Synergistic effects of calcium peroxide and Fe 3 O 4 @BC composites on AVS removal, phosphorus and chromium release in sediments

Black-odorous sediment pollution in urban have received widespread attention, especially pollution caused by acidied volatile sulde (AVS), phosphorus and heavy metals. Fe 3 O 4 was loaded onto the biochar (BC) by coprecipitate method to synthesize Fe 3 O 4 @BC composites, which were mixed with calcium peroxide (CP) to sediment pollution treatment. In this study, the removal of AVS was analyzed from three aspects: rstly, AVS was oxidized with oxygen produced by CP. And then, Fe 2+ could activate H 2 O 2 produced by CP to generate hydroxyl radicals which have strong oxidation property to oxidize AVS. Finally, AVS was removed by bacterial denitrication. The results showed that the AVS removal rate could reach 71% compared with the blank group on the 25 th day. With the addition of CP and Fe 3 O 4 @BC, the content of overlying water remained at 0.1 mg/L, which was due to the conversion of NH 4 Cl-P and Fe/Al-P in sediments into Ca-P to inhibit the release of phosphorus. At the same time, the acid extractable state and the reducible state of Cr in the sediment also decreased to 0.58% and 0.97%, which inhibited the release and migration of the heavy metal Cr. In addition, the results of high-throughput genetic test showed that the CP+Fe 3 O 4 @BC group had a great increase in the total number of microorganisms compared to other groups. The abundance of Sulfurovum increased while that of sulphate-Reducing Bacteria (SRBs) was inhibited. Moreover, an abundance of denitrifying bacteria (Dechlorominas, Acinetobacter and Flavobacterium) was increased. This study showed that the combined application of Fe 3 O 4 @BC composites and CP had a remarkable effect on the urban sediment treatment, which provided a new way to remove sediment pollution.


Introduction
In recent years, with the rapid development of industry and agriculture in China, a large number of pollutants have been discharged into water bodies, doing serious harm to urban rivers and forming blackodorous sediment (DeFu et al., 2015; Yu et al., 2020;Zhu et al., 2017). The main pollutants include AVS, phosphorus and heavy metals. Among three types of pollutants, AVS mainly cause of black and odorous water (Mai et al., 2021). The release of phosphorus and nitrogen in sediments can lead to eutrophication (Waajen et al., 2016a), which will affect water quality and aquatic ecosystem. The migration of heavy metals will affect living things and thus human health. Among these heavy metals, chromium (Cr) is the major cause of water pollution by anthropogenic and natural sources. Therefore, the treatment for AVS, phosphorus and Cr become a research hotspot.
The current approaches to remove AVS are chemical methods (e.g., chemical oxidation, chemical precipitation and occulation). AVS removal through denitri cation enhanced by nitrate addition has been considered as a cost-effective technology for black-odorous sediments control .
However, the addition of nitrate will lead to a great increase of nitrogen in the water, which leads to eutrophication and affects human health and safety (Alan R Townsend et al., 2003). At the same time, nitrate has a negative effect on the release of phosphorus and heavy metals from sediment . Calcium peroxide (CP) is another chemical substance that can remove AVS effectively, which doesn't produce other pollutants and is environmentally friendly. CP can slowly liberate hydrogen peroxide (H 2 O 2 ) and O 2 at a "controlled" rate when contacting with water (detailed as shown in Eqs. (1) and Eqs. (2)), during which H 2 O 2 can further generate produce free radicals . Therefore, CP can effectively control black-odorous sediments (Nykänen et al., 2012;Wang et al., 2019). At the same time, for phosphorus and chromium pollution, we mainly inhibit their release from sediment (Yin et  particles. Compared with other support materials, biochar (BC) is a readily available porous carbon-rich material with a higher speci c surface area (Dong et al., 2017). BC is widely applied in soil improvement (Ahmed et al., 2016;Huang et al., 2020), sediment treatment (Ahmed et al., 2016;Huang et al., 2020) and environment recovery (Ahmed et al., 2016;Huang et al., 2020). Humic acid is a carbon-rich material that contains a large number of functional groups, such as hydroxyl, carboxyl and aromatic groups. Biochar derived from humic acid not only has a larger speci c surface area, but also retains organic functional groups, which can solidify heavy metals though cation exchange and complexation . Therefore, we consider that Fe 3 O 4 loaded onto humic acid biochar can also solve the agglomeration problem of nano-Fe 3 O 4 . At the same time, it can adsorb heavy metals to reduce the harm of heavy metals. Up to now, it has not been reported that the Fe 3 O 4 @BC composite was produced, which was used to synergistically treat black and smelly sediment pollution with calcium peroxide.
In this study, humic acid was selected as the raw material to produce biochar at 600 ℃. Nano-Fe 3 O 4 @BC composites were successfully prepared by the coprecipitation method. The synergistic effect of CP and Fe 3 O 4 @BC composites was used to treat the black-odorous sediment. This study mainly focused on the removal effect and mechanism of AVS, and the effects on phosphorus and Cr release. The change of microbial community in the sediment was also explored deeply. This research proposes a safe, high e ciency and eco-friendly method to treat black and odorous sediment pollution in the urban river.

Reagents and materials
Calcium peroxide was purchased from Zhoujian Animal Health Technology Co., Ltd. (Guangdong, China), and the content of calcium peroxide is above 60%. Humic acid was purchased from the Changbai Mountain Nutrient Soil Plant in Dalian, China. All the other chemicals used were analytical and purchased from Chengdu Kelong chemical company (Sichuan, China). Deionized water was applied throughout the experiment.

Preparation of biochar
The humic acid was rst crushed and sieved, then put in a porcelain boat and heated in a tube furnace under N 2 atmosphere (200 mL/min) condition at 600 ℃ for 2 h, and the heating rate was kept at 10 ℃/min.

Preparation of Fe 3 O 4 @BC
The Fe 3 O 4 @BC nanocomposites were synthesized by the co-precipitation method (Zhu et al., 2011;Zhuang et al., 2015). Firstly, 10.2 g FeCl 3 ·6H 2 O and 4.975 g FeCl 2 ·4H 2 O were dissolved in 200 mL ultrapure water, then 7 g of pre-prepared BC powder was added into the solution and continued to stir 24 h. A peristaltic pump was used to slowly drop 5 M NaOH into the reaction solution (4 mL/min) until the solution pH reached 11. The whole preparation process was purged with nitrogen gas (N 2 ) to eliminate the interference of dissolved oxygen in the water. Finally, the black material obtained was ltered and washed several times, and placed in the 65°C vacuum oven to dry 24 h.

Characterization of the biochar and Fe 3 O 4 @BC
The surface morphology of the biochar and the Fe 3 O 4 @BC composites were observed using scanning electron microscopy (SEM, JSM-7500F, JEOL). Energy dispersive X-ray spectroscopy (EDS) was coupled with SEM to examine surface elemental composition and obtain surface elemental distribution maps.
The chemical structures were identi ed by Fourier-transform infrared spectroscopy (FT-IR, Nicolet 6700, USA). Analysis of the crystalline structures of the catalyst powders was performed by X-ray diffraction (XRD, EMPYREAN, UK).

Experiment design
The experiments were conducted in a series of 250-mL serum bottles and the total number of experimental serum bottles was 147. Seven sets of batch experiments were carried out simultaneously and the speci c amount was shown in the following table (Table 1).

Measuring method
The pH and ORP were analyzed with a portable analyzer (Multi3610, WTW, Germany). For the determination of AVS, 2.0 g wet sediments were added into a round-bottom ask lled with N 2 . Then the sediment suspension was stirred and acidi ed for 45 min with 20 mL of 6 mol HCl at room temperature. Finally, with N 2 as a carrier gas, the H 2 S was absorbed by 0.5 mol NaOH solution and determined via the methylene blue method, using a Techomp UV1000 spectrophotometer. According to the four-step extraction method proposed by Hieltjes and Lijklema (LIJKLEMA, 1980), different forms of P were extracted from the sediment and the content of P was analyzed by molybdate salt photometer (T3200, The different fraction of Cr in the sediment was measured by BCR (LIJKLEMA, 1980) three-step extraction method and the Cr was detected by Atomic absorption spectrophotometer (AA-6880G, Japan). Ammonia

Statistical analysis
Page 6/27 The results are presented as the means and standard deviations of three replicate samples. Signi cant differences among the means in different treatments were identi ed through one-way ANOVA, followed by Tukey's test.
All statistical analyses were performed using SPSS 21.0 (IBM, New York, USA), and signi cant levels were reported at p < 0.05.

SEM and EDS
As shown in Fig. 1 (a), the surface morphology of biochar mainly presents a aky structure with a few porous structure. However, it could be clearly observed that the surface was loaded with many nanoparticles when the Fe 3 O 4 was loaded on the biochar in Fig. 1

XRD and FT-IR
The crystalline properties of BC and Fe 3 O 4 @BC were analyzed by XRD (Fig. 2a).

pH and OPR
The variations of pH in overlying water were presented in Fig. 3a. There was nearly no difference between the CK and the CP+Fe 3 O 4 @BC regarding pH value during the experimental study (p > 0.05). The pH of each group with BC or CP increased and nally remained at about 8.0. In the CP+BC+Fe 3 O 4 group, the pH remained stable in the early stage and began to rise after the 6th day, eventually reaching 8.3.
From Fig. 3b, it was found that the ORP of all groups (excluding the CP+Fe 3 O 4 @BC group) remained positive during the entire process of the experiment. But in the CP+Fe 3 O 4 @BC group, the ORP was negative in the rst 8 days and then started to rise to a positive value after the 8th day.

Removal of AVS in the sediment
As shown in Fig. 4a  Phosphorus release from sediment is the main factor of overlying water eutrophication (Gao et al., 2005). The form of sediment P is divided into: weakly adsorbed phosphorus (NH 4 Cl-P), iron/aluminum bound phosphorus (Fe/Al-P), calcium bound phosphorus (Ca-P) and Residual Phosphorus (Res-P). NH 4 Cl-P and Fe/Al-P are easy to migrate, while Ca-P and Res-P are stable and di cult to migrate. On the 2th and 8th days, phosphorus morphological changes were shown in Fig. 6a and 6b. Although NH 4 Cl-P and Fe/Al-P contents decreased in the CP+Fe 3 O 4 @BC group, Ca-P content increased to 27.86%. Therefore, TP content of overlying water in the CP+Fe 3 O 4 @BC group was maintained at 0.1 mg/L. However, the NH 4 Cl-P content in the sediments of the CK group decreased from 23.25-10.61%, while the other contents remained unchanged, indicating that the release of NH 4 Cl-P into water led to the increase of TP content in the overlying. The content of Fe/Al-P in the BC group also decreased from 28.14-12.96%, while the other contents remained unchanged, which increased the content of TP in the overlying water.

Changes of Cr content of different fractions in the sediments
The release of Cr in sediment is an important cause of heavy metals in water.     Figure 9 shows the relative abundance of genus-level microorganisms in different experimental groups. The main bacteria in the CK group were Dechloromonas (5.6%), Arthrobacter (7.3%), Paenisporosarcina (3.5%), Sulfurovum (0.5%), Luteolibacter (2.5%) and Desulfobulbus (0.45%).

Microorganisms in sediments
Sulfurovum can obtain energy by oxidizing AVS

AVS removal mechanism
The excessive accumulation of AVS in the sediment could poison biological growth and impact environmental health . At the same time, AVS is the main cause of black and smelly water . More than 71% of the sediment AVS in the CP+Fe 3 O 4 @BC group had been removed, compared with the concentration of AVS in the CK group. The mechanism of AVS removal in this experiment is as follows: Firstly, CP can slowly decompose to release O 2 at a "controlled" rate when in contact with hydrous media. Zhang et al., 2020). The production of oxygen also led to the increase of Sulfurovum (Fig. 9), which promoted the removal of AVS. Therefore, the removal rates of AVS in the sediments of the CP group,  . (4)). In the CP+Fe 3 O 4 group, CP+Fe 3 O 4 +BC group and CP+Fe 3 O 4 @BC group, the removal rate of AVS was as high, as 26.26%, 38.58% and 52.77%, respectively. It is probably attribute to the Fenton reaction between Fe 2+ and calcium peroxide, which produced hydroxyl radicals (Fig. 10b)  . Nano-Fe 3 O 4 was loaded onto the BC by the coprecipitation method, which solved the agglomeration problem of nano-Fe 3 O 4 and made it fully contact reaction. Therefore, the removal of AVS from sediments in the CP+Fe 3 O 4 @BC group was 14.19% more than that in the CP+BC+Fe 3 O 4 group. At the same time, the by-product of the Fenton reaction would produce H + , which reacted with OH − to maintain pH value (Jian et al., 2021), so the pH of the CP+Fe 3 O 4 @BC group was unchanged (Fig. 3a). denitrifying bacteria also increased, which used nitrate to remove AVS (Fig. 10c).
Desulfobulbus is a type of SRB that can transform sulfate into sul de under anoxic environment. Desulfobulbus content in the CK and BC groups was 0.45% and 0.31%, respectively, thus increasing AVS content (Feng et  Phosphorus is an important nutrient, but excessive P will also lead to eutrophication of the water body, thereby destroying the water environment (Li et al., 2021). NH 4 Cl-P and Fe/Al-P are referred to as migrating phosphorus in this study because of their easy migration and release (Ribeiro et al., 2008;Rydin, 2000).
As shown in Fig. 11, the phosphorus migration state decreased from the 2nd day to the 8th day, but Ca-P content increased (except the CK group and the BC groups). The content of phosphorus migration state in the CK group and BC group decreased from 0.301mg/g and 0.273mg/g to 0.225mg/g and 0.16mg/g, while the content of Ca-P remained unchanged. This was attributed to the release of the phosphorus migration state in sediment, which led to the increase of phosphorus content in overlying water ( In the reduction environment, the dissimilatory nitrate reduction to ammonium (DNRA) was prone to occur (Zhu et al., 2018). In the oxidizing environment, ammonia nitrogen is converted to nitrite nitrogen by microorganisms and eventually to nitrate nitrogen (nitri cation reaction) (Xiao et al., 2010). In addition, denitrifying bacteria increased in the CP+Fe 3 O 4 @BC group, which promoted denitri cation.
In conclusion, the introduction of CP and Fe 3 O 4 @BC composite materials can effectively inhibit the release of phosphorus in sediment and promote denitri cation, which can effectively inhibit the eutrophication of water bodies (Waajen et al., 2016b).

Chromium
Cr is a well-known carcinogenic element present in drinking water. The high concentration of Cr also exerts strong toxic effects as it can diffuse through the cell membranes, oxidize biological molecules and create a potential risk of living being healthy. At the same time, Cr also is responsible for skin tumors in animals (Lal et al., 2020).
As shown in Fig. 12, the contents of acid exchangeable Cr (Ex-Cr) and reducible Cr (Reducible-Cr) in the CK and the BC groups were high, which were easy to release and migrate to water bodies. The content of Ex-Cr and Reducible-Cr in the experimental group with CP was lower than 0.5mg/kg. It was possible that O 2 was produced by CP, leading to the change of Cr morphology (Teuchies et al., 2011b). Adsorption, ion exchange, complexation and precipitation are the major mechanisms involved in the conversion of soluble and potentially soluble forms of heavy metals to geochemically stable solid phases by biochar (Shentu et al., 2022). However, the content of Ex-Cr and Reducible-Cr in the BC group did not decrease, which may be ascribed to that the BC had no effect on Cr xation. When Fe 3 O 4 was loaded onto BC by coprecipitation method, -OH functional groups were formed, which can solidify Cr (Fig. 2b). Therefore, the Ex-Cr and Reducible-Cr contents in the CP+Fe 3 O 4 @BC group were only 0.157mg/kg and 0.0942mg/kg at 25 days, respectively, which were far lower than those in other experimental groups (Fig. 12). which greatly reduced the release of phosphorus. At the same time, the increase of denitrifying bacteria (Dechlorominas, Acinetobacter and Flavobacterium) promoted the denitri cation reaction, which could promote the conversion of nitrogen. Therefore, the addition of CP and Fe 3 O 4 @BC could effectively inhibit the eutrophication of the water body. After CP and Fe 3 O 4 @BC composites were added, the content of the Ex-Cr and the reducible-Cr decreased to 0.157 mg/kg and 0.0942 mg/kg on the 25th day, respectively, which were lower than those in other groups. Thus, the harm of heavy metal Cr reduced. The addition of Fe 3 O 4 @BC composites could increase the total number of microorganisms and promote the growth of Sulfurovum and inhibit SRBs, which could effectively inhibit the production of AVS for a long time. Therefore, this study can be used to reference the restoration of sediments. It can be applied to practical engineering and provides a new idea to deal with black-odorous sediment.    The variation of TP in overlying water during the experimental study.

Figure 6
The variation of P in different forms in sediments: (a) the 2 nd day. (b) the 8 th day.  The relative abundance of microbial communities (c) Autotrophic sul de drives denitri cation.