Heavy Metals in Wastewater and Sludge Treatment Processes I-II [antikvár]

FOREWORD FOR VOLUME I The group of the transition and post-transition elements frequently referred to as "heavy metals" and generally accepted to include Cd, Cr, Cu, Hg, Ni, Pb, and Zn, plus the metalloids As and Se, have long been contaminants of sewages. With hindsight, it might have been preferable to have called the group "toxic elements"; this terminology permits the inclusion of elements such as aluminum which do not conform precisely to the physical or chemical meaning of "heavy metal" but which are nevertheless of concern for the same reasons. Even more appropriate might have been the description "exotic elements", a classification which could include Ag, Bi, Co, Mn, Mo, Sb, Sn, Te, Tl, and V. All of these elements can now be found in sewages, and in the environment generally, as a result of Man's activities. Although trade effluent controls have reduced the quantities of several of the toxic elements discharged to sewers, the increasing diversity of elements to be found in such discharges is testimony to the demands of technological innovation for new materials which previously had found little application. That these elements are of concem in the aquatic environment is implicit in the plethora of legislation which has arisen in recent years. Most of the elements referred to here are classified as Priority Pollutants by the United States Environmental Protection Agency (EPA) and have been accorded the status of Dangerous Substances by the Commission of the European Communities (CEC). As such, limits have been placed on their concentrations in potable water, in sources of abstraction, and in effluent discharges. The behavior of the elements of concem in sewage treatment processes is of central importance in achieving compliance with the legislation, because their removal minimizes the pollution of surface waters. However, this is at the expense of their accumulation in the sludges produced. The disposal of these sludges is also controlled by legislation on a national and intemational basis. Legislation is only of value if compliance can be checked; comprehensive legislation therefore demands a correspondingly comprehensive analytical capability. Recommended standard methods in Europe and the U.S. invariably specify atomic absorption spectrophotometry as the current method of choice, but little more than a decade ago, relatively insensitive, imprecise colorimetric methods were still in use. Now, a variety of methods are available, but many are not suited to wastewater matrices. Despite the recognition by Kirchoff and Bunsen of absorption in flames in the mid-19th century, it was not until a century later that the analytical possibilities of atomic absorption were described by Walsh in his 1953 patent application. By 1966, the technique using flame atomization was well-established. Although L'vov had designed a carbon tube furnace capable of greater sensitivity in 1961, it was the simpler design of Massman in 1968 that led to the appearance of several commercial designs a year later. The much greater sensitivity permitted not only the determination of a wider range of elements as low concentrations, but provided the necessary detection limits for metal speciation. It is remarkable that perhaps the most difficult matrix from an analytical point of view, sewage sludge, was the subject of some early attempts at metal speciation. This was certainly not because sewage sludge was considered an easy matrix to work with, but becasue there was a perceived need to obtain data amenable to biological interpretation so that the potential adverse effects of sludge use in agriculture could be quantified. The most expedient way of obtaining such data, that of chemical extraction, has acknowledged limitations and has not found widespread routine application. However, extraction techniques have contributed to an understanding of metal speciation in sewage sludges, the importance of which is now reflected in sludge disposal guidelines which make reference to factors such as cation exchange capacity, pH, and soil composition which can affect the bioavailability of metals. Considerable interest is currently being shown in metal speciation in natural waters.