti-CMV drug cidofovir.DHP can also be used in the synthesis of herbicides, fungicides, and organophosphorus flame retardants.At present, the main synthesis method is obtained through addition reaction followed by intramolecular rearrangement using diethyl phosphonate and paraformaldehyde or formaldehyde as raw materials.Thus, the DHP production process produces large amounts of industrial wastewater with high chemical oxygen demand (COD) (mg/L) ranged from tens of thousands to hundreds of thousands, and poor biodegradability by the low ratio value (0.1) of biochemical oxygen demand (BOD) to COD.It should be noted that the industrial wastewater is a kind of high concentration non-biodegradable organophosphorus wastewater with phosphorus content ranged from 100 to 3,000 mg/L.It is an important environmental problem in wastewater treatment, which poses a serious threat to natural environment and human health.Therefore, it is of great value to find a treatment method of DHP industrial wastewater with simple operation, good safety, low cost, and environment friendly.
Aimed at high concentration and hard-biodegraded DHP industrial wastewater, this study depicts a microchannel reaction first used to enhance the degradation process based on the advantages of rapid mixing, efficient heat and mass transfer, continuous sample injection and no amplification.A continuous treatment method for a kind of the high concentration and non-biodegradable organophosphorus industrial wastewater was constructed by heating hydrogen peroxide (H 2 O 2 ) to produce a large number of free radicals, and then fast reacting it with organophosphorus under a certain temperature and pressure, so as to meet the level of purification in compliance with emission standards.Meanwhile, the treatment method can carry out resource utilization for organophosphorus using the microchannel reactor.The results of the study will

A B
Treatment and resource utilization for a non-biodegradable organophosphorus wastewater using micro-channel reactor achieve rapid, efficient, safe, easy to control, and quantifiable continuous degradation of phosphorus-containing organic wastewater and will also provide new ideas and technologies for the efficient degradation of other non-biodegradable organic pollutants.

MATERIALS AND METHODS
The designed microchannel reactor

The treatment process
According to the process schematic shown in Figure 2, the DHP industrial was-

Materials and instruments
DHP industrial wastewater was originally collecting from Huangshi Fuertai Pharmaceutical Tech Co., Ltd., China, which is a leading producer and supplier of TDF intermediates and has been developing for over 10 years.In this study, the COD value of DHP production wastewater is 112,367 mg/L, the ammonia nitrogen (NH 3 -N) value is 1,742 mg/L, the ratio of BOD to COD is 0.15, and the phosphorus content is 2,500 mg/L.

Determination methods of total phosphorus
According to the Chinese standard GB 11893-89, TP content of DHP production wastewater before treatment and the residual solid after treatment were determined.The sample was digested with HNO 3 and HClO 4 in order to oxidize all phosphorus into orthophosphate.Then, the orthophosphate was reacted with (NH 4 ) 2 MoO 4 in the presence of potassium antimony tartrate to form molybdophosphate heteropoly acid, which was immediately reduced by ascorbic acid to form the blue complex.The phosphorus content in the blue complex was determined by spectrophotometer (UV-2000).The standard curve of phosphorus content was shown in Figure 3.  Liu, H. & Wang, Z.

RESULTS AND DISCUSSIONS
Optimum conditions for the treatment method In order to find the optimal reaction conditions to treat DHP industrial wastewater by the microchannel reactor, aimed at the oxidation reaction, a series of experiments were made by adjusting the reaction temperature (T), reaction time (t), reaction pressure (P), and sampling flow rate     Treatment and resource utilization for a non-biodegradable organophosphorus wastewater using micro-channel reactor Resource utilization for DHP industrial wastewater using the microchannel reactor After the oxidation and neutralization reactions of DHP industrial wastewater treatment using the microchannel reactor, the treated solution with yellow color was obtained.Then, it was distilled to separate the water and get the solid with the yellow color.Under the above optimum conditions, a series of parallel experiments were performed and the corresponding yellow solids were obtained.The mass of the yellow solid was weighed.According to Chinese standard GB 11893-89, the TP content of the yellow solid was determined.Detailed information were listed in Table 2.
As seen in Table 2, under the same reaction conditions, the yellow solids obtained appeared almost the same mass ranged from 13.9 to 14.1 g.Similarly, the TP contents of the yellow solids presented the same mass ranged from 12.7 to 12.9 g.By calculation, the recovery rate of P from the yellow solid was ranged from 90.7 to 92.1%.The result showed that the treated DHP industrial wastewater by microchannel reactor can obtain the relatively stable and high P content of the yellow solid.

CONCLUSIONS
Based on the advantages of rapid mixing, efficient heat and mass transfer, continuous sample injection and no amplification, microchannel reactor was first used to treat the high concentration and hard-biodegraded DHP industrial wastewater.According to the oxidation reaction of the DHP wastewater with H 2 O 2 and the neutralization reaction of the solution after oxidation reaction with NaOH, the DHP wastewater was treated with good degradation effect.Also, treated DHP wastewater can perform resource utilization with high P content.
• Optimal conditions for treating DHP industrial wastewater using microchannel reactor are as follows: lower temperature (140°C), longer reaction time (13.3 min), and slower sampling flow rate (1.5 mL/min); • Under ideal conditions, good degradation effect for DHP industrial wastewater can be reached with the high removal rate of NH 3 -N (97.3%) and COD (95.2%); • Inorganic phosphate solid was obtained with P content as high as 92.1%, which can perform the reuse of phosphorus resources for the DHP industrial wastewater.
The treatment process using the microchannel reactor for the DHP industrial wastewater indicated the advantages of simple operation, good safety, controllable conditions.Also, it can be amplified in parallel without amplification effect, which is suitable for industrial application.

Figure 1 -
Figure 1 -The device of industrial micro-channel reactor: (A) Inner structure of the microchannel reactor; (B) The high-throughput micro-channel reactor.
The microchannel reactor was designed by oxidation reaction of the DHP industrial wastewater with H 2 O 2 and neutralization reaction of the solution after oxidation reaction with NaOH.The specific experimental process schematic is shown in Figure2.Micromixer 1 (P-713) was set up for mixing the raw materials with the two analytical liquid chromatography pumps 5 and 6 (NP7010C).Connected with micromixer 1, microreactor 2, which is a tubular reactor, was designed for oxidation reactions.Microreactor 2 was made of stainless steel with an inner diameter of 0.5~1 mm and a volume of 20 mL.Followed by microreactor 2, back pressure valve 7 (BP-10) was used to adjust the pressure of the reaction solution.Next, micromixer 3 (P-713) was set up to mix reaction materials with the analytical liquid chromatography pump 8 (NP7010C).Connected with micromixer 3, microreactor 4, which is a tubular reactor, was designed for neutralization reactions.Like microreactor 2, microreactor 4 was made of stainless steel with an inner diameter of 0.5~1 mm and a volume of 20 mL.Following microreactor 4, container 9 was used to collect the waste solution.
tewater and H 2 O 2 were pumped by pump 5 and 6 and mixed with the volume ration of 1:1 in micromixer 1.Then, the oxidation reaction was run in microreactor 2 by adjusting reaction temperature, reaction time, reaction pressure, and sampling flow rate.After the oxidation reaction, in micromixer 3, the reaction solution was mixed with 0.1 mol/L sodium hydroxide (NaOH) pumped by pump 8 in a volume ratio of 1:1.The neutralization reaction was carried out in microreactor 4 at room temperature for about 1~5 min.The treated solution was collected by container 9 and distilled under reduced pressure.The distilled water determined the value of COD and NH 3 -N, and the residual solid determined the total phosphorus (TP) content.

Figure 2 -
Figure 2 -Process schematic of the treating DHP industrial wastewater using the microchannel reactor.

(
F).The corresponding value of COD and NH 3 -N were determined for each experiment.Detailed experimental data were listed in Table 1.The experimental results of DHP industrial wastewater treatment were illustrated in Figure 4.Under constant reaction pressure (500 Pa), reaction temperature, reaction time, and sampling flow rate, an important effect on the value of COD and NH 3 -N of the treated wastewater was observed.As for reaction temperature, with other conditions (reaction time and sampling flow rate) unchanged, both the COD and NH 3 -N value increased significantly with the rising reaction temperature.The result indicated that the relatively lower temperature can achieve better degradation effect.At the lowest reaction temperature (140°C), the value of COD and NH 3 -N was the lowest, with 3,000 and 83 mg/L, respectively.It can be speculated that higher temperature promotes the decomposition of H 2 O 2 , resulting in poor degradation effect of wastewater treatment.As for reaction time, with other conditions (reaction temperature and sampling flow rate) unchanged, both the COD and NH 3 -N value decreased significantly with the increasing reaction temperature.The result showed that relatively longer reaction time (13.3 min) can get better degradation effect.As for sampling flow rate, with other conditions (reaction temperature and reaction time) unchanged, both the COD and NH 3 -N value decreased significantly with the slowed flow rate.The result showed that the relatively slower sampling flow rate (1.5 mL/min) was conducive to the decomposition of wastewater.Taken the influence of reaction temperature for example, the removal rate of NH 3 -N and COD for the treatment DHP industrial wastewater using the microchannel reactor were shown in Figure5.When the reaction temperature increased from 140 to 180°C, the removal rate of NH 3 -N and COD decreased from 97.3 to 86.6% and from 95.2 to 90.2%, respectively.From the above analysis, the optimum conditions for treating DHP industrial wastewater using the microchannel reactor are as follows: lower

Figure 3 -
Figure 3 -The standard curve of determination of total phosphorus.ure 3 -The standard curve of determination of total phosphorus.

Figure 4 -
Figure 4 -Experimental results of DHP industrial wastewater treatment using the microchannel reactor.

Figure 5 -
Figure5-Removal rate of NH 3 -N and COD for DHP industrial wastewater treatment using the microchannel reactor.

Table 2 -
Information of the phosphorus recovery.