Salinity is one of the most important water quality issues in agriculture, which plays an important role in the production and productivity of crops. In this research, to reduce the salinity of agricultural drainage water simulated with sodium chloride salt, biochar was prepared from date palm leaves waste at different pyrolysis temperatures (350, 450, 550, and 650 ℃), and their performance was investigated for the adsorption of water salinity ions (sodium and chloride) using batch adsorption experiments in a completely randomized design with four replications. The results of this experiment showed that date palm leaves biochar prepared at 650 ℃ (DPLB650) had the highest adsorption. In the second step, the effect of independent variables pH (5, 6, 7, 8, 9), adsorbent dosage (10, 20, 30, 40, 50) g/L, contact time (30, 60, 120, 180, 240) minutes and EC (4, 8, 12, 16, 20) dS/m on the adsorption efficiency of sodium and chloride ions by DPLB650 adsorbent, using Taguchi experimental design method was evaluated and the most optimized variables and their levels was determined. The results showed that the maximum adsorption efficiency of sodium and chloride ions (27.43% and 27.56%, respectively) occurs at pH equal to 7, an adsorbent dosage of 30 g/L, a contact time of 180 minutes, and an initial EC of 20 dS/m. To validate this prediction, an experiment was conducted under the mentioned optimal conditions, and the adsorption efficiency of sodium and chloride ions was obtained as 27.61% and 27.65%, respectively. Investigating different adsorption kinetic and isotherm models showed the results followed the pseudo-second-order kinetic model and the Langmuir isotherm model.

EXTENDED ABSTRACT

Introduction

The world's growing population and water resource shortages make it necessary to utilize saline water, such as agricultural drainage water. However, the salinity of irrigation water has many destructive effects on crops and soil particles. Various methods are used to desalinate saline water, but relatively expensive. Thus, using low-cost methods such as using inexpensive adsorbents is important. Biochar is a carbon-rich solid produced from the pyrolysis of plant biomass and agricultural wastes. Biochar, Due to its physical and chemical properties, has shown promise in the adsorption of various pollutants from aqueous solutions. One potential source of biochar is agricultural waste, such as date palm tree leaves waste. By converting this waste into biochar, we can effectively manage agricultural waste.

Materials and methods
Agricultural drainage water was simulated using sodium chloride salt at five different salinity levels of 4, 8, 12, 16, and 20 dS/m. The biochars were produced from date palm tree leaves biomass at four different pyrolysis temperatures 350, 450, 550, and 650 ℃ with a retention time of 3 hours. The prepared biochars were sieved to 105-500 micron particle sizes. Then, their product yield and ash were determined. The EC and pH of biochars were measured. Using the BET method the specific surface area of biochars was measured. In the first step, the performance of biochars was investigated for the adsorption of sodium and chloride ions. Then, the most optimized biochar was selected for use in the next step. Batch adsorption experiments were conducted in a completely randomized design for each of the adsorbents with four replicates under the same conditions, pH equal to 7, an adsorbent dosage of 20 g/L, a contact time of 240 minutes, and an initial EC of 12 dS/m. In the end, the sodium and chloride ions adsorption capacity of the adsorbents was calculated. After selecting the optimized adsorbent, its elemental composition was determined using EDS analysis. Also, the pHpzc of the selected adsorbent was determined. In the second step, the Taguchi experimental design method was used to optimize the effective factors pH, adsorbent dosage, contact time, and initial EC at five different levels on the adsorption efficiency of sodium and chloride ions. 25 batch adsorption experimental combinations were determined and each of the experiments was conducted with three replicates. The evaluation of the signal-to-noise ratio (S/N) and analysis of variance (ANOVA) were conducted to analyze the experimental results. Given the goal of this research, the S/N based on the larger value is better chosen. Finally, kinetic and isotherm models were fitted on the adsorption experimental data.

Results and discussion
The results indicated that the production yield of prepared biochars has decreased with the increase of pyrolysis temperature from 350 ℃ to 650 ℃ while, their EC, pH, ash percentage, and specific surface area have increased. By comparing the performance of date palm leaves biochars, the DPLB650 adsorbent, which had the highest adsorption of sodium and chloride ions, was selected as the most optimized one and used in the second step of the research. In the second step, the results of Taguchi's experiments revealed that under optimal conditions (pH equal to 7, an adsorbent dosage of 30 g/L, a contact time of 180 min, and an initial EC of 20 dS/m), the maximum adsorption efficiency of sodium and chloride ions is 27.43% and 27.56%, respectively. For validation, an experiment was conducted under the mentioned conditions, where the adsorption efficiency of sodium and chloride ions was obtained as 27.61% and 27.65%, respectively. In addition, the pH, contact time, and initial EC at the p≤ .01 level and adsorbent dosage at the p≤ .05 level have a significant effect on the adsorption of sodium and chloride ions. The investigating results indicated that the pseudo-second-order kinetic model and the Langmuir isotherm model have the best fit on the experimental data of the adsorption of sodium and chloride ions.

Conclusion
In this study, reducing the salinity of agricultural drainage water simulated with sodium chloride salt at different salinity levels was investigated using date palm leaves biochar prepared at different pyrolysis temperatures (350, 450, 550, and 650 °C). The results indicated that DPLB650 had the highest performance on the adsorption of sodium and chloride ions. This adsorbent under optimal conditions (pH equal to 7, an adsorbent dosage of 30 g/L, a contact time of 180 min, and an initial EC of 20 dS/m), can reduce the electrical conductivity of agricultural drainage water by about 27%. The initial EC has the largest contribution to the adsorption of sodium and chloride ions. The pseudo-second-order kinetic model and the Langmuir isotherm model satisfactorily described the adsorption process.