Distribution and fractionation of rare earth elements in high fluoride groundwater from the North China Plain

doi:10.13745/j.esf.sf.2021.7.24

Abstract

Abstract:

High-fluoride groundwater is a major environmental problem that has caused global concerns. Although numerous studies have been done on the chemical characteristics, formation and migration mechanisms of high-fluoride groundwater, little is known about the concentrations and fractionation characteristics of rare earth elements (REE) in high-fluoride groundwater or if REE can be used as a tracer for studying the formation and distribution of high-fluoride groundwater. These uncertainties have limited REE application in high-fluoride groundwater research. In this paper, we investigated the fluoride and REE concentrations and distributions in groundwater collected along a flow path in the North China Plain (NCP) where strong fluoride anomalies in groundwater were observed. We found the groundwater fluoride concentrations ranged from 0.28 to 9.33 mg/L, with 55% groundwater exceeding the China drinking water standard (1 mg/L). According to PHREEQC calculation, fluorine occurs in groundwater as NaF, CaF⁺, MgF⁺ and, predominantly, F^-(85.42%-99.39%). High-fluoride groundwater is mainly distributed in the central alluvial lacustrine plain and the eastern alluvial marine plain, with 60% at depths below 180 m. Hydrochemical analysis indicated the formation of shallow high-fluoride groundwater is mainly controlled by evaporation and concentration, while deep high-fluoride groundwater is a result of mineral dissolution and competitive ion adsorption through water-rock interactions. The groundwater REE concentration is at pico to nanomolar level, and, according to PHREEQC calculation, REE species are mainly carbonate complexes ( $REECO 3 +$ and $REE(CO_{3})_{2}^{-})$, with 0-1.18% REE in complex with F^-(REEF²⁺ and $REEF 2 +$ ). The upper continental crust (UCC)-normalized REE patterns are characterized by enrichments of heavy REE (HREE) and middle REE (MREE) over light REE (LREE) and have significant negative Ce anomalies (0.11<Ce/Ce^*=Ce_UCC/(La_UCC×Pr_UCC)^0.5<2.29). The HREE enrichment in groundwater is mainly attributed to the preferential complexation of HREE over LREE or HREE with carbonate in forming more stable carbonate complexes. Along a groundwater flow path, both REE and fluoride concentrations in deep aquifers generally increase following similar trends. Besides, high-fluoride groundwater is more prone to HREE enrichment. These findings suggest that REE can potentially be used as a fluoride indicator for studying fluoride enrichment in natural aquifer systems.

Key words: hydrochemistry, water-rock interactions, elemental migration and transformation, enrichment and fractionation, North China Plain

CLC Number:

P641.3
P595

LIU Haiyan, LIU Maohan, ZHANG Weimin, SUN Zhanxue, WANG Zhen, WU Tonghang, GUO Huaming. Distribution and fractionation of rare earth elements in high fluoride groundwater from the North China Plain[J]. Earth Science Frontiers, 2022, 29(3): 129-144.

Figures/Tables 16

Fig.1 Geographical location of and Quaternary landforms in the North China Plain

Fig.2 Distribution of groundwater sampling locations in the study area

Table 1 Chemical equations and equilibrium constants used for simulating F--cation complexation reactions in WATEQ4F

反应	平衡常数	参考文献
Mg²⁺ + F^- = MgF⁺	1.82	[22]
Ca²⁺+ F^- = CaF⁺	0.94	[22]
Na⁺ + F^- = NaF	-0.24	[22]
Al³⁺ + F^- = AlF²⁺	7.0	[23]
Al³⁺ + 2F^- = Al $F 2 +$	12.7	[23]
Al³⁺ + 3F^- = AlF₃	16.8	[23]
Al³⁺ + 4F^- = Al $F 4 -$	19.4	[23]
Fe³⁺ + F^- = FeF²⁺	6.2	[22]
Fe³⁺ + 2F^- = Fe $F 2 +$	10.8	[22]
Fe³⁺ + 3F^- = FeF₃	14	[22]

Table 1 Chemical equations and equilibrium constants used for simulating F--cation complexation reactions in WATEQ4F

反应	平衡常数	参考文献
Mg²⁺ + F^- = MgF⁺	1.82	[22]
Ca²⁺+ F^- = CaF⁺	0.94	[22]
Na⁺ + F^- = NaF	-0.24	[22]
Al³⁺ + F^- = AlF²⁺	7.0	[23]
Al³⁺ + 2F^- = Al $F 2 +$	12.7	[23]
Al³⁺ + 3F^- = AlF₃	16.8	[23]
Al³⁺ + 4F^- = Al $F 4 -$	19.4	[23]
Fe³⁺ + F^- = FeF²⁺	6.2	[22]
Fe³⁺ + 2F^- = Fe $F 2 +$	10.8	[22]
Fe³⁺ + 3F^- = FeF₃	14	[22]

Table 2 Chemical equations and equilibrium constants added to the WATEQ4F database in PHREEQC calculation for REE- anion complexation reactions (ionic strength, 0; temperature, 25℃)

Table 3 Statistical table of chemical composition of groundwater in three study zones

区域	统计量		pH		地下水中各化学组分的指标参数统计值
区域	统计量		pH		TDS		$HCO 3 -$		Cl^-		$NO 3 -$	$SO 4 2 -$	F^-	K⁺	Na⁺	Ca²⁺	Mg²⁺
一区	最大值	7.97		963.00		461.16		376.63		115.85		76.08	1.33	1.86	37.49	220.71	68.14
	最小值	7.10		222.00		204.96		3.60		0.00		11.99	0.28	0.05	3.55	<检出限	24.55
	平均值	7.64		391.61		302.15		48.78		44.49		43.17	0.70	1.23	11.69	91.14	31.77
	标准差	0.20		155.61		65.88		83.97		37.81		17.98	0.24	0.53	7.06	43.04	9.61
	变异系数	0.03		0.40		0.22		1.72		0.85		0.42	0.34	0.43	0.60	0.47	0.30
二区	最大值	8.80		4 349.00		924.76		984.36		123.16		2 890.38	7.07	5.66	973.42	333.18	476.73
	最小值	7.08		274.00		31.72		4.71		0.44		17.99	0.71	0.33	28.14	3.59	1.50
	平均值	7.96		1 031.38		438.27		209.92		8.84		395.00	2.91	1.35	261.08	81.93	72.75
	标准差	0.55		1 047.07		193.14		263.43		27.14		735.41	1.81	1.26	234.02	98.49	113.32
	变异系数	0.07		1.02		0.44		1.25		3.07		1.86	0.62	0.93	0.90	1.20	1.56
三区	最大值	8.51		1 179.00		405.04		463.45		5.77		242.52	9.33	2.60	429.25	13.87	14.41
	最小值	8.20		796.00		275.72		56.01		1.72		40.70	1.39	0.76	277.00	7.32	1.62
	平均值	8.42		996.80		340.14		292.06		3.52		122.18	5.12	1.59	361.71	10.27	5.81
	标准差	0.12		132.50		41.77		134.64		1.58		94.54	2.87	0.60	52.50	2.24	4.44
	变异系数	0.01		0.13		0.12		0.46		0.45		0.77	0.56	0.37	0.15	0.22	0.76

Table 3 Statistical table of chemical composition of groundwater in three study zones

区域	统计量		pH		地下水中各化学组分的指标参数统计值
区域	统计量		pH		TDS		$HCO 3 -$		Cl^-		$NO 3 -$	$SO 4 2 -$	F^-	K⁺	Na⁺	Ca²⁺	Mg²⁺
一区	最大值	7.97		963.00		461.16		376.63		115.85		76.08	1.33	1.86	37.49	220.71	68.14
	最小值	7.10		222.00		204.96		3.60		0.00		11.99	0.28	0.05	3.55	<检出限	24.55
	平均值	7.64		391.61		302.15		48.78		44.49		43.17	0.70	1.23	11.69	91.14	31.77
	标准差	0.20		155.61		65.88		83.97		37.81		17.98	0.24	0.53	7.06	43.04	9.61
	变异系数	0.03		0.40		0.22		1.72		0.85		0.42	0.34	0.43	0.60	0.47	0.30
二区	最大值	8.80		4 349.00		924.76		984.36		123.16		2 890.38	7.07	5.66	973.42	333.18	476.73
	最小值	7.08		274.00		31.72		4.71		0.44		17.99	0.71	0.33	28.14	3.59	1.50
	平均值	7.96		1 031.38		438.27		209.92		8.84		395.00	2.91	1.35	261.08	81.93	72.75
	标准差	0.55		1 047.07		193.14		263.43		27.14		735.41	1.81	1.26	234.02	98.49	113.32
	变异系数	0.07		1.02		0.44		1.25		3.07		1.86	0.62	0.93	0.90	1.20	1.56
三区	最大值	8.51		1 179.00		405.04		463.45		5.77		242.52	9.33	2.60	429.25	13.87	14.41
	最小值	8.20		796.00		275.72		56.01		1.72		40.70	1.39	0.76	277.00	7.32	1.62
	平均值	8.42		996.80		340.14		292.06		3.52		122.18	5.12	1.59	361.71	10.27	5.81
	标准差	0.12		132.50		41.77		134.64		1.58		94.54	2.87	0.60	52.50	2.24	4.44
	变异系数	0.01		0.13		0.12		0.46		0.45		0.77	0.56	0.37	0.15	0.22	0.76

Fig.3 Piper plot for groundwater samples from the study area

Fig.4 Gibbs diagram for groundwater samples

Fig.5 (a) Horizontal distribution and (b) depth dependence of F- concentration in groundwater in the study area

Fig.6 Relationship between pH and fluorine species percentage (a) or total F- concentration (b) in groundwater

Table 4 REE concentrations and fractionation parameters in groundwater collected from the North China Plain (NCP)

区域	统计量		元素含量/(pmol·L^-1)
区域	统计量		La	Ce	Pr		Nd	Sm		Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu
一区	最小值		40	20	6		27	10		85	14	1	2	1	3	0	2	1
	最大值		242	444	57		218	38		589	43	7	44	13	45	9	65	13
	平均值		124	135	24		102	22		209	26	3	19	5	16	2	16	3
二区	最小值		36	27	6		16	8		18	10	1	5	1	4	1	2	1
	最大值		982	1 503	273		835	342		559	404	49	224	32	85	13	61	14
	平均值		212	306	46		171	45		197	51	6	33	7	20	3	18	4
三区	最小值		35	70	12		42	7		85	11	1	6	4	5	1	5	1
	最大值		1 648	2 545	377		1 471	282		309	291	38	183	39	109	16	95	16
	平均值		514	822	120		471	91		181	89	12	60	13	36	5	29	5
区域		统计量	元素含量/(pmol·L^-1)								(La/Sm)_UCC		(Gd/Yb)_UCC		Ce/Ce^*		Eu/Eu^*
区域		统计量	∑REE			∑LREE			∑HREE		(La/Sm)_UCC		(Gd/Yb)_UCC		Ce/Ce^*		Eu/Eu^*
一区		最小值	392			368			23		0.48		0.33		0.11		19.03
		最大值	1 176			1 039			236		1.08		3.51		2.29		89.81
		平均值	707			617			90		0.8		1.2		0.61		43.52
二区		最小值	174			113			31		0.33		0.22		0.31		2.27
		最大值	4 631			3 786			846		2.77		4.26		1.52		90.62
		平均值	1 119			977			142		0.74		1.62		0.8		43.04
三区		最小值	286			251			35		0.73		1.14		0.74		2.56
		最大值	7 266			6 480			786		0.81		1.82		0.81		45.79
		平均值	2 447			2 198			249		0.76		1.49		0.77		22.28

Fig.7 Horizontal variations of Nd content and ∑REE (a) and vertical variation of ∑REE (b) in groundwater in the study area

Fig.8 Simulated Nd, Sm and Yb speciations in shallow (a, b, c) and deep (d, e, f) groundwater in the study area

Fig.9 UCC-normalized REE pattern for groundwater in study zones I-III (a-c) and horizontal variations of Ce/Ce* and Eu/Eu* (d) in groundwater in the study area

Fig.10 Relationship between Eh and Ce/Ce* (a) or Eu/Eu* (b) in groundwater in the study area

Fig.11 Relationship between F- concentration and ∑REE (a) or (La/Sm)UCC (b) in groundwater in the study area

Fig.12 (a) Relationship between HCO 3 - and F- in groundwater and (b) variation of equilibrium constants for REE-carbonate complexation reactions with increasing atomic number (data from [26,28,39⇓-41])

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