A. L. Page

Zinc and cadmium uptake by barley in field plots fertilized seven years with urban and suburban sludge

Barley plants grown in 2.4− × 3.0-m field plots were fertilized annually with two sources of sewage sludge for 7 consecutive years. One sludge originating, from a suburban area, was composted and applied dry. The concentrations of N, Zn, and Cd were low and over the years averaged 1.1%, 496 mg kg-1, and 5.2 mg kg-1, respectively. The other sludge came from an industrial, urban area and was applied as a slurry. It had an N content three times as high, a Zn content that averaged 5 to 10 times as high, and a Cd content 5 to 20 times that of the suburban sludge. The applied sludge rates ranged from 0 to 225 metric tons (t) per hectare in increments of 45 t. Yields in each year peaked at 90 t with the suburban and at 45 t with the urban sludge. Yields increased during the first 4 years and remained stable for the last 3. No toxic effects were observed on any of the plants. During the first 2 years, little or no increase in concentrations of Zn and Cd in grain of barley was found with either sludge. In the last 4 years, Zn and Cd concentrations in grain and straw increased where urban sludge was used. In all cases, straw accumulated more Zn and Cd than did grain. A chemical fertilizer treatment of NPKS produced grain with Zn and Cd content comparable to that from an agronomic rate of suburban sludge over the entire 7-year period.

Surface loading effect on Cd and Zn sorption by kaolinite and montmorillonite from low concentration solutions

Studies of Cd and Zn sorption using Na-saturated kaolinite and montmorillonite, and low metal solution concentrations similar to those found in the environment, showed that metal sorption affinity (measured by Kd values) decreased markedly with increasing surface metal loading for both layer silicates.For equilibrium solution concentrations <0.1 μmol L−1 for Cd, and < 1 μmol L−1 for Zn, both metals were sorbed with greater affinity by kaolinite than montmorillonite. These results were probably due to the higher proportion of weakly acidic edge sites present on kaolinite surfaces. In the case of Zn there was an affinity reversal for equilibrium solution concentrations > 1 μmol L−1, which was attributed to the permanent charge sites of montmorillonite. Cadmium ions were sorbed, by kaolinite, with greater affinity than Zn for equilibrium solution concentrations between 0.3 to 1.5 μmol L−1. This result was attributed to retention of these metal ions through electrostatic attraction by permanent charge sites present on the kaolinite used in this work. According of these results it seems that metal sorption by these layer silicates involves predominantly edge weak acid sites at lower surface coverages (higher affinity sites), and permanent charge sites at higher metal coverages (lower affinity sites).It was concluded that Cd and Zn sorption by those two layer silicates is greatly influenced by surface metal coverage, and results cannot be extrapolated from low to high surface coverages, and viceversa.