Anthropogenic Organic Pollutants in Groundwater Increase Releases of Fe and Mn from Aquifer Sediments: Impacts of Pollution Degree, Mineral Content, and Ph

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Article Anthropogenic Organic Pollutants in Groundwater Increase Releases of Fe and Mn from Aquifer Sediments: Impacts of Pollution Degree, Mineral Content, and pH

Yuanzheng Zhai , Yifan Han, Xuelian Xia *, Xindai Li * , Hong Lu, Yanguo Teng and Jinsheng Wang

Engineering Research Center of Groundwater Pollution Control and Remediation of Ministry of Education of China, College of Water Sciences, Beijing Normal University, Beijing 100875, China; [email protected] (Y.Z.); [email protected] (Y.H.); [email protected] (H.L.); [email protected] (Y.T.); [email protected] (J.W.) * Correspondence: [email protected] (X.X.); [email protected] (X.L.); Tel.: +86-188-1148-5804 (X.X.); +86-010-58802736 (X.L.); Fax: +86-010-58802736 (X.L.)

Abstract: In many aquifers around the world, there exists the issue of abnormal concentrations of Fe and Mn in groundwater. Although it has been recognized that the main source of this issue is the release of Fe and Mn from aquifer sediments into groundwater under natural environmental conditions, there lacks enough reliable scientific evidence to illustrate whether the pollutants imported from anthropogenic activities, such as organics, can increase this natural release. On the basis of time series analysis and comparative analysis, the existence of an increasing effect was verified through laboratorial leaching test, and the impacts of aquatic chemical environment conditions, such as pH, on the effect were also identified. The results showed that the increase of organics in groundwater Citation: Zhai, Y.; Han, Y.; Xia, X.; made the release of Fe and Mn more thorough, which was favorable for the increase of groundwater Li, X.; Lu, H.; Teng, Y.; Wang, J. concentrations of Fe and Mn. The higher the contents of Fe- and Mn-bearing minerals in aquifer Anthropogenic Organic Pollutants in sediments, the higher the concentrations of Fe and Mn in groundwater after the release reaches Groundwater Increase Releases of Fe kinetic equilibrium. Lower pH can make the leaching more thorough, but the neutral environment and Mn from Aquifer Sediments: also increases the amount of Mn. It can be deduced that the pollutants such as organics imported by Impacts of Pollution Degree, Mineral anthropogenic activities can indeed increase the releases of Fe and Mn from aquifer sediments into Content, and pH. Water 2021, 13, 1920. groundwater, thus worsening the issue of groundwater Fe and Mn pollution. The findings provide a https://doi.org/10.3390/w13141920 deeper insight into the geochemical effects of Fe and Mn in the natural environment, especially in the

Academic Editor: groundwater system. Alexander Yakirevich Keywords: groundwater pollution; water–sediment interaction; leaching; release; Fe; Mn Received: 10 June 2021 Accepted: 7 July 2021 Published: 12 July 2021 1. Introduction Publisher’s Note: MDPI stays neutral Iron and manganese elements are widely distributed in the strata, including aquifer with regard to jurisdictional claims in sediments, and they can enter the groundwater through leaching and become the common published maps and institutional afﬁl- hydrochemical components [1]. In aquifers around the world, the concentrations of Fe and iations. Mn in groundwater vary very widely in space, from a few micrograms per liter [2,3] to tens of micrograms per liter or more [4]. Although this difference is related to the spatial distribution of Fe and Mn in the porous medium of the strata [5,6], it is also greatly related to the high complexities of the geochemistry processes of them [7,8], which are controlled Copyright: © 2021 by the authors. by the hydrogeochemical environment [9,10] and have not yet been well understood [11]. Licensee MDPI, Basel, Switzerland. When the concentrations of Fe and Mn in groundwater reach certain levels, the water intake This article is an open access article facilities, such as pumping wells, can be clogged [11,12], thus greatly shortening the service distributed under the terms and life of the water intake project [13]. In addition, although Fe and Mn are essential elements conditions of the Creative Commons of human bodies [14], high concentrations will not only affect the sense of groundwater Attribution (CC BY) license (https:// as drinking water (such as color and iron smell), but also can affect human body health creativecommons.org/licenses/by/ if drinking over a long period of time [15,16]. Therefore, the groundwater with too high 4.0/).

Water 2021, 13, 1920. https://doi.org/10.3390/w13141920 https://www.mdpi.com/journal/water Water 2021, 13, 1920 2 of 15

concentrations of Fe and Mn needs to be purified and qualified before drinking [17], which will obviously increase the water treatment process and consequently, increase the water supply cost [18]. As a result, it is not only of great scientific value, but also of great practical significance to study the hydrogeochemistry of Fe and Mn. Numerous investigations have focused on Fe and Mn pollution in groundwater, especially that for drinking, worldwide, such as in China [19], USA [20,21], European coun- tries [3], and India [22]. These studies have revealed the primary geochemical processes involved in the genesis of Fe- and Mn-rich groundwater [23]. The mechanism that causes the release and mobilization of Fe and Mn from aquifer sediments into groundwater in the natural environment is primarily the redox dissolution of Fe- and Mn-bearing minerals [8], such as magnetite, hematite, limonite, siderite, pyrite, manganite, braunite, pyrolusite, and psilomelane [24], sometimes with an impact on the desorption from sediment surface via competition with some other cations. Although Fe and Mn in groundwater are mainly introduced from the natural environment [25], they are also suspected of being influenced by anthropogenic activities [26]. With the socio-economic development, the impact of anthropogenic activities on groundwater has reached an unprecedented level in recent years [27]. The impact not only causes different degrees of pollution to groundwater by direct discharge of pollutants [28], but also changes the environmental conditions of groundwater (e.g., acid–base properties, redox conditions, pH, and microbiome) [29,30], both of which have been considered to have an impact on the natural environmental processes of Fe and Mn [31]. Organic matter is on the list of the anthropogenic pollutants [32]. The superposition of these anthropogenic pollutants and the chemical components in the natural environment make the hydrogeochemical process more complicated [33]. It has been found that the concentrations of Fe and Mn in groundwater with higher concentrations of organics are usually higher than those in groundwater without pollution of organics or with lower pollution degree [34]. For example, on the regional scale, such as in the important aquifers in USA [35], China [36], and some other counties [37,38], it has been found that the concentrations of Fe and Mn have increased in the past decades, simultaneously with the increased imports of organics (usually characterized by DOC, COD, or BOD), the anoxic and the acid environment. Simultaneously, on the site scale, such as in waste landfills [36] and polluted sites [39,40], it has also been found that the concentrations of Fe and Mn in the surrounding groundwater considerably increased after the operation of those sites for a period of time, while the background values of them in the groundwater were not high, and they were not discharged by those sites [41,42]. However, it should be noted that there were various organics, nitrogen species, or acid– base substances in the main pollutants of these sites [43]. Thus, it is an issue worthy of discussion, whether the positive correlation between Fe and Mn in groundwater and those pollutants introduced by anthropogenic activities is a coincidence or an inevitability in the physicochemical mechanism. The essence of the issue is the interaction between groundwater and sediments and the influencing factors of it, which may be physical, chemical, or even biological [44]. The investigation results show that up to now, few controlled experiments have conducted in-depth research on this interaction. The case studies mentioned above were only in the field investigation stage, and the corresponding conclusions were obtained from statistical analysis on this basis, but lacked mechanism exploration. Based on this, we presume that the increase of organics in groundwater can enhance the interaction between groundwater and sediments, thus increase the releases of Fe and Mn from sediments, and finally, increase their concentrations in groundwater. In addition, considering that physical and chemical effects are usually affected by aquatic environmental conditions, we also suspect that changes in pH will also impact the interaction and even the enhancement. To test the above hypotheses, the aims of this study were designed as follows: (1) to determine the presence of enhancing effects of organic pollution in groundwater on release of Fe and Mn from aquifer sediments; (2) to identify impacts of aquatic environmental conditions, such as pH, on the enhancing effects; (3) to reveal the mechanisms behind these effects and impacts. Water 2021, 13, x FOR PEER REVIEW 3 of 15

Water 2021, 13, 1920 Mn from aquifer sediments; (2) to identify impacts of aquatic environmental conditions,3 of 15 such as pH, on the enhancing effects; (3) to reveal the mechanisms behind these effects and impacts. This study will further the understanding of the roles of pollutants mainly introduced by anthropogenic activities, such as organics, on the geochemical processes of This study will further the understanding of the roles of pollutants mainly introduced by Fe and Mn naturally occurring in environment, especially in the groundwater system. In anthropogenic activities, such as organics, on the geochemical processes of Fe and Mn addition, we also sincerely hope that our efforts in this regard will be helpful for more naturally occurring in environment, especially in the groundwater system. In addition, we effective control of groundwater pollution. also sincerely hope that our efforts in this regard will be helpful for more effective control of groundwater pollution. 2. Materials and Methods 2.2.1. Materials In Situ Sediments and Methods Sampling and Characterization 2.1. InThe Situ in Sediments situ aquifer Sampling sediments and Characterizationwere sampled in Northeast China (Figure 1a), specifi- callyThe at a inpoint situ aquifer(45°45′52.58 sediments″ N, 126°30 were′ sampled12.05″ E) in located Northeast east Chinaof the (FigureSongnen1a), Plain specifically (Figure at1b), a pointwhich (45 belongs◦45052.58 to the00 N, Song 126◦hua30012.05 River00 E)Basin. located The eastsampling of the site Songnen is located Plain on (Figure the south-1b), whichern alluvial belongs flat to of the the Songhua Songhua River River Basin. (Figure The 1c), sampling which sitewas isformed located by on the the river southern alluvial al- luvialdeposits, flat ofspecifically, the Songhua coarse River sand (Figure and1 c),gravel which of wasthe Quaternary formed by the Holocene, river alluvial which deposits, are rel- specifically,atively loose. coarse There sand is strong and gravel hydraulic of the conne Quaternaryction between Holocene, the groundwater which are relatively and river. loose. In Therethe rainy is strong season, hydraulic the river connectioncan recharge between the groundwater the groundwater [45]. On and the river. contrary, In the ground- rainy season,water can the recharge river can the recharge river in the other groundwater periods [18]. [45 Controlled]. On the contrary, by the river–groundwater groundwater can rechargeinteraction, the the river depth in other of the periods groundwater [18]. Controlled level around by the the river–groundwater sampling site ranges interaction, from 3 to the5 m depth [46,47]. of In the many groundwater parts of the level river around basin, the especially sampling in site the ranges region froms close 3 to to 5 the m [ 46river,47]. and In manythe tributaries, parts of the the river concentrations basin, especially of Fe in can the reach regions tens close of milligrams to the river per and liter, the tributaries, while Mn theand concentrations COD can reach of several Fe can reachmilligrams tens of per milligrams liter in the per groundwater, liter, while Mn which and COD has had can reacha sig- severalnificant milligramsimpact on the per production liter in the and groundwater, life of the whichlocal residents has had [48]. a significant More details impact about on thethe productionhydrogeological and life conditions of the local of the residents sampling [48 ].site More can detailsbe referred about to the in the hydrogeological study of Zhu conditionset al. (2020) of [18]. the sampling site can be referred to in the study of Zhu et al. (2020) [18].

FigureFigure 1.1. Topographic mapmap ofof thethe samplingsampling site,site, ((aa)) China,China, ((bb)) SongnenSongnen Plain,Plain, ((cc)) SamplingSampling site.site.

TheThe samples of of sediments sediments were were manually manually drilled drilled in the in thephreatic phreatic aquifer aquifer using using a stain- a stainless-steelless-steel hand hand drill drillwith withlong longdrill drillpipe. pipe. To facilitate To facilitate comparative comparative analysis, analysis, samples samples were werecollected collected from from three three boreholes, boreholes, distributed distributed in ina straight a straight line line perpendicular perpendicular to to the the river. river. TheThe samplingsampling depthdepth waswas twotwo metersmeters underground,underground, whichwhich isis inin thethe unsaturatedunsaturated zone,zone, withwith thethe thicknessthickness ofof fourfour meters.meters. In orderorder toto makemake thethe samplessamples lessless disturbed,disturbed, thethe samplessamples were immediately put into impervious and opaque bags after being drilled out, sealed, and subsequently, saved in portable refrigerator.

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All sediment samples were transported to the lab immediately after sampling, followed by determination of the physiochemical properties of them in the lab (Table1). Before the physicochemical analysis, each sample was thoroughly stirred and mixed to make it as uniform as possible. The particle sizes of the samples were determined by the MS2000 laser particle size analyzer. The lithological characteristics of the sampling points were grayish black, with good sorting and good grinding. pH was determined by the potentiometric method; NO3-N and NO2-N were determined by the potassium chlo- ride solution extraction-spectrophotometry; organic matter was determined by potassium dichromate volumetric method by the electric furnace (1000 w) and characterized by TOC; and total Fe and total Mn were determined by the ICP-AES (PerkinElmer Optima 8000, Waltham, MA, USA) method.

Table 1. Physical and chemical properties of the aquifer sediments used for the experiment.

NO -N NO -N TOC Total Fe Total Mn Sediment pH 3 2 (mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg) I 5.87 25.5 0.105 1550 18540 384 II 6.24 22.6 0.134 6400 22664 451 III 6.11 27.5 0.12 7980 27749 506

2.2. Artiﬁcial Polluted Groundwater with Organics To avoid the interference of other chemical components, the groundwater polluted with organics used for the leaching test was manually made in the lab with ultrapure water and grade pure potassium hydrogen phthalate. Potassium hydrogen phthalate was chosen as the organic substance because it is a standard solution with COD, which makes our experimental results more universal and repeatable. In detail, according to the test needs, potassium hydrogen phthalate solutions of 10, 20, 40, 50, 60, 80, and 100 mg/L were prepared for use. In addition, the potassium hydrogen phthalate solution of 50 mg/L was divided into eight parts for use, pH of which was adjusted to 3, 4, 5, 6, 7, 8, 9, and 10 through adding HCl or NaOH, according to the needs. Ultrapure water was used for the blank test instead of the polluted groundwater.

2.3. Chemicals and Utensils Other chemicals and reagents used in the test included potassium nitrate, potassium sodium tartrate, Nessler’s reagents (sodium hydroxide, mercury iodide, and potassium iodide), 1:1 hydrochloric acid (HCl), sulfamic acid, ascorbic acid, ammonia solution, acetic acid–sodium acetate (Hac–NaAc) buffer solution, hydroxylamine hydrochloride, phenanthroline, and iron standard solution. Test vessels and equipment used in the test included conical flask (250 mL), centrifuge tube (10 mL and 50 mL), syringe (10 mL), volumetric flask (500 mL and 1000 mL), beaker (50 mL), graduated cylinder (50 mL), and 25 mm diameter hydrophilic filter (water system). All glassware used were carefully handled before use to ensure the quality of samples. All used glassware was soaked in 10% nitric acid lotion for 4 h, then rinsed with ultrapure water thrice, and dried for 4 h at 180 ◦C. The bottle caps and gaskets were cleaned using an ultrasonic cleaner for 30 min with 20% anhydrous ethanol aqueous solution, then rinsed with ultrapure water thrice, and dried naturally.

2.4. Leaching Test Procedures The leaching tests were achieved by the oscillation tests in the conical flasks carried out in the lab (Figure2). The ultrapure water (or organic solution) and the sediment samples for leaching were added into the conical flasks according to the ratio of liquid to soil of 10:1 in dark condition. Immediately after the addition, the conical flasks were sealed with sealing film and put into a constant temperature oscillator. The conical flasks were transferred to the centrifuge after oscillation, and centrifuged at 5000 r/min for 15 min. After centrifugation, the supernatant was taken out and filtered with a 0.45 µm water Water 2021, 13, x FOR PEER REVIEW 5 of 15

of 10:1 in dark condition. Immediately after the addition, the conical flasks were sealed Water 2021, 13, 1920 with sealing film and put into a constant temperature oscillator. The conical flasks were5 of 15 transferred to the centrifuge after oscillation, and centrifuged at 5000 r/min for 15 min. After centrifugation, the supernatant was taken out and filtered with a 0.45 μm water filter toﬁlter remove to remove interferences interferences such as such suspension, as suspension, from fromwhich which the leachate the leachate samples samples were were ob- tainedobtained for formeasurements. measurements. To Toensure ensure the the data data quality, quality, parallel parallel samples samples were were made made at at each each samplingsampling point. point.

Figure 2. Schematic diagram of the leaching experiment. Figure 2. Schematic diagram of the leaching experiment. To achieve the study aims, four groups of tests (named A, B, C, and D) with different To achieve the study aims, four groups of tests (named A, B, C, and D) with different conditions were carried out (Table2). In each group, three sediment samples were tested conditions were carried out (Table 2). In each group, three sediment samples were tested independently. In group A, 16 conical ﬂasks were used to test each sediment sample, independently. In group A, 16 conical flasks were used to test each sediment sample, cor- corresponding to 16 sampling times. Each test of group A lasted 3600 min to ensure responding to 16 sampling times. Each test of group A lasted 3600 min to ensure that the that the leaching processes were fully performed and the physicochemical processes in leachingthe sediment–water processes were system fully reached performed dynamic and the equilibrium. physicochemical The concentration processes in of the organics sediment–waterin the ultrapure system water reached used dynamic in group equilibrium. A was 50 mg/L,The concentration and the test of temperature organics in wasthe ultrapurecontrolled water at 20 used◦C. in group A was 50 mg/L, and the test temperature was controlled at 20 °C. Table 2. Control conditions for the leaching experiments. Table 2. Control conditions for the leaching experiments. Group Sediment Sampling Time (min) Organics (mg/L) pH Number of Samples I Organics Number of Group0, 5, 10, 30, Sediment 60, 90, 120, 180, 360, Sampling 540, Time (min) pH A II 50 16 720, 1080, 1440, 1800, 2700, 3600 (mg/L) Samples III Ⅰ I 0, 5, 10, 30, 60, 90, 120, 180, 360, B II A II 540, 720, 1080,10, 20,1440, 40, 60, 1800, 80, 100 2700, 50 6 16 III III 3600 I Ⅰ C II 3600 10, 20, 40, 3 III B II 6 50 60, 80, 100 I III D II 3, 4, 5, 6, 7, 8, 9, 10 8 III Ⅰ A: long-time leaching with continuousC monitoring;II B: leaching with organics3600 solutions with different concentrations; C: leaching with3 different pH; sediment: three different sedimentsIII (seen in Table1); sampling time: the time after reaching dynamic equilibrium, except for 50 group A; pH: that of the solution. Ⅰ 3, 4, 5, 6, D II 8 In the groups B, C, and D, the leaching processes were also7, fully8, 9, 10 performed, all III of which lasted 3600 min, and this duration was based on the results of group A. The A:concentrations long-time leaching of organics with continuous in the ultrapure monitoring; water B: leaching used inwith group organics B were solutions 10, 20, with 40, differ- 60, 80, ent concentrations; C: leaching with different pH; sediment: three different sediments (seen in and 100 mg/L, respectively, and the test temperature was also controlled at 20 ◦C. The Table 1); sampling time: the time after reaching dynamic equilibrium, except for group A; pH: that ofconcentration the solution. of organics in the ultrapure water used in the group C was also 50 mg/L, the test temperature was controlled at 20 ◦C, while pH of the solution was 3, 4, 5, 6, 7, 8, 9, and 10, respectively.In the groups B, C, and D, the leaching processes were also fully performed, all of whichFor lasted the 3600 purpose min, ofand comparative this duration analysis, was based all theon the above results tests of weregroup accompanied A. The con- centrationsby blanktests, of organics that is, in thethe ultrapure solution forwate leachingr used in was group ultrapure B were water10, 20, instead40, 60, 80, of and the 100polluted mg/L, groundwater. respectively, and the test temperature was also controlled at 20 °C. The 2.5. Leachate Measurements The leachate samples were immediately tested for hydrochemistry after preparation.

In consideration of the possible physiochemical changes in the leaching processes, the Water 2021, 13, 1920 6 of 15

following hydrochemical indicators were measured for the collected leachate samples: 2+ − − 2− Fe , total Fe, Total Mn, NO3 -N, Cl , pH, K, Na, Ca, Mg, and SO4 (Table3).

Table 3. Items, detection methods, and statistical information of monitoring indicators.

Item (Unit) Detection Method Level of Detection (Unit) Fe (II) O-phenanthroline spectrophotometry 0.03 mg/L TFe ICP-AES (PerkinElmer Optima 8000) 0.01 mg/L TMn ICP-AES (PerkinElmer Optima 8000) 0.001 mg/L − NO3 Ion chromatography (Thermo ICS-2100) 0.02 mg/L Cl Ion chromatography (Thermo ICS-2100) 0.02 mg/L pH Water quality analyzer (HANNA-HI9828) — K ICP-AES (PerkinElmer Optima 8000) 0.007 mg/L Na ICP-AES (PerkinElmer Optima 8000) 0.004 mg/L Ca ICP-AES (PerkinElmer Optima 8000) 0.0002 mg/L Mg ICP-AES (PerkinElmer Optima 8000) 0.001 mg/L 2− SO4 Ion chromatography (Thermo ICS-2100) 0.09 mg/L

COD was determined using potassium dichromate volumetric method. Fe2+ was determined using O-phenanthroline spectrophotometry. Total Fe, total Mn, K, Na, Ca, and − − Mg were determined using the ICP-AES (PerkinElmer Optima 8000). NO3 -N, Cl , and 2− SO4 were determined using ion chromatography (Thermo ICS-2100, Waltham, MA, USA). pH was determined using portable multi-parameter rapid water quality analyzer (HANNA- HI9828, Beijing, China). An indicative standard sample was tested every 10 samples with an error of less than 10%. All the analytical procedures are in conformance with quality requirements. The error was less than 10%, and the pass rate was 100%.

3. Results 3.1. Changes of Fe/Mn with Sediments The three samples of the aquifer sediments mainly consisted of ﬁne sand and sand (Figure3) including quartz, sodium feldspar, and potassium feldspar. The Fe-bearing minerals in the three samples were mainly hematite and bixbyite; and the Mn-bearing minerals were mainly pyrolusite and bixbyite. The three samples were weak acidic with pH of 5.87–6.24 (Table1). NO 3-N and NO2-N of the samples were 22.6–27.5 mg/kg and 0.105–0.134 mg/kg, and TOC was 1550–7980 mg/kg. Total Fe was 21,360–27,749 mg/kg, and total Mn was 484–506 mg/kg, which is about one ﬁftieth of that of total Fe. For the convenience of discussion, the three samples are arranged in ascending order according to the Fe contents in the samples, and named I, II, and III (Table1), respectively. Coincidentally, from sample I to III, the Mn contents are also in ascending order. Water 2021, 13, x FORWater PEER 2021 ,REVIEW 13, x FOR PEER REVIEW 7 of 157 of 15

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10 10 (a) Summary graph (b)II Summary graph 10 10 (a) Summary graph (b)II Summary graph 8 Average with +/－1 standard deviation error bars 8 Average with +/－1 standard deviation error bars 8 Average with +/－1 standard deviation error bars 8 Average with +/－1 standard deviation error bars 6 6 6 6 4 4 Volume (%) Volume Volume (%) Volume 4 4 2

2 (%) Volume Volume (%) Volume

2 0 2 0 0.01 0.1 1 10 100 1000 0.01 0.1 1 10 100 1000 0 Granularity (μm) 0 Granularity (μm) 0.01 0.1 1 10 100 1000 0.01 0.1 1 10 100 1000 Granularity (μm) Granularity (μm)

10 (c)III Summary graph

8 Average with +/－1 standard deviation error bars 10 (c)III Summary graph 6 8 Average with +/－1 standard deviation error bars 4 6 (%) Volume 2 4

Volume (%) Volume 0 0.01 0.1 1 10 100 1000 2 Granularity (μm)

0 0.01 0.1 1 10 100 1000 Granularity (μm) Figure 3. Particle sizes of the collected sediment samples, (a) I sediment, (b) II sediment, (c) III sediment Figure 3. Particle sizes of the collected sediment samples, (a) I sediment, (b) II sediment, (c) III sediment. Figure 3. Particle sizes of the collected sediment samples, (a) I sediment, (b) II sediment, (c) III sediment AfterAfter reaching reaching dynamic dynamic equilibrium equilibrium in the inorganic the organic solution solution systems systems of the three of the sed- three imentsediment samples, samples, the concentrations the concentrations of Fe of ofI, II, Fe and of I, III II, samples and III samples were 2.00, were 2.44, 2.00, and 2.44, 3.89 and mg/L, respectively, and the values of Mn were 0.039, 0.041, and 0.051 mg/L, respectively. After reaching3.89 mg/Ldynamic, respectively, equilibrium and in the the values organic of Mn solution were 0.039, systems 0.041, of and the 0.051 three mg/L, sed- respec- That is to say, the higher the content in the sediment, the higher the corresponding con- iment samples, tively.the concentrations That is to say, theof higherFe of I, the II, contentand III in samples the sediment, were the2.00, higher 2.44, the and corresponding 3.89 centration in the leachate after reaching equilibrium (Figure 4). Specifically, the concen- mg/L, respectively,concentration and the values in the leachateof Mn were after 0.039, reaching 0.041, equilibrium and 0.051 (Figure mg/L,4). respectively. Speciﬁcally, the con- trationcentration of Fe in of the Fe leachate in the leachate increased increased by 2%–37% by 2–37% for every for 1000 every mg/kg 1000 mg/kgincrease increase in the in That is to say, the higher the content in the sediment, the higher the corresponding con- contentsthe contents of Fe-bearing of Fe-bearing minerals minerals in the aq inuifer the aquifersediment. sediment. Mn’s corresponding Mn’s corresponding value was value centration in7%–200% thewas leachate 7–200% for every after for 10 every reaching mg/kg 10 increase mg/kg equilibrium increasein the co (Figure inntents the contentsof 4). Mn-bearing Specifically, of Mn-bearing minerals the concen- mineralsin the aqui- in the tration of Fefer in sediment. aquiferthe leachate sediment. increased by 2%–37% for every 1000 mg/kg increase in the contents of Fe-bearing minerals in the aquifer sediment. Mn’s corresponding value was 12 0.3 7%–200% for every 10(a) mg/kg increase in the contents of Mn-bearing(b) minerals in the aqui- organic sollution organic sollution fer sediment. 10 ultrapure water ultrapure water

8 0.2

12 0.3 6 (a) (b) organic sollution organic sollution 10 4 ultrapure water 0.1 ultrapure water

8 2 0.2 Concentration of Fe in solution (mg/L) Concentration ofin Mn solution (mg/L) 6 0 0.0 20000 25000 30000 480 490 500 510 Concentration of Fe in sediment (mg/kg) Concentration of Mn in sediment (mg/kg) 4 0.1 FigureFigure 4. Relationships 4. Relationships between between equilibrium equilibrium concentratio concentrationsns of Fe of and Fe andMn and Mn andmineral mineral contents contents of of 2 ◦

Concentration of Fe in solution (mg/L) the sediment samples (temperature = 20 C), (a) Fe, (b) Mn. the sediment samples (temperature = 20 °C), (aConcentration ofin Mn solution) (mg/L) Fe, (b) Mn. 0 0.0 200003.2. Changes 25000 of Fe/Mn with 30000Organics 480 490 500 510 3.2.Concentration Changes of Fe of in Fe/Mn sediment with(mg/kg) Organics Concentration of Mn in sediment (mg/kg) The equilibrium concentrations of Fe and Mn in the leachate increased linearly with the The equilibrium concentrations of Fe and Mn in the leachate increased linearly with Figure 4. Relationshipsincrease between of organics equilibrium concentration concentratio of the solutionns of Fe forand leaching Mn and (Figuremineral5), contents that is, theof higher the increase of organics concentration of the solution for leaching (Figure 5), that is, the the sediment samplesthe concentration (temperature of= 20 organics °C), (a) of Fe, the (b solution,) Mn. the higher the equilibrium concentration of higher the concentration of organics of the solution, the higher the equilibrium concentra- Fe and Mn in the leachate. For the three samples, the concentration of Fe in the leachate tion of Fe and Mn in the leachate. For the three samples, the concentration of Fe in the 3.2. Changes of Fe/Mnincreased with by Organics 0.3–2.0 mg/L for every 10 mg/L increase in the concentration of organics of the leachatesolution. increased The corresponding by 0.3–2.0 mg/L value for ofevery Mn 10 was mg/L 0.003–0.05 increase mg/L. in the What concentration is more, compared of or- The equilibrium concentrations of Fe and Mn in the leachate increased linearly with ganicswith of the solution. blank tests, The Fe corresponding and Mn concentration value of Mn in was the experiment0.003–0.05 mg/L. tests What was signiﬁcantly is more, the increasecompared of higher,organics with for concentration example, the blank even tests, of in the Fe and testsolution thatMn hadconcentration for onlyleaching 10 mg/L in(Figure the organic experiment 5), solutionthat is, tests the added, was the higher the concentrationconcentration of organics was one andof the a half solution, times thatthe inhigher blank the test. equilibrium concentration of Fe and Mn in the leachate. For the three samples, the concentration of Fe in the leachate increased by 0.3–2.0 mg/L for every 10 mg/L increase in the concentration of organics of the solution. The corresponding value of Mn was 0.003–0.05 mg/L. What is more, compared with the blank tests, Fe and Mn concentration in the experiment tests was

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Water 2021, 13, 1920 significantly higher, for example, even in the test that had only 10 mg/L organic solution 8 of 15 significantly higher, for example, even in the test that had only 10 mg/L organic solution added, the concentration was one and a half times that in blank test. added, the concentration was one and a half times that in blank test.

11 0.4 11 sediment Ⅰ 0.4 10 (a) sediment Ⅰ sediment sediment Ⅰ Ⅱ (b) 10 (a) sediment sediment Ⅰ Ⅱ 9 sediment sediment Ⅱ Ⅲ (b) sediment sediment Ⅱ Ⅲ 9 sediment Ⅲ 0.3 8 sediment Ⅲ 0.3 8 7 ） 7 ））） 6 0.2 6 mg/L mg/L 5 0.2 mg/L （ mg/L 5 （（

（ 4 Fe 4 Mn Fe 3 Mn 0.1 3 2 0.1 2 1 1 0 0.0 0 0 20406080100 0.0 0 20406080100 0 20406080100organic sollution（mg/L） 0 20406080100organic sollution（mg/L） organic sollution（mg/L） organic sollution（mg/L） Figure 5. Relationships between equilibrium concentrations of Fe and Mn and organic solution, (a) Fe, (b) Mn. Figure 5.Figure Relationships 5. Relationships between between equilibrium equilibrium concentratio concentrationsns of Fe and of Mn Fe andand Mnorganic and organicsolution, solution, (a) Fe, (b (a)) Mn. Fe, (b) Mn. 3.3. Changes of Fe/Mn with Time 3.3. Changes3.3. Changes of Fe/Mn of with Fe/Mn Time with Time In the early period (within 90 min from the beginning) of the leaching tests (group A In the early period (within 90 min from the beginning) of the leaching tests (group A in Table 1),In concentrations the early period of (withinFe and 90Mn min in the from leachate the beginning) of the three of the sediment leaching samples tests (group in Table 1), concentrations of Fe and Mn in the leachate of the three sediment samples increasedA in sharply; Table1), concentrationsin the followed ofperiod, Fe and ge Mnnerally in the from leachate 90th minute of the three to 1800th sediment minute, samples increasedincreased sharply; sharply; in the infollowed the followed period, period, generally generally from from90th 90thminute minute to 1800th to 1800th minute, minute, the the concentrations still increased on the whole, but the speed slowed down obviously; the concentrationsconcentrations still still increased increased on onthe the wh whole,ole, but but the the speed speed slowed slowed down down obviously; obviously; while while from 1800th minute to the end (3600th minute) of the tests, the concentrations were while fromfrom 1800th 1800th minute minute to to the the end end (3600th (3600th mi minute)nute) of of the the tests, the concentrations were almost almost unchanged, which could be divided into the final period (the third period; Figure almost unchanged, whichwhich couldcould be be divided divided into into the the ﬁnal final period period (the (the third third period; period; Figure Figure6). In the 6). In the whole period, the concentration curves of the three sediment samples showed a 6). In thewhole whole period, period, the the concentration concentration curves curves of of the the three three sediment sediment samples samples showed showed aa similar similar change trend. The concentration curves of the blank tests also showed a similar similar changechange trend.trend. The concentration curves of the blank tests tests also also showed showed a a similar similar trend. trend. The curves of Fe and Mn showed a similar trend. The concentration of Fe of the trend. TheThe curves curves of of Fe Fe and and Mn Mn showed showed a a similar similar trend. trend. The The concentration concentration of of Fe Fe of of the the three three sediment samples (I, II, and III), in which 50 mg/L COD had been added, after reach- three sedimentsediment samples samples (I, (I, II, II,and and III), III), in inwhic whichh 50 50mg/L mg/L COD COD had hadbeen been added, added, after after reach- reaching ing dynamic equilibrium was 6.20, 8.00, and 9.50 mg/L, respectively, while the corre- ing dynamicdynamic equilibrium equilibrium was was 6.20, 6.20, 8.00, 8.00, and and 9.50 9.50 mg/L, respectively, whilewhile the the corresponding corresponding values of Mn were 0.072, 0.18, and 0.25 mg/L, respectively. As for the blank tests, spondingvalues values of of Mn Mn were were 0.072, 0.072, 0.18, 0.18, and and 0.250.25 mg/L, respectively. respectively. As As for for the the blank blank tests, tests, the the corresponding values of Fe were 2.00, 2.50, and 3.90 mg/L, respectively, while the val- the correspondingcorresponding values values of Fe of Fewere were 2.00, 2.00, 2.50, 2.50, and and 3.90 3.90 mg/L, mg/L, respectively, respectively, while while the the val- values of ues of Mn were 0.039, 0.041, and 0.051 mg/L, respectively. It is easy to see that the values ues of MnMn were were 0.039, 0.039, 0.041, 0.041, and 0.051 mg/L, respectively. respectively. It It is is easy easy to see that the values varied varied with the sediments, elements, and the degree of organic pollution of water. All in varied with the sediments, elements, and the degree ofof organicorganic pollutionpollution ofof water.water. AllAll inin all, the all, the curves of experiment tests and blank tests show the similar change law. The dif- all, the curvescurves ofof experimentexperiment tests and blankblank tests show the similar change law. The The dif- difference ference between them is the amplitude of Fe and Mn concentration changes. The range of ferencebetween between themthem isis thethe amplitudeamplitude ofof FeFe andand MnMn concentrationconcentration changes.changes. TheThe rangerange ofof Fe and Fe and Mn concentrations in experiment tests is more dramatic, which will be three times Fe and Mn concentrations in experiment tests is more dramatic, which will be three times that of that of the blank tests. that of the blank tests.

18 0.5 0.06 18 (a) The first 90 minutes 0.5 (b) The first 90 minutes 3.0 0.06 (a) The first 90 minutes (b) The first 90 minutes Ⅰ alturapure water 3.0 Ⅰ ultrapure water 0.04 Ⅰ alturapure water 2.0 Ⅰ organic solution 0.4 Ⅰ ultrapureⅠ organic watersolution 0.04 Ⅰ organic solution 2.0 Ⅱ alturapure water 1.0 0.4 Ⅰ organic Ⅱ ultrapure solution water 0.02 Ⅱ alturapure Ⅱ organic watersolution 1.0 Ⅱ ultrapure water 0.02 12 0.0 Ⅱ organic solution Ⅱ Ⅲorganic alturapure solution water 0 20406080 Ⅱ organic solution 0.00 020406080 12 0.0 0.3 Ⅲ ultrapure water Ⅲ Ⅲalturapure organic solutionwater 0 20406080 0.00 020406080 0.3 Ⅲ Ⅲultrapure organic water solution Ⅲ organic solution Ⅲ organic solution

0.2 Fe (mg/L) Fe 0.2(mg/L) Mn

Fe (mg/L) Fe 6 6 (mg/L) Mn

0.1 0.1

0 0.0 0 0 600 1200 1800 2400 3000 3600 0.0 0 600 1200 1800 2400 3000 3600 0 600 1200 1800 2400 3000 3600 0 600 1200 1800 2400 3000 3600 Time (min) Time (min) Time (min) Time (min) Figure 6. Changes of concentrations of Fe and Mn in the leachate with time(min), (a) Fe, (b) Mn. Figure 6.Figure Changes 6. Changes of concentrations of concentrations of Fe and of Mn Fe and in the Mn leachate in the leachate with time(min), with time (a (min),) Fe, (b (a) )Mn. Fe, (b) Mn. 3.4. Changes of Fe/Mn with pH 3.4. Changes3.4. Changes of Fe/Mn of with Fe/Mn pH with pH The equilibrium concentrations of Fe and Mn in the leachate decreased linearly with The equilibriumThe equilibrium concentrations concentrations of Fe and of FeMn and in the Mn leachate in the leachate decreased decreased linearly linearly with with the increase of pH (Figure 7), that is, the higher the pH of the solution for leaching, the the increasethe increase of pH (Figure of pH (Figure 7), that7 ),is, that the is,higher the higher the pH the of pHthe ofsolution the solution for leaching, for leaching, the the lower the equilibrium concentration of Fe and Mn in the leachate. For the three samples, lower thelower equilibrium the equilibrium concentration concentration of Fe and of FeMn and in the Mn leachate. in the leachate. For the Forthree the samples, three samples, the concentration of Fe in the leachate decreased by 0.2–0.6 mg/L for every one pH the concentrationthe concentration of Fe ofin Fe the in theleachate leachate decreased decreased by by 0.2–0.6 0.2–0.6 mg/L mg/L for for every oneone pH pH increase of the solution. The corresponding value of Mn was 0.003–0.02 mg/L. Speciﬁcally, the concentration of Fe in the leachate decreased by 10–60% for every 1% pH increase of the

Water 2021, 13, x FOR PEER REVIEW 9 of 15

Water 2021, 13, 1920 9 of 15 increase of the solution. The corresponding value of Mn was 0.003–0.02 mg/L. Specifically, the concentration of Fe in the leachate decreased by 10%–60% for every 1% pH increase of thesolution. solution. And And the the average average decrease decrease was was 45%. 45%. The correspondingThe corresponding value value of Mn of was Mn 11–80%, was 11%–80%,and the averageand the decreaseaverage decrease was 36%. was 36%.

11 0.4

(a) (b) 10 sediment Ⅰ sediment Ⅰ sediment Ⅱ sediment Ⅱ sediment Ⅲ 0.3 sediment Ⅲ 9 ）） 8 0.2 mg/L mg/L

7 （（ Fe Mn 6 0.1

4 0.0 234567891011 234567891011 pH pH FigureFigure 7. 7.RelationshipsRelationships between between equilibrium equilibrium conc concentrationsentrations of ofFe Feand and Mn Mn and and pH, pH, (a) ( aFe,) Fe, (b ()b Mn.) Mn.

4. 4.Discussion Discussion 4.1. Input of Organics Increase Fe/Mn Release 4.1. Input of Organics Increase Fe/Mn Release This set of tests revealed the influences of different physicochemical conditions, such This set of tests revealed the influences of different physicochemical conditions, such as Fe and Mn contents in the aquifer sediment, organics concentration of the solution for as Fe and Mn contents in the aquifer sediment, organics concentration of the solution for leaching, duration of the leaching, and pH, on the leaching process of Fe and Mn from the leaching, duration of the leaching, and pH, on the leaching process of Fe and Mn from the sediment to the leachate. All these conditions have an impact on the concentrations of Fe sediment to the leachate. All these conditions have an impact on the concentrations of Fe and Mn in the leachate. In a certain period, the longer the duration, the more fully the and Mn in the leachate. In a certain period, the longer the duration, the more fully the leaching, and the higher the concentrations of Fe and Mn in the leachate, until they reach a leaching, and the higher the concentrations of Fe and Mn in the leachate, until they reach dynamic equilibrium between the leachate and the sediment. The previous study indicated a dynamic equilibrium between the leachate and the sediment. The previous study indi- that the average concentration of Fe and Mn in the studied area under natural conditions is cated that the average concentration of Fe and Mn in the studied area under natural con- 3.5 mg/L and 0.06 mg/L, respectively [18]. The average concentration comparing with our ditionsstudy is is 3.5 a little mg/L higher. and 0.06 The mg/L, concentration respectively of Fe [18]. and The Mn average in whole concentration Songnen Plain comparing is relatively withhigh, our and study the is alluvial a little flat higher. is particularly The concentration high in the of region.Fe and TheMn in alluvial whole flat Songnen is a hyporheic Plain is zonerelatively with high, a strong and exchange, the alluvial accumulation flat is particularly of organic high matter, in theand region. human The influence, alluvial flat which is a hyporheicmay be why zone Fe andwith Mn a strong content exchange, in the alluvial accumulation flat is particularly of organic high matter, in the and region human [25 ]. influence,The increases whichof may Fe be and why Mn Fe contents and Mn in content the sediment in the alluvial and the flat organics is particularly concentration high inin thethe region solution [25]. for The leaching increases is favorable of Fe and to Mn the contents increases in of the Fe andsediment Mn concentrations and the organics in the concentrationleachate. When in the the solution Fe and for Mn leaching contents is in favorable the sediment to the were increases 21,360–27,749 of Fe and mg/kg Mn con- and centrations484–506 mg/kg, in the respectively,leachate. When and the the Fe reaction and Mn temperature contents in was the set sediment at 20 ◦C, thewere equilibrium 21,360– 27,749concentrations mg/kg and of 484–506 Fe and mg/kg, Mn increased respectively, by 0.3–2.0 and mg/L the reaction and 0.003–0.05 temperature mg/L, was respectively, set at 20 °C,for the each equilibrium 10 mg/L concentrations increase of organics of Fe and concentration Mn increased of the by solution 0.3–2.0 mg/L for leaching. and 0.003–0.05 mg/L, respectively,In the existing for environmental each 10 mg/L increase hydrogeological of organics investigations, concentration organic of the solution pollution for of leaching.groundwater has been found to enhance the release of Fe and Mn from aquifer sediments intoIn groundwaterthe existing environmental [49], especially hydrogeological in those aquifers investigations, with abundant organic Fe and pollution Mn contents. of groundwaterThis enhancement, has been induced found to by enhance the groundwater the release of pollution Fe and Mn with from organics, aquifer increases sediments the intoconcentrations groundwater of [49], Fe and especially Mn in groundwater in those aquifers to different with abundant degrees, and Fe evenand Mn makes contents. ground- Thiswater enhancement, in some regions induced unpotable by the consideringgroundwater Fe pollution and Mn. with For example,organics, McMahonincreases the et al. concentrations(2019) [2] revealed of Fe that and the Mn concentration in groundwater of Mn to was different related todegrees, DOC from and the even soils makes in USA. groundwaterBroclawik et in al. some (2020) regions [50] found unpotable that the considering presence of Fe methane and Mn. contributes For example, to the McMahon depletion et ofal. the(2019) sediment [2] revealed in ferric that iron the compounds, concentration on of the Mn other was words,related increasingto DOC from the the iron soils ions in in USA.the solution.Broclawik et al. (2020) [50] found that the presence of methane contributes to the depletion of the sediment in ferric iron compounds, on the other words, increasing the iron4.2. ions Cause in the of Enhanced solution. Releases of Fe and Mn It can be seen from the results that the leaching of solution with pH of 7 can also 4.2.release Cause Feof andEnhanced Mn from Releases sediment of Fe and to water,Mn which can be mainly attributed to the leaching andIt dissolutioncan be seen function from the of results the water that [ 50the]. Theleaching common of solution physicochemical with pH of conditions 7 can also for releasegroundwater Fe and Mn leaching from sediment and chemical to water, reactions which are can as be follows. mainly First, attributed the existence to the leaching of soluble components is the decisive factor. There are easily soluble Fe and Mn components in the sediment, and the higher the content of the component, the more conducive it is to leaching.

Water 2021, 13, x FOR PEER REVIEW 10 of 15

and dissolution function of the water [50]. The common physicochemical conditions for groundwater leaching and chemical reactions are as follows. First, the existence of soluble components is the decisive factor. There are easily soluble Fe and Mn components in the sediment, and the higher the content of the component, the more conducive it is to leach- Water 2021, 13, 1920ing. Second, data from this study demonstrate that acidic conditions are more conducive 10 of 15 to the leaching. The lower pH gives rise to more complete leaching, for example, the acid pickling and rust removing. Third, time is an essential factor during the leaching and chemical reactions.Second, The data sufficiency from this of study leaching demonstrate and reaction that acidic is directly conditions proportional are more conduciveto the to time. Therefore,the we leaching. could conclude The lower that pH leaching gives rise occurs to more with complete minerals. leaching, This forexplains example, why the acid Fe and Mn arepickling detected and in rustsome removing. natural gr Third,oundwater time is without an essential impacts factor of during anthropogenic the leaching and activities [51]. chemical reactions. The sufficiency of leaching and reaction is directly proportional to the Previous studiestime. Therefore, have shown we could that conclude organic that matter leaching can occursredox with[52] minerals.and complex This explains with why Fe and Mn are detected in some natural groundwater without impacts of anthropogenic metal minerals [34] or co-precipitate with metal elements [44]. Specifically, it reduces the activities [51]. redox potential in groundwater,Previous studies which have creates shown thatan environment organic matter more can redoxsuitable [52 for] and the complex pres- with ence of reductivemetal substances minerals [[53,54].34] or co-precipitate In other words, with the metal high-valent elements [oxidation44]. Specifically, of Fe and it reduces Mn in the sedimentsthe redox transfers potential to inthe groundwater, lower reduced which state creates by the an effects environment of organics more suitablematter. for the As for our study,presence small of amounts reductive of substances K, Ca, Na, [53 and,54]. Mg In otherwere words, also detected the high-valent in the leachate oxidation of Fe along with theand release Mn inof the Fe sedimentsand Mn from transfers the sediment to the lower (Figure reduced 8), state which by theindicated effects ofthat organics there were water–sedimentmatter. As for interactions our study, small occurri amountsng. In of addition, K, Ca, Na, the and reason Mg were of the also unusually detected in the high concentrationleachate of K along was with the theadding release of of the Fe potassium and Mn from hydrogen the sediment phthalate. (Figure8 ),The which redox indicated that there were water–sediment interactions occurring. In addition, the reason of the was supposed the major reaction occurring [55]. Potassium hydrogen phthalate is the re- unusually high concentration of K was the adding of the potassium hydrogen phthalate. ducing agent, Theproviding redox was electrons supposed to thepromot majore reactionthe reduction occurring and [55 dissolution]. Potassium hydrogenof minerals phthalate [34]. Noteworthy,is the the reducing complex agent, associates providing with electrons Fe to toimprove promote the the stable reduction upper and limit dissolution of of Fe(III) [56], whichminerals leads [34 to]. higher Noteworthy, concentration the complex of Fe(III) associates in the with solution Fe to improve [57,58]. theIn stableaddi- upper tion, organic-associatedlimit of Fe(III) Fe [and56], whichMn are leads more to higherstable concentrationthan dissolved of Fe(III) Fe and in theMn solution ions [44], [57, 58]. In which can alsoaddition, be beneficial organic-associated to the metal Fe ox andides Mn dissolution are more stable (Figure than dissolved9). Therefore, Fe and potas- Mn ions [44], sium hydrogenwhich phthalate can also will be beneficialpromote tothe the Fe metal and oxidesMn release. dissolution (Figure9). Therefore, potassium hydrogen phthalate will promote the Fe and Mn release.

15 20 (a)ultrapure water (b)organic sollution

15 10

5 Cl－ NO －N 3 5 － 2－ concentration(mg/L) Cl NO －N concentration(mg/L) SO4 pH 3 2－ SO4 pH 0 0 0 600 1200 1800 2400 3000 3600 0 600 1200 1800 2400 3000 3600 Time(min) Time(min)

15 60 (c) ultrapure water 50 (d) organic sollution

10 40

5 20 K Ca K Ca

concentration(mg/L) 10 Na Mg Na Mg concentration(mg/L)

0 0 0 600 1200 1800 2400 3000 3600 0 600 1200 1800 2400 3000 3600 Time (min) Time (min)

Figure 8. Changes of other concerned indicators (K, Na, Ca, Mg, and pH) in the leachate with time (take I as an example, the other rulesFigure are 8. roughly Changes the of same other as I),concerned (a,c) ultrapure indicators water, ((K,b,d )Na, organic Ca, Mg, solution. and pH) in the leachate with time (take I as an example, the other rules are roughly the same as I), (a)(c) ultrapure water, (b)(d) organic solution.

Water 2021, 13, 1920 11 of 15 Water 2021, 13, x FOR PEER REVIEW 11 of 15

Figure 9. MechanismFigure 9. Mechanism of the increased of the releases increased of Fe releases and Mn of from Fe and sediments Mn from to groundwatersediments to groundwater induced by inputs inof organics. duced by inputs of organics. Results signify that pH influences the release of Fe and Mn. Lower pH results in Results signifyfurther that adsorption, pH influences causing the more release complexation of Fe and Mn. and Lower more release,pH results which in fur- is consistent with ther adsorption,our causing experimental more complexation results. Specifically, and more when release, the pH which is below is consistent the PZC with (the point of zero our experimentalcharge), results. there Specifically, are electrostatic when interactions the pH is below between the the PZC positively (the point charged of zero metal oxides and charge), there negativelyare electrostatic charged interactions phthalic, whichbetween leads the topositively adsorption. charged On the metal contrary, oxides when the pH is and negativelyabove charged the PZCphthalic, [59], theywhich will leads repel to each adsorption. other, causing On the desorption. contrary, Noteworthy,when the adsorption pH is above thecan PZC accelerate [59], they the complexation will repel each reaction, other, andcausing the product desorption. desorbs Noteworthy, little as the pH increases. adsorption can accelerateAfter analyzing the complexation the data, wereaction, find that and there the isproduct liner relationship desorbs little between as the the pH and the pH increases. concentrations of Fe and Mn. The formulas show that the pH has more significant impact. After analyzing the data, we find that there is liner relationship between the pH and 2 the concentrations of Fe and Mn. The CformulasFe = −0.442pH show that + 10.366 the pH (R has= 0.9635) more significant (1) impact. 2 CMn = −0.0090pH + 0.2236 (R = 0.983) (2) CFe = −0.442pH + 10.366 (R2 = 0.9635) (1) 4.3. Implications for Pollution Control 2 With theC rapidMn = − social0.0090pH and economic+ 0.2236 (R development, = 0.983) anthropogenic activities (2) make the total amount of activated organics on the earth increase dramatically [60,61]. The artificial 4.3. Implicationsinput for Pollution of organic Control pollutants will not only lead to organic pollution of groundwater, but will also potentially worsen other water quality indicators and the surrounding environment of With the rapid social and economic development, anthropogenic activities make the groundwater by changing the natural cycle of geochemical elements [62] and temperature, total amount of activated organics on the earth increase dramatically [60,61]. The artificial such as demonstrated with Fe and Mn in our study. Therefore, the implications of con- input of organic pollutants will not only lead to organic pollution of groundwater, but trolling organic pollution are not limited to organic pollution itself. Some other elements will also potentiallyincluding worsen Fe and other Mn water would qu beality affected indicators by the and organic the surrounding pollution andenviron- then influence the ment of groundwaterenvironment by changi [63].ng the natural cycle of geochemical elements [62] and temperature, such as demonstratedTo monitoring with organic Fe and pollution, Mn in our improving study. Therefore, the methods the forimplications assessing groundwater of controlling organic pollution contamination are not is limited necessary. to organic There pollution are some itself. methods Some that other have ele- been proposed ments includingrecently Fe and [64 Mn,65 would]. Based be on affected our study, by the we organic suggest pollution more relevant and then indicators influence should be brought the environmentinto [63]. the test list, such as Fe and Mn, and some recent field studies have taken them into the To monitoringindicators organic [66 ].pollution, In recent improvin years, theg number the methods of studies for assessing on regional groundwater Fe and Mn investigation organic contaminationhave increased is necessary. worldwide There [2]. ar Ite will some be themethods guidance that for have the groundwaterbeen proposed remediation and recently [64,65].also Based should on be our put study, in the we pollutant suggest inventory. more relevant In addition, indicators there should has been be a considerable brought into theamount test list, of research such as intoFe and the Mn, effects and of some biogeochemical recent field processes studies have on organics taken [67–69]. The them into the indicatorsrelationship [66]. between In recent organic years, pollutionthe number and of microbialstudies on communities regional Fe and in groundwaterMn also investigation havewillbe increased a focus forworldwide future research, [2]. It will which be the will, guidance in addition, for havethe groundwater positive effects on finding remediation andthe also source should of pollution. be put in the pollutant inventory. In addition, there has been a considerable amountAlthough of research we innovate into the techniqueseffects of biogeochemical constantly, the processes remediation on processorganics itself still has a [67–69]. The negativerelationship impact between on the organic groundwater pollution environment. and microbial Minimizing communities this negative in impact will groundwater alsobe a will focus be ina focus the future, for future so remediating research, which groundwater will, in addition, by controlling have positive the biogeochemical effects on findingreactions the source between of pollution. minerals, microbes, and different elements in groundwater becomes

Water 2021, 13, 1920 12 of 15

more and more valued [70]. All in all, because of the active chemical reactions in the environment, using the interaction between organic pollution microbial and primary chemical components of the environment, such as Fe and Mn, to conduct environmental remediation may be a worthy direction for future environmental remediation, which has actually caught the attention of scientists in recent years [31,59]. Our study can be a reference for related research. The factors affecting Fe and Mn and organics have been studied in our research, which may become the basis of future feasible solutions.

5. Conclusions This study reveals that Fe and Mn can release from Fe- and Mn-bearing minerals of aquifer sediments through groundwater leaching. It also further proves the hypothesis that the input of pollutant such as organics into groundwater through anthropogenic activities indeed can considerably enhance the leaching, which can cause the concentrations of Fe and Mn to increase exponentially, far more than those only obtained from natural groundwater– sediments interactions. The higher the contents of Fe- and Mn-bearing minerals in aquifer sediments, the higher the concentrations of Fe and Mn in groundwater after reaching dynamic equilibrium between the sediment and the groundwater. Acidification of the groundwater environment can make the leaching more thorough. The study explains well why the groundwater concentrations of Fe and Mn present great spatial differences in the natural environment worldwide. It is also deduced that the abnormal increase of groundwater concentrations of Fe and Mn in some aquifers can be mainly attributed to the anthropogenic activities, especially those related to organics emissions. In addition, the acidification of the water environment, which is mainly caused by anthropogenic activities, should be also responsible for the abnormal increase. Thus, groundwater pollution control may be not only effective to those pollutants discharged directly by anthropogenic activities, but may also be related to those species existing in the natural environment. Increased attention should be paid to the significance of organics on the geochemical processes of Fe and Mn in the natural environment, especially in the groundwater–sediment system, which will also be favorable for the further development of groundwater pollution control measures.

Author Contributions: Methodology, Y.Z. and X.X.; formal analysis, X.L. and Y.H.; investigation, X.X.; data curation, X.L. and H.L.; writing—original draft, X.L.; writing—review and editing, Y.Z.; supervision, J.W.; funding acquisition, Y.T. and Y.Z. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Major Science and Technology Program for Water Pollution Control and Treatment (No. 2018ZX07101005-04), the National Natural Science Foundation of China (No. 42077170 and 41831283), Beijing Advanced Innovation Program for Land Surface Science of China, and the 111 Project of China [B16020]. Data Availability Statement: Not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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