INCREASED EXTRACTABLE METAL CONCENTRATIONS FOLLOWING SEWAGE SLUDGE ADDITIONS TO GRASSLAND SOILS.

 

Jeffrey R. Bacon*, Malcolm C. Coull, Irene J. Hewitt and Pat Cooper.

Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB15 8QH, UK

*e-mail: j.bacon@mluri.sari.ac.uk

 

ABSTRACT

Soil samples have been collected on an annual basis from experimental field plots in which the concentrations of single metals (Cd, Cu or Zn) have been built up over a four-year period through the addition of metal–containing sewage sludges. The soils have been analysed for total metal concentrations (HF digestion and aqua regia extraction) and extractable concentrations (ammonium nitrate single extraction and the modified BCR sequential extraction procedure). The relative extractability of metals not only increased upon addition of sludge but also continued to increase in the two years following the last addition. Although the total concentrations of some metals (for example, Ni) did not increase after the addition of sludge, the extractable concentration did increase indicating an interaction between soil and sludge which can lead to increased availability of metals already present in the soil.

 

INTRODUCTION

Application to agricultural land has long been considered a beneficial use of sewage sludge but concern also exists that application of sludges irreversibly introduces potentially toxic compounds into the soil. The UK regulations on addition of sewage sludge to agricultural land, based on EU directives, are regularly reviewed and in 1994 a network experiment, funded by several UK agencies, was started at a number of sites in the UK in order to study the long term effect on soil fertility of additions of metal-containing sludges. Sludges were introduced into the experimental plots in four annual additions so that by 1997 a range of concentrations for individual heavy metals (Cd, Cu, Zn) had been established with the top concentrations being set about 25-30% above the permitted maximum soil concentration for sludge additions.

 

Although the focus of the network study was the microbial activity in the soils, the heavy metal status plays an important role in interpreting the data and in setting appropriate limits. Soil samples were taken on an annual basis during the metal build up and again in 1999, two years after final sludge addition, and have been analysed by a range of microbial and chemical techniques. As part of the common protocol for the network experiment, the aqua regia and ammonium nitrate extractable heavy metal concentrations have been determined for the samples taken from all the sites. In addition for the MLURI grassland site at Hartwood in Scotland, total heavy metal concentrations were determined using an HF dissolution procedure and selected soils were analysed using the modified BCR sequential extraction procedure. Both sets of samples for which data are presented were collected (in 1997 and 1999) after the final addition of sewage sludge so it was to be expected that the total metal concentrations should remain the same. The focus of this study was to investigate any changes in metal availability that have occurred following addition of sludge and in the two years since the last addition.

 

METHODS

Site

The experimental site was set up in 1994 at the Hartwood Research Station located approximately 10 km to the east of Motherwell in central Scotland. The site was chosen to represent a hill farm predominantly under permanent grassland and typical of the agricultural land likely to receive sewage sludge. The experimental plots consisted of three replicate blocks each consisting of 23 plots (8x6 m) laid out in a random order.

Metal addition

Metal–rich sludges were added to the experimental plots over four successive years (1994–1997) following a well-defined protocol (Chambers et al., 1998) in order to raise the total metal concentrations to and beyond the maximum permitted under current legislation controlling the addition of sewage sludge to agricultural land. The sludges were chosen to contain a high concentration of a predominant single metal (Cd, Cu or Zn) in order to study the effect of a single metal on soil fertility.

Sampling

Composite soil samples were taken for each plot on an annual basis by bulking together at least 20 random cores taken to a depth of 20 cm. Fresh soil was air dried (30°C) and sieved (2–mm mesh). Soils were stored in polyethylene bags prior to analysis.

Soil digestion and extraction

Total metal concentrations in soils were determined following HF digestion of ground and ignited soil (Bacon et al., 1995). Aqua regia extractions were carried out on ground soils using the procedure of McGrath and Cunliffe (1985). The ammonium nitrate extractable metal concentrations were determined using the German standard procedure DIN 19730.

Sequential extraction procedure

The modified BCR three step sequential extraction procedure (Rauret et al. 2000) was used for selected soil samples. This is an operationally defined extraction procedure that apportions the metals to four different fractions: acetic acid extractable, reducible, oxidizable and residual. The final residual fraction was digested by the HF procedure used to determine the total metal concentration on the same sample.

Determination of concentrations

The concentrations of Cd, Cu, Ni, Pb and Zn in soil digests and extracts were determined either by inductively coupled plasma atomic emission spectrometry (ICP–AES) or, if the concentrations were below the detection limits of ICP–AES, by graphite furnace atomic absorption spectrometry (GF–AAS). Calibration and check standards were made up in the same matrix as the sample solution. The analyses were carried out in an accredited laboratory under strict quality control procedures. In–house standard soil samples were included with each batch of analyses as a check. The precision of analysis was generally ±2–3%. All concentrations are expressed relative to air–dried soil.

 

 

RESULTS AND DISCUSSION

 

The total heavy metal concentrations in sludged soils determined by three independent methods (HF digestion, aqua regia extraction and the sum of the sequential extraction extracts) were in close agreement (Table 1). This gave ground for confidence in the analytical data, in particular in the sequential extraction data which are subject to errors at each of the extraction stages. It is to be noted that virtually all the added metals were aqua regia extractable. Although the sludges were chosen because they had high concentrations of a predominant single metal, other metals were also present in lower concentrations. In particular lead was present in all the sludges to different degrees and the zinc sludge also contained other metals.

 

Table 1. Metal concentrations (mg g–1) in selected plots.

element

Cu

Cu

Pb

Pb

Pb

Pb

Zn

Zn

plot

control

Cu

control

Cd

Cu

Zn

control

Zn

HF digestion

26

211

48

 84

 95

 92

72

391

Aqua regia extr.

25

190

45

 94

 98

102

79

394

Sum fractions

25

217

55

101

115

116

73

420

 

A notable effect has been that although the total concentrations in the soils have remained the same since 1997, the extractable concentrations, both using single ammonium nitrate extraction and the sequential extraction approach, have increased. In some cases, the ammonium nitrate extractable metal concentrations now represent a substantial proportion of the total metal concentrations (Figure 1). These findings suggest that changes in availability of metals continue to occur well after the addition of sludge.


 


Figure 1. Change in percentage of total metal extractable by ammonium nitrate from 1997 to 1999. Plots are denoted by c (control) or the metal treatment.

 

Although heavy metals other than Cd, Cu and Zn were at relatively low concentrations in the applied sewage sludges, the extractability of these metals was seen to increase. This demonstrated that the addition of sludge not only adds metals to the soils whose availability will be greater than the metals native to the soil but also increases the availability of metals already present in the soil. This is illustrated (Table 2) by 1999 data for nickel in all the treatments. Although addition of Cu sludge did not increase the soil Ni concentration, extractability of Ni both by ammonium nitrate and acetic acid (first step of the sequential extraction procedure) was seen to increase. Similar effects were observed for the Cd and Zn treatments but a small increase in total metal concentration was also observed. Data for Pb followed a similar trend.

 

Table 2. Total and extractable concentrations (mg g–1) of Ni in different treatments.

Treatment

control

Cd plot

Cu plot

Zn plot

Total (HF) concentration.

20

26

18

51

NH4NO3 extractable

   0.4

    1.3

    0.6

 6

Acetic acid extractable

   1.7

    6.3

    2.7

20

 

Current legislation is based entirely on total metal concentrations and does not take into account the variable degree of availability of metals in sewage sludge and soils. For example, lead is more extractable from the Cu–enriched soils than from the Cd–enriched soils. The preliminary data from this study demonstrate that amendment of soils with sewage sludge can significantly increase the available fraction of metals within the soil. This results not only from the added metals being more available than the metal native to the soil but also from changes in availability of the metals already present in the soil. When considering the impact of sewage sludge amendment, it is probably insufficient to measure total metal concentrations in the soil and sludge prior to sludge addition. It is also important to monitor changes in the soil resulting from interactions between sludge and soil.

 

REFERENCES

Bacon JR, Berrow ML and Shand CA (1995), Chem. Geol. 124: 125–134

 

Chambers BJ, Garwood WD, Chaudri AM, McGrath SP, Carlton–Smith CH, Hall JE, Hallett JE, Bacon J, Campbell CD, Coull MC and Aitken MN (1998), Effects of sewage sludge applications to agricultural soil microbial activity and the implications for agricultural productivity and long–term soil fertility. Final report (CSA 2566). London, UK Water Industry Research Ltd

 

DIN 19730 (1997). Extraction von Spurenelementen mit Ammoniumnitratlösung. Berlin, Deutsches Institut für Normung.

 

McGrath SP and Cunliffe CH (1985), J. Sci. Food Agric. 36: 794–798

 

Rauret G, Lopez–Sanchez JF, Sahuquillo A, Rubio R, Davidson C, Ure A and Quevauviller P (2000), J. Environ. Mon. 1: 57–61