Sorption and transport of Mn2+ in soil amended with alkali-modified pomelo biochar

Owing to its effectiveness and being environment-friendly, biochar has been used for adsorbing and immobilizing pollutants in soil in recent years of studies, which is also suitable for manganese pollution in soil caused by manganese mining and processing activities. In this research, alkali-modified pomelo biochar (MBC) was regarded as a soil amendment, and the improvement of soil physicochemical properties and Mn2+ sorption and transport in soil by modifying with MBC were investigated. In incubation experiment, 0–10% (w/w) MBC addition amount significantly improved the physicochemical properties of soil. Due to the amelioration of soil physicochemical properties along with the oxygen-containing functional groups and the developed pore structure of MBC itself, the adsorption capacity of MBC modification soil towards Mn2+ (qe) was enhanced in batch adsorption experiment, and qe increased by 10–108% when MBC ratio grew from 0 to 10% at 300 mg·L−1 Mn2+ solution. In column transport experiment, the Mn2+ retention rate climbed by 13–106% from 0 to 10% MBC addition proportion when adopted the MBC filling way that placed MBC on the soil upper layer, and the reinforced restriction on Mn2+ transport in soil amended with MBC might ascribe to the enhanced qe as well as the reduced saturated hydraulic conductivity. These results proved that MBC effectively augmented adsorption ability and suppressed transport of Mn2+ in soil, which could provide an available mind on prevention and remediation of soil Mn contamination.


Introduction
Manganese (Mn), as the fourth metal in terms of average annual consumption , is extensively used in the production of stainless steel, dry batteries, glass, and specialty chemicals (Duan et al. 2010;Zhang et al. 2020). In recent decades, with the rapid development of modern industry, the global mining and processing of Mn ore gets more and more frequent. During these activities, a large amount of Mn will enter the topsoil around the mining area through surface runoff, wind-borne transportation, and atmospheric deposition . Part of the Mn accumulated in the soil surface will infiltrate into the deep soil with rainfall, and even migrate to the aquifer. Excessive Mn will be eventually be adsorbed by the organism via the food chain (Duan et al. 2011). Although Mn is one of the essential trace elements for the organism, when the Mn uptake is too high, there will be symptoms of Mn poisoning in organism, examples include iron deficiency in algae, death of fish embryos, adverse effects on terrestrial plants, and human neurological diseases (Du et al. 2019;Sabina et al. 2019). Therefore, how to effectively inhibit the migration of Mn in the soil remains to be resolved.
Chemical immobilization has been used for stabilizing soil pollutants to restrict their transport extensively for which is of convenience and effectiveness and can be applied on a large scale in recent years (Gong et al. 2018). Compared with other cheap chemical stabilizer, biochar, a high-temperature pyrolysis product of waste biomass, which mostly possesses higher specific surface area (SSA), pH, cation exchangeable capacity (CEC), total organic carbon (TOC) and richer functional groups, redox couples, etc., has demonstrated a unique superiority to mitigate the mobility of pollutants in soil by growing studies. Khorram et al. (2015) found that migration of fomesafen was restrained due to the enhanced adsorption capacity in biochar-amended soil. In a column leaching test, Daryabeigi Zand and Grathwohl (2016) observed that after the biochar was added to contaminated soil, the leaching of polycyclic aromatic hydrocarbons was remarkably decreased. Qin et al. (2018) reported that biochar derived from pig significantly reduced the mobility of cadmium, lead, and dibutyl phthalate in the low organic carbon soil, and it could attribute to the high SSA, surface alkalinity, pH, and mineral contents of biochar itself.
Adsorption is a decisive factor in controlling the transport of pollutants in the soil (Lei et al. 2020), and adsorption of Mn by soil is closely related to the physicochemical properties of the soil itself, and pH as well as redox potential (Eh) is the most critical among all the soil physicochemical properties since they control the mutual transformation of divalent Mn (soluble and high mobility) and Mn oxide (insoluble and relatively stable). Hue et al. (2001) claimed that Mn toxicity and mobility often occurred in those high Eh soil for long-term flooded or excessive use of organic amendments and acid soil. In addition, similar to other heavy metals, soil CEC and TOC might also affect Mn adsorption (Bradl 2004).
On the one hand, it is universally acknowledged that biochar is both pollutant adsorbent and soil amendment, which may improve the adsorption performance of soil through its own excellent adsorption ability and modification on soil physicochemical properties to weaken pollutant transport. Vu et al. (2015) observed the adsorption capacity of soil for rhamnolipid was enhanced as the biochar was added to soil, which result from the improvement of soil organic matter caused by biochar; thus, the migration of rhamnolipid was mitigated. Kim et al. (2015) explained that decline in the NH 4 NO 3 -extractable heavy metal of biochar modification soil was caused by the increment in soil pH and heavy metal adsorption ability induced by biochar in a soil incubation experiment. On the other hand, biochar also affects soil hydraulic characteristics to influence the transport of pollutants as well. Lei et al. (2020) thought it was one of the reasons for the restricted migration of 3,5,6-trichloro-2pyridinol in biochar-modified purple soil that the soil diffusion coefficient and convection velocity were decreased by biochar addition. According to Trinh et al. (2017), an intensified interaction between pollutant and soil following biochar addition caused by the declining pore-water flow rate gave rise to a nonreversible retention of pollutant.
Few studies have focused on the influence of biochar on Mn 2+ sorption and transport in soil so far on the basis of collected literature. The type, dosage, and application method of biochar are three influence factors needed to be considered carefully affecting pollutant sorption and transport in biocharmodified soil . Alkali-modified pomelo biochar (MBC) exhibited an excellent adsorption for Mn 2+ in aqueous solution, whose saturated adsorption capacity was up to 163.194 mg·g −1 according to a previous study ). In the present study, MBC was further selected as a soil amendment, and several tasks were completed: (1) the impact on soil properties with different dosage MBC addition; (2) the adsorption of Mn 2+ by soil amended with MBC; and (3) the influence of MBC addition amount and filling way on Mn 2+ migration in soil. Based on the former study, this study aimed to further explore the application value of MBC in soil Mn contamination.

Soil and MBC
The soil was collected from a 0-to 20-cm layer of an uncontaminated field in Chongqing Municipality (29°32′ N, 106°2 7′ E), China. The soil was thoroughly mixed, air-dried for days, and crushed to pass through a 2-mm sieve to remove coarse particles and plant residues before initiating experiments (Xu et al. 2016).
Details of MBC preparation were described in a previous study . In short, pomelo peels modified with NaOH solution were pyrolyzed at 500°C for 2 h under oxygen-free environment in a furnace (GF11Q-B, Nanjing Boyuntong Instrument Technology Co., Ltd., China), then the pyrolysis products were collected and sieved for less than 0.38 mm, which was named MBC.
Determination of samples' (soil and MBC) physicochemical property: deionized water was added to sample at 2.5:1 (v/w) for soil and 20:1 (v/w) for MBC, respectively, agitating them by a glass rod for 1 min, then standing for 30 min, and pH value and Eh value were measured using a desktop acidimeter (PHS-3C, Shanghai INESA Scientific Instrument Co., Ltd., China). In the case of CEC, hexamminecobalt trichloride solution-spectrophotometric method (Ministry of Ecology and Environmental of PR China, HJ 889-2017) was applied. TOC was determined by K 2 Cr 2 O 7 -H 2 SO 4 oxidation method (Ministry of Agriculture of PR China, GB 9834-88). The bulk density of undisturbed soil was tested by the cutting ring method (Ministry of Agriculture and Rural Affairs of PR China, NY/T 1121NY/T .4-2006, while dividing the dry mass of MBC by the packed volume to estimate that of MBC. Soil texture was measured according to a simplified method described by Kettler et al. (2001). The elemental composition was conducted by energy-dispersive spectroscopy (EDS; VEGA 3, TESCAN Inc., Czech Republic). The detailed data are listed in Table 1. Part of the data about MBC was derived from a previous study ).

Incubation experiment
Soil was treated in a glass container at 25 ± 2°C with different ratios of MBC: 0, 1, 3, 5, 10% (w/w). To ensure relative constant water content (60% field water holding capacity), spraying water every 3 days during incubation period for 30 days (Xu et al. 2016).
The modified soils were air-dried then crushed to pass through a 2-mm sieve once incubation experiment was finished, and they were here referred as B0-S, B1-S, B3-S, B5-S, B10-S corresponded to the MBC ratios of 0, 1, 3, 5, and 10% (w/w) respectively. pH, Eh, CEC, and TOC of modified soils were determined, and micro-morphology of B0-S and B10-S was measured by a scanning electron microscope (SEM; VEGA 3, TESCAN Inc., Czech Republic).

Adsorption experiment
Adsorption kinetics experiment: Mn 2+ solution in here was prepared with MnCl 2 ·4H 2 O. Mn 2+ concentration was set to 300 mg·L −1 , and the pH value of Mn 2+ solution was adjusted to 7.0±0.1 by 0.1 M HCl and NaOH solutions. One gram of modified soils (including B0-S, B1-S, B3-S, B5-S, B10-S) was added to 50 mL Mn 2+ solution in a conical flask, which were next oscillated for 10,30,60,120,180,300,420,540,720,960,1200, and 1440 min in a constant temperature oscillator (ZWY-2102C, Shanghai Zhicheng Analytical Instrument Inc, China) at a temperature of 30°C and a rotating speed of 120 rpm respectively. Once the adsorption time reached the set value, the solution was centrifuged for 10 min at 3000 rpm with centrifuge (TG16-WS, Xiangyi Centrifuge Instrument Co., Ltd., China) and then filtered through a 0.45-μm filter membrane, and Mn 2+ concentration in the filtrate was determined by flame atomic absorption spectrophotometer (AA800, PerkinElmer Inc, America) based on the standard (Ministry of Ecology and Environmental of PR China, GB 11911-89).
Adsorption isotherm experiment: Mn 2+ concentration gradient was set to 50, 100, 150, 200, 250, and 300 mg·L −1 . 0.1 M HCl and NaOH solution were used to adjust the initial pH of the Mn 2+ solution to 7.0 ± 0.1. One gram of modified soils (including B0-S, B1-S, B3-S, B5-S, B10-S) was added to 50 mL Mn 2+ solution in a conical flask, which were next oscillated for 24 h in a constant temperature oscillator at a temperature of 30°C and a rotating speed of 120 rpm. After the adsorption was completed, the solution was centrifuged, filtered, and determined as the above procedure. In addition, soil itself would not release Mn 2+ , which was proved by an adsorption experiment with soil and deionized water.
The adsorption capacity of soil towards Mn 2+ was expressed as follows : where q e (mg·g −1 ) is the amount of adsorbed Mn 2+ per unit mass of soil in the adsorption equilibrium stage, V (L) is the volume of Mn 2+ solution, C 0 and C e (mg·L −1 ) are the initial and equilibration Mn 2+ concentration, respectively, and M (g) is the mass of soil.
To analyze the adsorption mechanism of B0-S for Mn 2+ , attenuated total reflection Fourier transform infrared spectroscopy (FTIR; Nicolet iS5, Thermo Fisher Scientific Inc., USA) and X-ray diffraction (XRD; D/MAX 2500pc Rigaku Instrument Co., Japan) were applied to measure the change of functional groups and mineral crystal structure in B0-S before and after adsorption of Mn 2+ .

Adsorption model fitting
Pseudo-first-order, pseudo-second-order, and intra-particle diffusion model were utilized to describe the adsorption kinetics, which were written as Eqs.
(2)-(4) : where t (min) is adsorption time; q t (mg·g −1 ) is the amount of adsorbed Mn 2+ per unit mass of soil at time t; k 1 (min −1 ), k 2 (g·mg −1 ·min −1 ), and k (mg·g −1 ·min −0.5 ) represent pseudo-firstorder, pseudo-second-order, and intra-particle diffusion rate constant, respectively; and b (mg·g −1 ) is intercept. For adsorption isotherm, Langmuir and Freundlich model, two of the most typical models, were applied, and they were as Eqs. (5) where q max (mg·g −1 ) is the saturated adsorption capacity, K L (L·mg −1 ) is the affinity constant between adsorbent and adsorbate, K F (mg·g −1 ·L n ·mg −n ) is the Freundlich equilibrium constant, and n is the constant that reflects the strength of adsorption.

Transport experiment
The filling way (MBC was laid on the upper, lower layer and uniformly mixed with soil, which were denoted as LUP, LLO, and UMS, respectively, and CK meant no MBC addition.) and dosage (0, 1, 3, 5, 10%) of MBC were taken into account in Mn 2+ transport experiment, whose detailed designs are listed in Table 2. A total of 3 g of natural soil and MBC were filled in a 12×100 mm (D×H) chromatography column. In order to evenly distribute the influent solution and prevented the loss of substance in column, the bottom and top of the column were both filled with 2 g of fine quartz sand, a 300-mesh polyester screen was additionally packed between quartz sand in the column bottom and the mixture of soil and MBC (Chen et al. 2017). The bulk density of the mixture of soil and MBC was calculated by the respective bulk density of the soil and MBC, then the column filling height could be determined according to the mixture bulk density . The filling of the materials in the column was performed by wetpacked method to ensure uniformity and no interspace (Chen et al. 2017). Prior to the transport experiment, materials in the column were incubated for 1 week like the above incubation test. Deionized water was pumped slowly into the column for 12 h to saturate it by a peristaltic pump (BT-600EA, Chongqing Jieheng Peristaltic Pump Co., Ltd., China), then the pore volume and porosity of the column could be gravimetrically calculated (Chen et al. 2018). After the soil column was saturated, 300 mg·L −1 of Mn 2+ solution with a volume of 200 mL was continuously injected into the column. During the test, the flow rate of the peristaltic pump was adjusted so that the height of the liquid level above the column was always maintained at 1~2 cm (Tan et al. 2015a), and the effluent was collected regularly to measure the Mn 2+ concentration by flame atomic absorption spectrophotometer. The transport experimental device is shown in Fig. S1.

Transport parameters calculation
In the above Eq. (7), K s (cm·min −1 ) is the saturated hydraulic conductivity, Q (mL·min −1 ) is the flow rate provided by a peristaltic pump, A (cm 2 ) is the cross-section area of the column, L (cm) is the filling height of the column, and ΔH (cm) is the water head difference between the water level and the outlet of the column, (Tan et al. 2015a).
Equations (8)-(9) were used for estimating the mass balance of Mn 2+ , where q total (mg) is the amount of Mn 2+ Saturated hydraulic conductivity was denoted as mean ± standard derivation, and different letters behind the figure represent a significant difference at P<0.05 retained in column, t total (min) is the total time of an integrated test, C t (mg·L −1 ) is the Mn 2+ concentration in effluent at time t, m total (mg) is the total amount of Mn 2+ injected in column, and R (%) is the Mn 2+ retention rate in column. Furthermore, V b and V s (mL) are the breakthrough and saturation volume which are defined as an accumulated volume when C t /C 0 reached 0.05 and 0.90 respectively (Singh et al. 2012).

Statistical analyses
All treatment and measurement were carried out in triplicate. Experimental data were analyzed by SPSS software. One-way analysis of variance was used to compare mean values, and significant differences were statistically considered when P < 0.05. Pearson's correlation coefficients were calculated to determine the relationship between parameters.

Results and discussion
The impact of MBC on soil physicochemical properties The chemical properties of soil were significantly changed by modified with MBC (P<0.05), and the variation increased with the augment of MBC addition ratio except for CEC (Fig. 1). MBC modification increased the soil pH by 14-67%, which could attribute to the reason that high alkalinity and pH of biochar caused the release of alkali metal salts after biochar was applied to soil, and the hydrolysis of alkali metal salts increased the soil pH (Kelebemang et al. 2017). Biochar was composed of mineral phase, amorphous carbon, graphite carbon, and unstable organic molecules, many of which could be electron donors or acceptors in the soil (Joseph et al. 2015), and applying MBC greatly descended the soil Eh by 21-59%, which suggested MBC possess stronger reducibility than soil. When MBC addition rate enhanced from 0 to 10%, the soil TOC grew by 47-578%, which might be due to the high carbon content and abundant TOC of biochar itself (Ali et al. 2020). What's more, biochar could adsorb organic molecules of soil and facilitate the generation of organic matter via the surface catalytic activity (Ren et al. 2020;Wu et al. 2019). The soil CEC rose by 13-26% after MBC treatment, and this was likely to be relevant to the growing surface variation charges, which resulted from the increased pH, and resulted in the intensified adsorption affinity for cation (Liu et al. 2018). It was also observed that MBC improved the physical properties of soil, as is revealed in Fig. S2, the surface micromorphology of B0-S was flat and homogenous, while an irregular and porous structure was found in that of B10-S. The improvement on soil physical properties was linked to the porous characteristic of biochar itself, which could interact with the soil aggregate to establish a relatively developed pore structure (Tan et al. 2015b). Abdelhafez et al. (2014) found the physicochemical properties of a metal-polluted soil (including the soil aggregate stability, water holding capacity, CEC, and TOC) amended with biochar originated from organic wastes were significantly ascended after incubation. As stated by Tan et al. (2015b), incubated with biochar for 1 year, the pH, CEC, and micro-morphology of ultisol were ameliorated a lot. Zheng et al. (2020) reported that it was an effective measurement to augment the soil fertility that applied biochar to soil since the physicochemical properties of soil would be polished.

Adsorption kinetics
The adsorption of soil towards 300 mg·L −1 Mn 2+ solution in different reaction time within 24 h was investigated. As revealed in Fig. 2(a), regardless of the MBC dosage, the adsorption rate dropped rapidly with time, which was likely to relate to the available adsorption sites. Because of the finite adsorption sites, the unit time consumption of the adsorption sites would gradually decrease, which corresponded to the reduction of the adsorption rate with time ). The adsorption equilibrium time of each group of soil was about 700 min, except for B10-S, there were more adsorption site in B10-S, so, it took more time for Mn 2+ to occupy them. The kinetics fitting parameters are listed in Table S1; according to the correlation coefficients (R 2 ), pseudo-second-order (R 2 =0.620-0.970) and intra-particle diffusion model (R 2 =0.702-0.952) were fitting better. The former result indicated there was chemical sorption, while the latter consequence implied a diffusion mechanism, which belonged to physical sorption ); hence, a combination of physical and chemical sorption existed in the adsorption process. It was noteworthy that the effect of intra-particle diffusion was reinforced with the augment of MBC dosage according to the trend of R 2 of intra-particle diffusion model, which suggested the pore structure of modified soil was developed with the increase of MBC addition rate, and the result was in keeping with the conclusion obtained from the incubation experiment.

Adsorption isotherm
To examine the adsorption capacity of soil amended by different addition amount of MBC towards Mn 2+ solution with varying initial concentration from 50 to 300 mg·L −1 , the adsorption isotherm test was carried out. As shown in Fig. 2(b), a higher q e was obtained in a larger C 0 , when C 0 increased from 50 to 300 mg·L −1 , q e grew by 180% for B0-S, 181% for B1-S, 182% for B3-S, 197% for B5-S, and 259% for B10-S, because a larger C 0 could provide more Mn 2+ to adsorbent . The Mn 2+ adsorption capacity of soil was strengthened by MBC modification, and the soil amended with more MBC dosage exhibited a larger q e . When MBC addition ratios increased from 0 to 10%, q e increased by 10-60%, 22-92%, 13-80%, 14-69%, 13-86%, and 10-108% in C 0 of 50, 100, 150, 200, 250, and 300 mg·L −1 respectively. It was apparently logical that biochar amendment could boost the adsorption capacity of soil, for biochar was a specific adsorbent whose adsorption capacity was much larger than that of soil. This result was observed in other studies (Vithanage et al. 2014;Liu et al. 2015). The fitting parameters of Langmuir and Freundlich model are displayed in Table S2. In general, the Freundlich model (R 2 =0.929-0.998) fitted the adsorption isotherm better than the Langmuir model (R 2 =0.818-0.976), which showed that the adsorption was a kind of multilayer adsorption that occurred on a heterogeneous surface. Both adsorption ability and affinity were augmented as the MBC addition amount increased certified by the change trend of K f and n respectively .  From Fig. 3, no matter how MBC was added to the soil, its BTC was lower than that of CK, which showed that when MBC was added to the soil, the transport of Mn 2+ was suppressed. When the MBC dosage was fixed, whatever the MBC filling methods were, the Mn 2+ BTC of every filling method were basically identical. Ming et al. (2014) also reported that it was close to lock Cr(VI) in soil to place biochar on soil superstratum or to mix biochar with soil evenly. However, judging from Table 3, in spite of the MBC dosage, relative retention parameters of Mn 2+ in column of LLO were significantly inferior to UMS and LUP (P<0.05), which demonstrated that LLO got the worst limit on Mn 2+ migration, and the phenomenon might be connected with the contact between MBC and Mn 2+ solution. Because of the presence of the quartz sand on the column, Mn 2+ solution could be evenly dispersed on the material on the column's upper layer, whereas the continuous downward infiltration of Mn 2+ solution might trigger preferential flow (Zhang et al. 2019), causing the formation of specific channel for Mn 2+ penetration, which resulted in Mn 2+ to inadequately be in contact with the material in the lower layer of the column. Obviously, LLO made the contact between Mn 2+ solution and MBC the least, which lead to the worst Mn 2+ retention effect. Moreover, as displayed in Table 2, K s of LLO was remarkably higher than that of the other two filling methods (P<0.05).  filled the apple tree biochar into the silty clay soil with the same application ways as this article, and the largest K s was observed with LLO filling way as well when the biochar addition ratio was 1 or 2%. The K s value determined the residence time of solution in soil, and the solution always attained more sufficient contact with those soil with lower K s value (Singh et al. 2012). Therefore, another reason for the worst Mn 2+ retention effect of LLO might be that the K s value was the highest in this filling method. In conclusion, the LLO filling way was not advisable in practical application.

Effect of MBC dosage
The BTC of different MBC dosage with UMS, LUP, and LLO filling methods are depicted in Fig. 4. It could be seen from the figure that the BTC with a larger MBC addition ratio was often below the BTC with a smaller MBC addition ratio, which illustrated that as the MBC dosage grew, the retention of Mn 2+ in the column enhanced, and the migration of Mn 2+ was more inhibited. Khorram et al. (2015) found that biochar effectively reduced the mobility of fomesafen in soil, and this inhibitory effect strengthened with the augment of biochar dosage. From the concrete parameters in Table 3, when the filling ways were unaltered, in addition to the V b , the q total , R, and V s values were improved significantly (P<0.05) with the augment of MBC addition rate. With the MBC dosage increased from 0 to 10%, the q total and R grew by 13-95%, 13-106%, and 4-85%; the V s climbed by 115-175%, 115-175%, and 115-175%; but the increment was only 18-35%,  Different letters behind the figure represent a significant difference at P<0.05. Data was denoted as mean ± standard derivation 24-35%, and 12-29% for V b in filling way of UMS, LUP, and LLO, respectively, and this improvement effects perhaps attributed to the following two reasons. On the one hand, since the adsorption ability of MBC towards Mn 2+ was much better than that of soil, the increase of MBC dosage would enhance the adsorption affinity of the mixture for Mn 2+ . The adsorption experiment has already proved that the soil amended by larger MBC ratio possessed greater adsorption capacity towards Mn 2+ . On the other hand, in the same MBC filling method, with the MBC addition ratio that grew from 0 to 10%, the K s values dropped by 10-21%. The reduced K s could owe to the high SSA and low bulk density of biochar, which made its pores, especially large pores, easy to be blocked, thereby reducing water permeability according to Lei et al. (2020). Consequently, on the premise of being economically feasible, increasing the MBC dosage as much as possible could better inhibit the migration of Mn 2+ .
The intensified mechanism on Mn 2+ adsorption and retention in soil by MBC addition To figure out the mechanism of intensification for adsorption MBC exert on soil towards Mn 2+ , the distinction of adsorption of Mn 2+ by B0-S and MBC must be identified firstly. For B0-S, FTIR and XRD analysis were conducted to elucidate its adsorption towards Mn 2+ (Fig. 5). From Fig. 5a, in terms of B0-S before adsorption towards Mn 2+ , the peaks at 3621 and 3424 cm −1 were both associated with the stretching vibration of O-H in the clay minerals (Xu et al. 2020). The band centering at 1638 cm −1 originated from the C=C and C=O stretching vibration of amides and aromatics (Churchman et al. 2010). Those vibrations of peaks all represented Si-O at 1030, 797, 694, 525, and 468 cm −1 , which belonged to the quartz, feldspar, and kaolinite (Langford et al. 2011;Bernier et al. 2013;Chandrasekaran et al. 2015). After the adsorption of Mn 2+ by B0-S, the bands at 797 and 525 cm −1 became weakened and shifted to a lower wavenumber at 522 cm −1 , respectively, which revealed that Si-O was involved in the adsorption process. According to MDI jade software, the main mineral crystal structure of B0-S was SiO 2 (PDF# 99-0088), after adsorption, Mn 2 SiO 4 (PDF# 02-1327) was observed at peaks of 31.1°, 35.1°, and 50.2°, corresponded to the crystal face of (111), (200), and (220) respectively (Fig. 5b). Combining FTIR results, the SiO 2 in B0-S was hydrolyzed and reacting with Mn 2+ to form Mn 2 SiO 4 , Yan et al. (2012) also drew the same conclusion. On the one hand, Mn 2+ could react with CO 3 2− and -COO − on MBC to form MnCO 3 and -COOMn + respectively . Hence, MBC modification might increase the oxygen-containing functional groups to enhance the Mn 2+ adsorption ability of soil. On the other hand, the pore structure of the soil modified with MBC was ameliorated, which might also play a positive role in Mn 2+ adsorption.
Furthermore, the improvement on soil physicochemical properties with MBC might be a response to the reinforced sorption capacity of soil towards Mn 2+ as well. There was a significant correlation between q e and pH (P < 0.05), and q e and TOC (P < 0.01) (Table S3). Some scholars also obtained that the improvement on soil physicochemical properties with biochar was one of the reasons for the augment of soil adsorption capacity towards heavy metals via correlation analysis (Gondek et al. 2016;Liang et al. 2017;Hailegnaw et al. 2020). The increase of pH overwhelmingly affected the solubility of Mn and could heighten the electrostatic adsorption on metal cations (Sparrow and Uren 2014). For TOC, there were more organic matters in those high TOC soil, which might complex with metal cations to adsorb them (Palansooriya et al. 2020).
To sum up, owing to the nature of MBC (high SSA, pH, TOC, rich oxygen-containing functional groups and developed pore structure), the physicochemical properties together with the adsorption capacity were improved, and the detailed mechanism is illustrated in Fig. 6. The results of adsorption and transport experiments were used for elucidating the intensified mechanism on Mn 2+ retention in soil amended with MBC. As shown in Table S4, the significant positive correlation between R and q e (P < 0.01) and the significant negative correlation between R and K s (P < 0.01) were observed, which implied Mn 2+ transport was closely connected with both adsorption capacity and hydraulic characteristics of soil. To explain the intensified inhibition mechanism on Mn 2+ in soil by MBC addition, the distinction of Mn 2+ migration between natural and MBC modification soil is illustrated in Fig. 7. Compared to natural soil, due to the good performance of MBC for Mn 2+ sorption and significant improvement for soil physicochemical property, the MBC modification soil had stronger adsorption ability towards Mn 2+ , which allowed more Mn 2+ to be adsorbed and immobilized. Moreover, the porous structure of MBC was easily blocked, which lead to a low water permeability and K s in the modification soil, and the contact and residence time of Mn 2+ with soil would be reinforced, then the Mn 2+ migration was inhibited. In a word, both strengthened soil adsorption capacity for Mn 2+ and decreased soil K s might be a response to the restrain mechanism on Mn 2+ transport in soil by MBC.

Conclusions
MBC which presented prominent adsorption ability towards Mn 2+ in a past research was further to serve as soil amendment to adsorb and immobilize Mn 2+ in soil in this work. The chemical properties (pH, Eh, CEC, TOC) of soil modified with serval MBC dosage were significantly adjusted (P < 0.05), and its pore structure was also improved through incubation. The chemical and physical sorption both existed in adsorption of Mn 2+ by modified soils suggested by the R 2 of pseudo-second-order and intra-particle diffusion model. The adsorption isotherm was well fitted by the Freundlich model (R 2 =0.929-0.998), which implied the sorption of soil modified with MBC for Mn 2+ was multilayer adsorption. MBC modification affected the adsorption of Mn 2+ by soil relies on not only its excellent adsorption capacity but also improvement of soil pH and TOC. It Fig. 6 Intensified adsorption mechanism of soil amended with MBC towards Mn 2+ was sensible to fill more MBC with the filling method of LUP or UMS on the basis of economic feasibility, which could inhibit Mn 2+ migration in soil to the utmost extent. The mechanism on Mn 2+ restricted transport in soil amended with MBC might be associated with intensified adsorption ability and declined K s . The study gives some theoretical guidance and advice for soil Mn contamination remediated with chemical stabilization.