Solid phase speciation of Zn and Cd in zinc smelter effluent-irrigated soils

Figures & data

Figure 1. Map of the study area showing the sampling points.

Table 1. Effect of long-term irrigation with zinc smelter effluent on important properties and metal content in soil.

Parameter		Effluent-irrigated	Tubewell water irrigated
Soil pH		6.85–9.00 (7.84)**	7.70–8.40 (8.15)
Soil organic carbon (%)		0.28–1.81 (0.77)**	0.41–0.74 (0.58)
EDTA extractable metal content (mg kg^-1)	Zn	4.50–2520 (179)**	0.90–6.30 (3.08)
	Cd	0.02–31.9 (3.96)**	ND–0.13 (0.05)
Total metal content (mg kg^-1)	Zn	64.5–21,700 (1268)**	29.7–60.7 (47.0)
	Cd	0.06–214 (13.4)**	0.02–0.19 (0.08)

Notes:

Figures in parentheses indicate the mean values. (**) indicates that the differences between means are significant at 1% probability level. ND indicates non-detectable.

Table 2. Important properties of the experimental soils used for identification of solid phases.

Soil	Texture	pH	EDTA Zn (mg kg^-1)	EDTA Cd (mg kg^-1)	Total Zn (mg kg^-1)	Total Cd (mg kg^-1)
Contaminated-1	Sandy loam	7.76	2520	29.7	21,700	193
Contaminated-2	Clay loam	7.52	215	9.70	915	14.0
Non-contaminated	Sandy clay loam	7.70	2.90	0.05	43.7	0.08

Table 3. Input data on the concentration of metals (mg L^-1) and pH in Baker soil extract used for estimating the free ion activity of Zn²⁺ and Cd²⁺ in different soils.

Soil	Ca	Mn	Ni	Cu	Zn	Cd	Pb	Mg	Al	Fe	K	pH
Contaminated-1	339	0.75	0.05	0.46	22.6	4.15	0.72	60.6	0.008	0.11	39.7	7.75
Contaminated-2	162	1.25	0.05	0.17	13.8	0.89	1.80	41.1	0.014	0.39	11.3	7.60
Non-contaminated	192	3.10	0.05	0.13	1.50	0.12	0.26	35.0	0.014	0.58	11.3	7.58

Table 4. Free ion activity of Zn (pZn²⁺) and Cd (pCd²⁺) in different soils as estimated by Baker’s soil test computer programme.

Soil	pZn^2+	pCd^2+
Contaminated-1	6.491	8.102
Contaminated-2	9.854	11.991
Non-contaminated	11.072	13.115

Table 5. Solubility equations of some important minerals of Zn, Cd, Fe and Si in soil.

Element	Minerals	Equilibrium equations	log k°
Zinc (Zn)	Soil-Zn	Soil-Zn + 2H^+ ⇌ Zn^2+	5.8
	Franklinite (ZnFe2O4)	ZnFe2O4 + 8H^+ ⇌ Zn^2+ + 2Fe^3+ + 4H2O	9.85
	Willemite (Zn2SiO4)		13.15
	Smithsonite (ZnCO3)	ZnCO3 + 2H^+ ⇌ Zn^2+ + CO2 (g) + H2O	7.91
	Zincite (ZnO)	ZnO + 2H^+ ⇌ Zn^2+ + H2O	11.16
	Zn(OH)2 (amorphous)	Zn(OH)2 + 2H^+ ⇌ Zn^2+ + 2H2O	12.48
	α-Zn(OH)2	α-Zn(OH)2 (c) + 2H^+ ⇌ Zn^2+ + 2H2O	12.19
	ß-Zn(OH)2	ß-Zn(OH)2 (c) + 2H^+ ⇌ Zn^2+ + 2H2O	11.78
	γ-Zn(OH)2	γ-Zn(OH)2 (c) + 2H^+ ⇌ Zn^2+ + 2H2O	11.74
	ε-Zn(OH)2	ε-Zn(OH)2 (c) + 2H^+ ⇌ Zn^2+ + 2H2O	11.53
Cadmium (Cd)	Soil-Cd	Soil-Cd ⇌ Cd^2+	−7.00
	Octavite (CdCO3)	CdCO3 + 2H^+ ⇌ Cd^2+ + CO2 (g) + H2O	6.16
	Cadmium silicate (CdSiO3)		7.63
	Monteponite (CdO)	CdO + 2H^+ ⇌ Cd^2+ + H2O	15.14
	ß-Cd(OH)2	ß-Cd(OH)2 (c) + 2H^+ ⇌ Cd^2+ + 2H2O	13.65
Iron (Fe)	Soil-Fe (Fe(OH)3)	Fe(OH)3 + 3H^+ ⇌ Fe^3+ + 3H2O	2.70
	Maghemite (γ-Fe2O3)	1/2γ-Fe2O3 + 3H^+ ⇌ Fe^3+ + 3/2 H2O	1.59
	Goethite (α-FeOOH)	α-FeOOH + 3H^+ ⇌ Fe^3+ +2H2O	−0.02
Silicon (Si)	Quartz (α-SiO2)		−4.00
	SiO2 (amorphous)		−2.74

Source: Lindsay [24].

Figure 2. Solubility diagram of Zn containing minerals in soil.

Figure 3. Solubility diagram of Cd containing minerals in soil.

Lindsay WL. Chemical equilibria in soils. New York: Willey inter sciences; 1979.

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