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This article in JEQ

  1. Vol. 29 No. 6, p. 1835-1839
     
    Received: Nov 30, 1999


    * Corresponding author(s): hae1@psu.edu
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doi:10.2134/jeq2000.00472425002900060014x

Molybdenum Content of Water Treatment Residuals

  1. H. A. Elliott * and
  2. M. Taylor
  1. Agricultural and Biological Engineering Dep., Pennsylvania State Univ., University Park, PA 16802.

Abstract

Abstract

Molybdenum (Mo) content of water treatment residuals (WTRs) was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) to evaluate suitability for land application under state regulatory policies that limit Mo to 18 mg kg−1. Samples of WTR were collected from 32 Pennsylvania facilities that employ aluminum salts, ferric chloride, and/or polymers for coagulation. The mean Mo content of all samples was 3.1 mg kg−1, with 78% having Mo levels <5 mg kg−1. The WTRs from plants using ferric chloride as a coagulant averaged 5.6 mg Mo kg−1, significantly higher (p = 0.02) than the 1.6 mg Mo kg−1 for utilities using alum. Differences were related to coagulant purity: Mo content in liquid ferric chloride was 10.0 mg L−1 but below detection by ICP-AES for alum. The initial sample from one facility, collected from the filter backwash basin, contained 26.4 mg Mo kg−1. Elevated Mo in backwash solids was attributed to filtration capture of extremely fine, Mo-enriched AI hydrous oxide particles and erosion of anthracite filter media during backwashing. Combined backwash and coagulation solids from this facility's storage lagoon averaged 6.3 mg Mo kg−1, underscoring the need for consistent sampling procedures. The mean Cu to Mo ratio in these WTRs was >100, well above the minimum dietary ratio (2:1) considered protective of grazing animals.

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