The mean Cd leachable concentrations were reduced from 4.52 to 0.89 and 0.52 mg/l
(80 and 88% reduction) after 7 and 14 days of stabilization. Cu leachable concentrations
were reduced from 295 to 79.1 and 37.2 mg/l (73 and 87% reduction). At the 7th and 14th
days, the mean Ni leachable concentrations were reduced to 8.9 and 9.1, respectively, from
19.0 mg/l (52% reduction). Pb leachable concentrations were reduced from 956 to 3.9 and
1.5 mg/l (>99.5% reduction). This shows that phosphate is particularly promising in immobilizing
Pb contaminated soil. Zn leachable concentrations were reduced from 3153 to
460 and 344 mg/l (85 and 89% reduction). Many researchers have examined the stabilization
of metals in contaminated soils. Chowdhury et al. [26] reported a 22,388 mg Pb/kg
contaminated soil treated with 5% phosphate-based additive. Pb leaching concentration
was reduced from 180 to 3.6 mg/l. Basing on a geochemical modeling, Ruby et al. [15]
reported that the addition of phosphate amendment to Pb-bearing soil resulted in aqueous
lead solubility of 0.1 g/l in soil. Ma et al. [17] reported that phosphate rocks reduced
water-soluble Pb from a contaminated soil by 56.8–100%. Ma [20] reported that hydroxyapatite
(Ca10(PO4)6(OH)2) effectively immobilized (>71%) of the aqueous Pb in four sites
of PbHAsO4 contaminated soil. Chlopecka and Adriano [18] reported that the ameliorants
of lime and apatite (Ca5(PO4)3OH) decreased the leaching concentration of Zn in a contaminated
soil. Berti and Cunningham [11] investigated three sites of Pb contaminated soil
from 1200 to 3500 mg/kg. The leachable soil Pb was significantly reduced in all cases from
as high as 30 mg Pb/l to below the regulatory limit of 5 mg/l after treatment of 0.5% of
P to the soil. Chen et al. [21] reported that a Pb, Cd, and Zn contaminated soil was stabilized
with apatite. The removals were about 0.729 mmol of Pb, 0.489–1.317 mmol of
Cd, and 0.596–2.187 mmol of Zn/g of apatite. Eighmy et al. [23] added 1.2 mol of H3PO4
to 1 kg dry scrubble residue and the leachable concentrations of heavy metal reductions
were Cd (38%), Cu (58%), Pb (98%), and Zn (28%). Zhang et al. [22] reported that a Pb
contaminated soil was added apatite and the soluble Pb concentration was 96% reduced in
suspension. Hettiarachchi et al. [24] added 5 g of P to five sites of Pb contaminated soil
ranging from 4463 to 42,592 mg/kg soil. The reductions in bioavailable Pb ranged from 15
to 41%. These results showed that phosphate-based additive was able to form less soluble
minerals with heavy metals in aqueous soil and can reduce leachable concentration of heavy
metals, particularly Pb.